Essay: Experimental Methodology, Equipment and Materials

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3.1 Introduction

Chapter three presents an overview of the experimental methodology and materials used in the study reported in this thesis. The first part of the chapter shows the method, the experimental details and a description of the test vehicles used in carrying out the study. The second part describes the innovative materials, equipment and procedure employed in this work. The third part presents the manufacturing process of solder joints stencil printing used in the study. The fourth section concerns a description of the reflow profile for the formation of solder joints and the thermal ageing processes respectively. The fifth and last part present the metallographic preparation of test samples for the measurement of CSH and IMC, and for the microstructural analysis of solder joints.

3.2 Methodology, Experimental Details and Description of Test Vehicles

3.2.1 Methodology

The method used in this investigation is the scientific and experimental approach. It comprises of Experimental Details, Test Vehicle Preparation/Assembly Procedure, Equipment and Materials. The process of innovative design as a method involving both quantitative and qualitative techniques is adopted in the data analysis. The experimental data was validated using results from the literature. Figure 3.1 presents the flow chart of the experimental methodology used in this work.

Figure 3.1: Flow chart of the experimental methodology

3.2.2 Experimental Details

This section presents a description of the experimental details. Figure 3.2 illustrated in the form of a shuttle card flowchart gives an overview of the pictorial representation of the entire chapter ranging from experimental set up to the conclusion. The significant points include for example test vehicles, used in a sequential manner to achieve the results reported in this thesis. Test vehicles with Sn-Ag-Cu lead-free solder paste and component chip resistors are prepared using the Benchmarker II stencil filling apertures shown in Figure 3.3. The experimental details, however, affords the reader an opportunity to have at a glance of what is contend in the overall chapter without necessarily going through all. All the experimental tools and equipment used in this study are located at the Engineering Science and Manufacturing Systems Laboratory of the University of Greenwich at Medway, UK.

Figure 3.2: Experimental details

3.2.3 Test Vehicles Description

In this study, five types of test vehicle designs were made in-house using SMT materials commonly utilised in the manufacture and assembly of electronics components. Cu and Sn-plated (surface finishes) substrates were used to fabricate the test vehicles. The test vehicles were designed using half-automated stencil printing machine or done manually by using Benchmarker II stencil as presented in Figure 3.3. Details of the experimental test vehicles are shown in Figures 3.5, 3.9, 3.12, 3.13, 3.14, and Figure 3.16 respectively.

Figure 3.3: Benchmarker II showing areas of interest & enlarged test vehicle

3.2.4 Test Vehicle 1: Effect of Reflow Profile Verification

Test vehicle 1 (Figure 3.5) was designed to verify the effect of reflow profile parameter setting on the shear strength of solder joints in SMT chip resistors assembly. The test vehicle consists of a fabricated single sided 100% copper clad FR4 board strips with a thick film metallisation and a substrate dimension of (80 x 120 x 1.6) mm. Lead-free solder paste with alloy composition of 96Sn-3.8Ag-0.7Cu was used to complete the fabrication with the help of Benchmarker II stencil printing apertures (Figure 3.5 (i)). Three different pad sizes replicating pad sizes of typical SMT component resistors were used, these include 1206, 0805 and 0603 resistors. The experimental test Procedure in Figure 3.4 is for the ‘non-aged’ and ‘aged’ samples. It comprises of five steps plus one additional step for the isothermally aged samples.

The steps employed in the experimental procedures include:

‘ Cleaning the substrates with isopropanol and placing them under the stencil strapped over with solder paste; such that it could stick to the substrate in the same pattern to form solder pads.

‘ As soon as the model of the solder paste was formed, and stencil removed, components were then, picked and placed. The placement uses a needle-like pen, which dips in the adhesive flux to have enough grip for picking and placing the components.

‘ Finally, substrates were used to assemble the three different types of components to form the test vehicles employed in this study.

‘ Steps 1 to 2 repeated to replicate five substrates each with 71 IC components. This same process was used to achieve the experimental results from test vehicle 2.

Figure 3.4: Experimental procedure of test vehicles

Figure 3.5: Test Vehicle 1 used for the effect of reflow profile parameter setting

3.2.5 Test Vehicle 2: Effects of Strain Rate Verification

Test vehicle two was for the effect of strain rate on the Thermomechanical Reliability (TMR) of surface mounted solder joints in electronic manufacturing.
Figure 3.9 shows test vehicle two which consists of three different types of surface mount chip resistors (namely ‘1206’, ‘0805’ and ‘0603’) as in test vehicle one shown in Figure 3.5.The resistors are soldered on the copper substrate according to the parameters shown in Table 3.1. The assembled surface mount components used five bare Cu boards for their fabricated substrate.

Two of the test vehicles were ‘aged’ isothermally for 24 hours at 150”C and a constant humidity of 35% RH. Each substrate contains 142 components; for ‘non-aged’ samples, a total number of 50 components of (1206 and 0805) and 42 for (0603) are used. For the aged samples, same numbers of ‘components’ allotted for it. An ‘aged’ populated Cu board at the ageing temperature of 1500C for ten days is presented in Figure 3.6. A schematic of a standard SMT chip resistor is in Figure 3.7. The SMT resistors is widely used in automotive applications (Lau, 1991; De Gloria, 2014; Johnson et al., 2004; RS Components for Automotive, tape recorders, 2014; Middelhoek, 1994; Otiaba, Okereke and Bhatti, 2014).

Figure 3.6: Cu PCB Sample with SMT Components Aged at 1500C for 10 Days

Figure 3.7: Schematic of a standard SMT chip resistor
Source: (RS Components for Automotive, tape recorders, 2014)

Table 3.1: Dimensions of the chip resistors (in mm)
Source: (RS Components for Automotive, tape recorders, 2014)
Type Length, L Width, B Thickness, D Width of wrap
around, T Weight (g)
(1000pcs)
1206 3.10 ” 0.10 1.55 ” 0.10 0.55 ” 0.10 0.50 ” 0.20 8.947
0805 2.00 ” 0.10 1.25 ” 0.10 0.50 ” 0.10 0.40 ” 0.20 4.368
0603 1.60 ” 0.10 0.80 ” 0.10 0.45 ” 0.10 0.30 ” 0.20 2.042

3.2.5.1 Solder pad land pattern, size chart and shear area

The design of an SMT component uses the right solder pad size and land pattern upon which the shear area depends. During reflow soldering, however, the land width must be smaller than the chip resistor width to control the solder volume properly. For this purpose, usually the land width is set at 0.7 to 0.8 times (W) of the width of chip resistor while for reflow soldering solder size can be adjusted with a land width set to 1.0 to 1.3 times chip resistor width (W). These settings and land pattern measurements vary according to manufacturers’ specifications and use environments (found from their data sheet) and which might differ slightly from the information provided in the chart given in Figure 3.8.

Figure 3.9: Test vehicle 2 used for the effect of strain rate on TMR

3.2.6 Test Vehicle 3: Effects of CSH Verification
Test vehicle 3 is adopted for the effect of CSH on the thermomechanical reliability of BGA solder joints. Two experiments were conducted to check for the effect of pad size and temperature variation on solder joint reliability. Also, a third test was performed using a copper surface finish (CuSF) pad with board area dimension of 23 ” 23mm and 1.55mm thick. It was prepared using surface mount assembly process shown in Figure 3.10.

Figure 3.10: PCB Test vehicle assembly process

3.2.6.1 Test Vehicle Preparation (BGA81)
180 BGA81 components and 10 FR4 PCBs of (101.78 x 138.58 mm2) size are used. Four BGA components of Sn-Ag-Cu solder alloy composition; 1.00 mm pitch dimension and 0.36 mm ball diameter are placed on each of the six different pad sizes: 19, 20, 21, 22, 23 and 24 mil diameters with two different surface finishes. The assembly process comprised of application of flux on PCB, component placement by pick-n-place machine for alignment of the BGA Die, reflow soldering of the assembly using a convection oven at a peak temperature of 235 ”C and finally, visual inspection was carried out. The study design for CSH is given in Figure 3:11.

The test samples for isothermal ageing placed in the environmental chamber operated at a temperature of 1500C and relative humidity of 35% for ‘2, 4, 6 and 8’ days respectively. The chamber program was to operate for 200hrs. Ten test vehicles were made in-house, five ‘as-soldered’ samples are used for shear tests, other five samples, which were ‘Aged’, were cross-sectioned, and metallographic prepared for the measurement of CSH using SEM. Just as mentioned previousely, Figure 3.11 presents also a step- by-step method of measuring the CSH; while Figure 3.12 (i) and (ii) represented the assembled test vehicle, and the BGA81 component used.

Figure 3.12: Test vehicle 3(a) – for effect of BGA CSH on TMR of SJs

3.2.6.2 Test Vehicle Preparation (BGA169)
For producing the test vehicles with BGA169 components, a similar process was followed (as prescribed earlier for BGA81 components). The main difference here was to use a constant pad size and different reflow peak temperatures (to achieve different CSHs).

3.2.6.3 Reflow and Ageing of BGA169 Assemblies
The assembled packages shown in Figure 3.13 were then reflowed using convection reflow oven (Novaster 2000 NT) described in section 3.4.3, which enables uniformity in the transfer of heat across all areas of the assembled packages. The purpose of the reflow process was to allow the convection heating of the solder alloy substance to attain a temperature which is a little above the melting point of the alloy itself to enable soldering of the BGA169 solder balls onto the substrates to establish mechanical and electrical bonding of the assembly. Care was taken not to use temperature profiles, which could totally melt the solder alloy causing it to flow and causing bridging across the boards. Ramp-to-spike reflow profile at 225”C, 235”C, 245”C, and 255”C Peak temperatures with a tolerance level of ” 5 was used to reflow the BGA169 devices for a duration time of 480s.
The soldered assemblies are then halved, and one part was subjected to isothermal ageing at 150”C for 200 h (8 days) in an environmental ageing chamber (Espec ARS-0680). The ageing of the test vehicles at the same isothermal temperature and time duration was carried out to ensure that IMC growth is constant across all the test vehicles.

Figure 3.13: Test vehicle 3(b) – BGA169 on FR4 SnSF board for CSH.

3.2.7 Test Vehicle 4: Effect of Voids Verification
Test vehicle 4 presented in Figure 3.14 was fabricated to determine the effect of solder type, reflow profile and PCB surface finish on the formation of voids in BGA lead-free solder joints. The effect of surface finish on the PCB is the factor under investigation and two different pad surface finishes, Ni and copper boards were used to determine its effect on the formation of voids in the BGA solder joint.

Figure 3:14: Test vehicle 4- for SB x-ray analysis on effects of voids in SJs

The Test vehicle 4 board’s dimension is (80 x 120 x 1.6) mm and each board consists of 20 soldered bumps. The boards were cleaned using Methylated spirit and Isocline Isopropanol to increase wettability and minimise voids formation. The same two-stage cleaning process was employed to clean the stencil and squeegee utilised during solder printing process. The paste types are 96SC LF 318 and 97SC LF700. The former is level 1 while the latter is level 2. Also, the nickel (Ni) PCB pad finish used was tagged level 1 and copper (Cu) is level 2. The third factor is the reflow profile, and only one parameter of it was under consideration, and this is the activation energy. Activation energy identified as a critical factor in this investigation has its effect on the formation of a quality solder joint. During activation stage of reflow profile, the flux in the solder paste and other soluble contents was driven off. It has been reported by (Beddingfield and Higgins, 1998) that the amount of flux including other solvents matter in the solder paste mix determines the percentage by volume of voids in the solder joint. The activation temperature has a direct influence on the degree to which the flux matrix is driven off the paste mix. Thus, two different activation temperatures are used for the reflow profile. Level 1 is a 190”C, and level 2 is a 200”C centigrade temperatures. Table 7.1 presented in Chapter 7 section 7.3 the designated solder paste as A, reflow profile as B and PCB surface finish as C. It also shows the levels, as ‘1 and 2’. The experiment was conducted using full factorial design. Stencil printing/bumping of the solder paste on the pad surface was the next process, followed by component placing and formation of the solder joint through reflow soldering process described in section 3.4.3. The reflow profile used has been presented in Chapter 7, Figures 7.3 – 7.6.

3.2.8 Test Vehicle 5: Effect of ATC on Long Term Reliability of Solder Joint
Test vehicle 5 was used to investigate the effect of Accelerated Thermal Cycles (ATC) on the long-term reliability of solder joints. The test vehicle was prepared by manual placement of BGA solder balls on flexible substrates with the aid of halide flux, which serves as adhesives and oxide remover. The solder ball is lead-free, 0.76 mm in diameter and has alloy composition of Sn-4.0Ag-0.5Cu (SAC405). The board consists of electroplated Au/Ni-Cu pad. A total number of 100 pads were used to achieve this work. The assembled boards are divided into five groups with one kept for ‘As-reflowed’ sample and the rest thermally cycled for 33, 66, 99 and 132 hours. The parameters and the temperature profile used for this investigation are selected according to JEDEC standard, JESD22-104D (JEDEC, 2009). The test condition for the temperature cycling and the thermal chamber was set to operate at 43 minutes per cycle and has a temperature range of 0oC to 150oC with a ramp rate of 3.5oC/minute (150oC/43mins) resulting in a dwell time of 10 minutes, ramp down and ramp up of 11.5 minutes each respectively. The parameters and the temperature profile are given in Table 3.2 and Figure 3.14. The flexible substrate test vehicle and materials for its preparation are shown in Figure 3.15, while the same test vehicle, equipment and experimentation processes are presented in the form of a flow system illustrated in Figure 3.16.

Table 3.2: Thermal Cycling Parameters
Low Temperature High Temperature Ramp
Rate Dwell
Time Cycle
Period
00C 1500C 11.50C/ min 10 min. 43 min.

Figure 3.14: Thermal Cycling Profile measured for 43 mins per period

Figure 3.15: Test vehicle 5 – showing its material constituents from (a-c)

Figure 3.16: Test vehicle, equipment and processes used in the study
3.3 Materials and Processes
The key materials used in this investigation most of which were discussed in the previous sections consist of three different sizes of surface mount chip resistors (R1206, R0805 and R0603). They include multicore Sn-Ag-Cu lead-free solder paste, FR4 copper substrate, BGA components, halide flux, conductive bakelite powder and monocrystalline diamond suspensions, only a few of them will be discussed here.

3.3.1 Sn-Ag-Cu Lead-free Solder Paste
A commercially available Tin-Silver-Copper lead-free solder paste sample with type 3 particle size distributions and alloy composition of (95.5w%Sn-3.8w%Ag-0.7w%Cu) weight percent (as previously discussed) are used in this investigation as the jointing material. It has a metal content of 88.5% by weight, and a melting point of 2170C. The paste and its container are represented in Figure 3.17, while the dimensions of the chip resistors are given in Table 3.1 above. The particle size is acquired from the manufacturer’s data sheet, and the paste sample is stored in a fridge at -4”C. The details of the respective samples is provided in Table 3.3. It can be observed from the table that the size of the R1206 is the largest while that of the R0603 is the least. The size variation is introduced to study the impact of miniaturisation of electronic components and devices on the thermomechanical reliability of their solder joints. Other materials used for the purpose of achieving the required research studies in this thesis are found in the chart provided in Figure 3.21.

Figure 3.17: Lead-free solder paste consisting of 95.5Sn 3.8Ag 0.7Cu alloy

Table 3.3: Solder paste details
Materials Content
Solder Alloy 95.5Sn-3.8Ag-0.7Cu
Particle Size Distribution, ”m 25-45
Metal Loading, weight % 88.5
Flux Type No-clean and Halide-free

3.3.2 Universal FR-4 Board and BGA Flexible Substrate
The Universal FR-4 PCB is commercially available and lead-free components compliant. Three types of the FR-4 PCBs plus the flexible substrate are used in this study. Two of the FR-4s and the flexible substrate have surface finishes made of Tin while the other was made of copper. The first of the two has a tin-plated surface finish (Figure 3.19) with different pad diameters ranging from 19mil to 24mil for the sole purpose of the research findings and design. The substrate is designed to mount up to twenty-four BGA81 components per side with pads having 1.0mm and 0.8mm pitches. However, the 1.0 mm pitch pad sizes was used for the experiments. Four (4) components are placed on each of the different pads. Altogether, a total number of ten boards were used for the BGA81 CSH experiment as earlier described. The second is also a Tin Surface Finish (SnSF) FR-4 board with dimensions of area and thickness of 23×23 mm and 1.55 mm, respectively. Its pitch is 1.5 mm while the diameter of its pad is 0.584 mm (23 mils).

Figure 3.18: Image of the lead-free universal FR4 BGA printed circuit board

Similarly, the Copper Surface Finish (CuSF) board has an area of 23x23mm and thickness of 1.55mm. The flexible substrate mainly was designed to provide solutions to more fragile and highly miniaturised electronics components and integrated circuits. It is a technique, which greatly simplifies the making of interconnections between various planar portions of an assembly. The use of flexible substrate may include compact packaging configurations that enhance dynamic performance and ensure a cost-effective production part. The component placement on PCBs and test vehicle preparations are preceded by flux application or by stencil printing of solder paste on the tin-plated surface finish board, (or on bare Cu boards as was required). The components were mounted on all four types of (substrate) PCBs to form the test vehicles used. The two types of FR4 PCBs with SnSF and CuSF and the flex circuit described in the preceding discussions i s given in section 3.3.3. The full description of the Benchmarker II is found in a chart given in Figure 3.21.

3.3.3 Benchmarker II Laser-cut Stencil
A stencil provides the openings for all the components on the board or substrate so that the printing of paste can be through the apertures. The number of openings on a stencil matches the number of openings required for the surface mount components on the board. Stencils are uniquely made to match specific PCB designs and maynot hence be used for others. As shown in Figure 3.19 (a), a laser-cut stencil with a thickness of 0.125 mm was used in this study, while
Figure 3.19 (b) shows a close view of the Benchmarker II discussed earlier.

Figure 3.19: (a-b) Benchmarker II laser-cut stencil

3.3.4 Solder Flux
Flux acts as a temporary adhesive, holding the component in position before reflow soldering process. The solder flux utilised for this experiment is the no-clean type either rosin or halide flux (Figure 3.20 (d)), which is applied directly onto the surface of the printed circuit board during ‘Test vehicle’ preparation. The rosin flux is comprised primarily of refined natural resins extracted from the Oleoresin from pine trees. However, Rosin fluxes are inactive at room temperatures but become active when heated to soldering temperatures. The melting point of the resin is 1720C to 1750C. Rosin fluxes are used purposely to reduce solder balling and bridging, as well as aid proper solder paste flow and increased wetting of desired areas (Prasad, 1989; Ning-Cheng, 2002; Xun-ping et al., 2010). Another type of flux commonly in use in the laboratory where the experiments were conducted is the Halide flux. However, in electronic packaging and solder interconnect, halides flux whose content is of halogenated compounds, usually from bromides or chlorides have been in use for years to reduce metallic oxides. In previous years, there was great concern that ionic halide left on the PCB as residues could cause corrosion or dendritic growth in the solder joint of assembled components, and for this reason, the packaging industry of electronics solder alloy has begun to use covalently bonded halides, which are much more reliable and profitable.

3.3.5 Other Materials Used

3.3.5.1 Conductive Bakelite Powder
The conductive Bakelite (Figure 3.20 (e)) is a moulding powder developed specifically for use in hot mounting processes. The powder comes in different colours and is particularly useful for electron microscopy, with sufficient electrical conductivity to provide a real solid earth leakage from the specimen (Azeem and Zain-Ul-Abdein, 2012; Muir Wood et al., 2003).

3.3.5.2 Monocrystalline Diamond Suspensions
The monocrystalline diamond suspensions (Figure 3.20 (f)) used for the specimen’s metallurgical preparation are those of 6 microns and 1 micron respectively. The suspensions are applied onto the grinder via a nozzle or injection system before diamond polishing of the specimens. This suspensions provides a chemo-mechanical polishing (CMP) action that significantly increases removal rates, reduces subsurface damage and improves surface finish (Muir Wood et al., 2003; Tighe, Worlock and Roukes, 1997).

Figure 3.20: SMT materials used for the studies carried out in this thesis

3.3.6 Ball Grid Array Components and Their Geometric Representations
The BGAs used in this work are of two types: BGA81 and BGA169. For a full description, the BGA 81 has 9×9 full matrix array, 10x10mm in size and 0.36mm/0.46mm bottom/top ball diameters. Its pitch is 1.0 mm while the composition of its lead-free solder alloy is 95.5%Sn-3.9 percentage Ag-0.6%Cu (SAC405). The other component, BGA169, consists of 13×13 full matrix arrays and is 0.76mm in diameter. It has 1.5mm pitch dimension and the same composition of solder alloy as the BGA 81 component. Figure 3.21 (a) and (b) presents the lead-free BGA components while (c-d) show for example design settings for typical BGA81 and BGA169 components.

Figure 3.21: Pb-free BGA81 & 169 displaying (a-d) Top and bottom Side View

Nevertheless, Figure 3.22 presents the design configurations of BGA81 & 169 top and bottom ball view. The configurations are employed generally by component manufacturers during assembly.

Figure 3.22: Design configurations of BGA81 & 169 top and bottom ball view
SOURCE: [ToplineBGA.com].
3.4 Equipment and Process
This section presents a brief description of the state-of-the-art laboratory equipment, experimental setup and their parameter values used for the investigations in this thesis. In ensuring that the data cum results obtained to comply with the IPC/JEDEC standards, the equipment used is not different from those commonly used in SMT packaging industries. The first set of the equipment includes DEK 260 Stencil Printing machine used when printing solder paste onto a substrate, Gold-Place L20 Pick and Place (PnP) device used to place components (e.g. chip resistors and BGAs) on boards. The second phase comprises Novastar 2000HT ‘Horizontal Convection’ reflow oven for the reflow soldering process, ARs-0680 Climatic (Temperature and Humidity) chamber for ‘thermal and isothermal ageing’. The third part includes Struers Accutom-5 precision and Guillotine manual cutting machine, Dage Bond Tester (DEK 4000PXY series) for destructive ‘shear testing’. The final phase includes Struers polishing machine, X-Ray machine for ‘analysis of voids’ in solder joints and the Benchtop SEM for evaluation of the microstructure and fracture analyses of solder joints. Figure 3.23 presented a summary of the experimental equipment and processes and described in details afterwards. A description of the thermal cycling and vibration chamber and the Reichert microscope was not given, but the metallographic equipment and processes are elaborately dealt with towards the end of this chapter.

Figure 3.23: Equipment and Processes used in the study
3.4.1 Machine for Stencil Printing of Solder Paste
Stencil printing is a vital step in electronics assembly because an appropriate selection of solder paste volume, stencil aperture and squeegee pressure would contribute substantially to the quality and reliability of the solder joint. Stencil printing of solder paste used in the assembly of chip resistors is one of the critical steps in surface mount manufacturing. This machine works with both metal and the rubber squeegee. The metal (stainless steel) squeegee used for this experiment has an angle of 45”. Other specifications for this machine include the use of semi-automatic standalone screen printer, use of DEK Align 4 vision system with a print area of 440 x 430mm (17, 3’x16, 93′). The maximum board size is 500 x 450mm (16.69’x17.72′) and screen frame is 508 x 508mm (20’x20′) internal. A programmable control of process variables comes with this machine which ensures accuracy and repeatability in most demanding and busy situation (Mallik et al., 2009; Durairaj et al., 2002).
The printing process involves as previously mentioned a squeegee mechanism, which directly affects the product yield and quality of the final assembly. Moreover, with ‘Fine Pitch’ technology as today’s technology demands, it is more prone to residual defects. However, the majority of the soldering defects encountered after the reflow process include delamination, open/short circuits as well as circuit bridging problems. They are contingencies attributive to defects originating from solder paste disposition process. The solder paste printing is achieved following IPC/JEDEC standard printing process by using Benchmarker II stencil apertures and by appropriately selecting the right choice of (Sn-Ag-Cu) solder paste/volume, substrate selection with appropriate surface finish pad and quality rubber or metal squeegee.
The parameters used in achieving the stencil printing at no or insignificantly little defects are presented in Table 3.4 while the stencil printing machine used throughout the experiments is DEK 260 SERIES as shown in Figure 3.24.

Table 3.4: Stencil printing parameters used
Parameters Values
Forward Print Pressure 20mm/s
Pressure 8.0kg
Batch count 235 cycles
Vision alignment 0
Print mode DBL squeegee
Reverse print speed (RPS) 20mm/s
Print stroke 342mm
Inspect rate 0
Separate speed 100%
Print gap 0.0mm

Figure 3.24: Stencil printing machine -DEK 260 series.

3.4.2 The APS Gold-place L20 Pick and Place (PnP) Machine
This equipment was used to place the components onto the PCB terminations or land areas to form test vehicles temporarily before reflow soldering. Solder flux was initially applied onto the PCB surface to hold the materials in position tentatively after being placed by the PnP machine to form the test vehicle as shown in Figure 3.25.

Figure 3.25: (a) PnP machine (b) Enlarged test vehicles after comp. placement.

The computer uses the advanced planning and scheduling (APS) software to control the pick and place machine, which was pre-programmed for the printed circuit board and components placement. The programming of the device enables components’ mounting lacing with the aid of a vision fitted camera from its tray to the set location on the FR-4 PCB. The machine also uses a vacuum pick up tool to hold the component as it positions in a central squaring and vision-assisted alignment basin before precision placement accomplishment. For test vehicles with BGA81 (designated test vehicle 3a), a total number of five PCBs were used, with four components placed on each pad size (i.e. 19, 20, 21, 22, 23 and 24mil diameters). The various pad dimensions were used to fabricate the test vehicles used for the ‘As-soldered’ and ‘Aged’ shear strength test samples. In the case of test samples for SEM examination, five (5) PCBs were also utilised with only two components placed on each of the pad sizes. The total number of components configured by the PnP machine on each board for the shear strength and SEM test samples were twenty-four (24) and twelve (12) ‘components’ respectively. However, for test vehicle with BGA169 (designated test vehicle 3b), a total of forty-eight (48) components were placed, two (2) components per board on sixteen (16) boards (PCBs) for SnSF pads and eight (8) boards for CuSF pads which represent test vehicle 3c. In all, one-half was used for ‘as-soldered’ while the other for ‘Isothermal ageing’.
3.4.3 Convection Reflow Oven for the Reflow Soldering Process
The Novastar (2000HT horizontal) convection oven was used for the reflow soldering process. The process includes conveying the test vehicle (substrate with components already placed on it) through an oven with successive heating elements of varying temperatures. In the oven, each board typically goes through the stages of gradual pre-heating, brief duration at high soldering temperature ramp, controlled collapse (occurring at liquidus temperatures), and cooling process. This process lasted for about seven to eight minutes where the samples had to cross the six different heating zones and one cooling zone of the oven, each of them having their set temperature according to the set reflow profile. Soldering temperatures require appropriate temperature profiles for a given experimental design. The Novastar model is a production scale reflow soldering machine type, which operates on a forced convection heating system using heating elements, which can attain a maximum temperature range of 350 ”C for each of the heating zones. The PCB test vehicle is carried on an 1829mm long conveyor belt system, whose speed is adjusted between 0.05 to 0.99m/min. Figure 3.26 shows a reflow oven in which components were reflow-soldered. Once the reflow process is completed, three substrates are separated for isothermal ageing. The two substrates left were kept for the shear strength test, which was next in the list.

Figure 3.26: Convection reflow oven for components soldering.
3.4.3.1 Temperature Profile Used for the Reflow Soldering
The peak reflow temperature for the chip resistors solder joint was kept at around 245 ”C as shown in Figure 3.27. This step allowed the solder to melt and form the joint. The Ramp-To-Spike (RTS) reflow profile was used because of the uniform heating of the test vehicle, thereby ensuring that the thermal profile increase continuously along preheats and soak regions up to the peak temperature, before cooling down rapidly. The reflow duration was set at 8 minutes. Another purpose of the reflow profile was to enable the heating of the solder- alloy material to a temperature which is a little above the melting point of the alloy to allow soldering of the BGA or chip resistor solder balls/alloy onto the substrates to establish mechanical and electrical bonding of the assembly. Care is taken (following the optimisation process described in chapter four of this thesis) not to use temperature profiles which could totally melt the solder alloy causing it to flow and cause circuit bridging across the boards.
For the effect of CSH verification of BGA81.1.0T1.ISO component using 19-24 mil pad size variation (Test Vehicle 3a), a Ramp-to-spike reflow profile peak temperatures of 235”C was used. The peak temperature profiles of 225”C, 235”C, 245”C, and 255”C were used to verify the effect of BGA169.1.5T1.ISO CSH under temperature variation (Test vehicle 3b), both with a tolerance level of ” 50C. The RTS profile has the further advantage of producing brighter and shiner joints that have lesser solderability problems due to the availability of flux vehicle in the solder paste during the preheat stage of the reflow process. The utilisation of peak temperature variation at various and critical stages of reflow allows the component resistors cum BGAs solder joints approach their dissolution state, dissolve and metallise to form a joint with the Cu base metals. The soldering process comprised the following reflow stages:
‘ Pre-heating stage – The Pre-heat stage is the point when the solder particles heat up before getting to their melting point level.
‘ Activation stage – Activation stage is the stage when the oxides in the flux evaporate.
‘ Reflow stage – The Reflow stage is the stage when the flux reaches its melting point at liquidus temperature.
‘ Cooling stage – The Cooling stage is the point when the samples cool down at a ramp-down rate.
Figure 3.4 and Figure 3.16 discussed earlier in this chapter present the experimentation process and equipment. The display of the RTS profile employed in this study is in Figure 3.27. The reflow temperature profiles for the BGA test vehicle 3a and 3b are presented in Figure 3.28 and Figure 3.29.

Figure 3.27: Sample of the chip resistors reflow profile

Figure 3.28: Reflow profile for test vehicle 3a

Figure 3.29: Reflow profile for test vehicle 3b

3.4.4 Climatic Chamber for Isothermal Ageing
Isothermal ageing of part of the test vehicles (for ageing temperature) was carried out in a Temperature and Humidity Chamber of the model (Espec ARS0680) with dimensions W1050”H1955”D1805. It has a programmable control unit to set the required temperature range in Celsius and time in hours. The control unit has a touch screen user Interface with which input was given to establish the parameters for the ageing process. The isothermal ageing process used involved placing three substrates of the test samples in the chamber, setting the control unit for 250 hours at 150 0C and saving it for all the components. The programme then ran to perform the process. Through the interface application of the chamber during process operation, and after every 24 hours the humidity level was continuously monitored and checked. The device application chamber switched off automatically after 250 hours of operation, and the samples removed from the hot enclosure for further analysis. The photograph of the climatic chamber is presented in Figure 3.30 (a-c).

Figure 3.30: (a) Temperature and Humidity chamber, (b) Programmable screen user interface and (c) Samples inside the chamber

3.4.5 Dage Bond Tester (DEK 4000PXY Series) for Test & Measurement
The Multipurpose 4000 series Dage Bond Tester is capable of performing all pull and shear test applications. The tester is configured as a simple wire pull tester, which is upgraded to provide ball shear, die shear, and bump pull tests. The equipment uses frictionless load cartridges and air bearing technologies, which ensure maximum accuracy, repeatability and reproducibility. The cartridges are designed for different applications and are readily exchanged to match a chosen operation. Its function is automated also with sophisticated electronic and software controls.
The test specimens were held in position within a sizeable fixture before the components were sheared at standard shear speed and shear height of 200”m/s and 60 ”m for BGAs and 30”m for the chip-size resistors respectively. This equipment was used in this work to obtain the shear force required for the destructive tests on solder joints of BGAs and the components of the chip-size resistors. It was also used to verify the integrity of the soldered assembly by using the set parameters in controlling the shear tool when shearing the SMD components from the substrate through the solder joints.
Dage bond tester mechanism involves the use of micro force to shear a soldered joint on the printed circuit board permanently; by so doing, the strength of the solder joint obtained reports and records on the screen. The overall aim of this task was to measure shear strength of solder joints for each component type at a designated or varying shear speeds. The Dage Bond tester is illustrate in Figure 3.31, and the enlarged form of the test vice, shear tool cartridge and shear tool position on test vehicle during shear testing is presented in Figure 3.32(a-b); while the process steps for the shear test are outlined in section 3.4.5.1 respectively.

Figure 3.31: Dage Series 4000, Shear Testing Machine.

Figure 3.32: (a) Shear tool/sample holder (b) Shear testing position.

3.4.5.1 Process Steps Used in Shear Test and Data Collection
The process steps involve cutting the substrate into the right shape that can fit into the vice by removing the unwanted parts, and then apply shear force on each of the components as shown in Figure 3.33; with shear height and shear direction clearly indicated. The detailed steps are as follows:

‘ The metallic board is trimmed and made of right shape so that they could fit into the test ‘Vice’ of the Dage Bond Tester.
‘ After fixing and tightening the substrates correctly on the test vice, the shear test is performed by applying shear force through the tool of the tester at different shear speeds.
‘ The shear height is set at 30”m for a resistor and 60”m for a BGA solder joint. The shear height is the distance between the tool and the surface of the substrate.

Shear force values for each component was noted, and results were accordingly tabulated and categorised as there had to be segregation in the results for aged and non-aged samples respectively.

Figure 3.33 The schematic showing shear height and test direction of BGA solder ball

3.5 Precision Cutting of Samples for Metallography Preparation
The aim of this task was to prepare the test samples for further analysis using an image capture so that both the fractured and the non-fractured but cross-sectioned surfaces could be examined and analysed. PCBs of selected samples from the as-reflowed and the isothermally aged test vehicles were cut to size with manual guillotine machine and then cross-sectioned using the ‘Struers Accutom-5’ precision cutting machine. The assemblies are sectioned along the centre path of the solder joint in such a manner that the solder joint becomes revealed enabling the microstructure analyses, CSH and IMC measurements to be carried out for as-soldered and aged samples using Scanning Electron Microscopy Examination (SEM), and before the metallographic phase of the laboratory experiment. The SEM preparatory task processes for chip resistors solder joints test are presented in Figure 3.34 (a-d), while the precision cutter with the respective BGA solder joints strips are shown in Figure 3.35.

Figure 3.34: (a-b) Manual and precision cutter, (c-d) Test vehicle and sliced PCB

Figure 3.35: Precision Cutter & strips of cross-sectioned BGA components

3.5.1 Metallography Preparation
In preparation for the CSH as well as IMC measurements and microstructure study of the sectioned solder assemblies, the sectioned strips of the test vehicles were moulded using conductive Bakelite powder described in section 3.3.5.1 which allows the flow of electrons in the Mould while using the SEM, which areonly used for conductive materials. To remove roughness from the surface of the moulded test vehicles and to enable better quality view on the SEM, the surface was polished using the roll grinder and surface polisher. The various equipment or machines and the respective methodologies is discussed in the next sections.

3.5.2 The Buehler Compression Mounting Press
The Buehler compression machine produces the Mould after a period of about ten minutes of operation. This device operated pneumatically or assisted in making test samples for metallurgical moulds before carrying out the electron microscopy investigation. The test samples were initially, placed on top of a clean ram on the machine before they were mounted on the Mould, with the solder joints side facing downward before being released into the machine using a ram control. Two and a half quantities of the bakelite powder measured with a standardised cup are directly poured into a space above the test specimen. The moulding chamber was air tightened and subjected to high pressure by covering it with a plunger before switching the machine on. The ram control was pushed up at this point to build up pressure within the moulding chamber. This process was de-gasified after about five minutes of running, by pulling down the Ram control to release some built-in air or gases that might interfere with the moulding process. A picture of the mould-making process using the already described Bakelite powder is presented in Figure 3.36.

Figure 3.36: Images displaying the mould-making process
Source: [UoG-2015]

3.5.3 The Buehler Abrasive Paper Rolls
The Buehler equipment has four different abrasive rolls as shown in Figure 3.37, each of which has different surface finishes of 240, 320, 400 and 600grits respectively. The purpose of applying this equipment is to reduce the surface roughness of the moulded sample by polishing it consecutively on each of the paper rolls. The duration of time spent on the 240 grit is dependent on the surface roughness of the sample before proceeding to the other rolls having finer surface finishes. The surface texture of the abrasive paper rolls is finer, as it progresses from the 240-grit roll to the 600-grit roll. The face of the moulded sample showing the solder joints was hand polished by moving it haphazardly on the surface of the various abrasive paper rolls.

Figure 3.37: Image of abrasive paper rolls
Source: [UoG-2015]

3.5.4 Metaserv 2000 Grinder/Polisher
The final phase of the metallographic preparation of test specimens was conducted using this equipment. The grinding machine consists of two chambers, operated simultaneously depending on the kind of Monocrystalline Diamond Suspension (MDS) in use. For the purpose of this chapter, both grinding chambers were utilised owing to the use of two different monocrystalline diamond suspensions (6”m and 1”m) respectively. Once the equipment is switched on, the diamond suspensions were applied directly onto the moving Grinder (abrasive stone) before the moulded specimen was held firmly by the hand and placed in a stationary position while it spins and polishes with the suspensions, the surface to be examined. The 6”m suspension was applied firstly onto the first grinder before the application of the 1”m suspension onto the second grinder (Figure 3.38).The test specimen was washed properly with water after the first grinding before proceeding to the second grinder. This process provides a chemo-mechanical polishing (CMP) action on the surface of the specimens, which significantly increases removal rates of foreign particles or grains, reduces subsurface damage as well as improves the surface finish of test specimens for efficient electron microscopy examination.

Figure 3.38: Metaserv 2000 grinder with polisher and MDS
Source: [UoG-2015]
3.6 Benchtop SEM for Fracture Analysis
The fractured surfaces of the components solder joints are investigated for both brittle and ductile fractures under a SEM. This SEM operates at a high magnification level of 10kV and produces images of a sample by scanning it with a focused beam of electrons injected from an electron gun filament. The injected electrons interact with atoms in the sample, thereby producing various detectable signals. These contain information about the sample’s surface topography and composition. Moreover, the atoms excited by the electron beam thus emit secondary electrons. Other forms of particles originating from the electron beam are the Back-Scattered-Electrons (BSE), which consist of high-energy electrons that arising from reflected or backscattered specimen volume interaction with specimen atoms. This equipment was specifically used for its high-powered capability of measuring and examining the CSH and the failure mode of solder joints. Figure 3.39 shows (a) the JEOL5000 series Neoscope Benchtop SEM used for the fracture surface analysis and (b) the internal structure showing specimen platform and its movable tray. The process steps used in achieving the SEM analysis are outlined in section 3.6.1, and a photograph of the JEOL Neoscope process analysis steps with appropriate labels is presented in Figure 3.40.

Figure 3.39: (a) JEOL Neo-Scope Benchtop SEM and (b) SEM internal structure.

3.6.1 Process Steps Used in SEM Analysis
Fractured surfaces of each component (only the substrate side) was observed in this part, and necessary images were taken to support the further explanation. Slicing of the PCB was required because SEM has a limited viewing space inside its chamber to accommodate the sample. The following steps are followed,
‘ The PCB is trimmed into smaller pieces by using the metal board cutter known as Guillotine. Sliced parts having solder pad are placed on the observatory platform in the SEM chamber.

‘ Once the placement is done, the door of the chamber will be closed and pushed tightly for a few seconds, so that, the vacuum pump to hold the tray could create enough vacuum.

‘ The settings of the image for clearer vision are completed by adjusting the object’s position, brightness, sharpness and contrast to get the clearest possible image of the fractured surfaces.

‘ Four images are captured from each solder pad. The type R1206 and R0805 pictures were taken at 50” and 220” magnification, whereas type R0603, which has the smallest image is captured at 70X and 220X.
Figure 3.40: Images displaying the SEM process analysis step
Source: [UoG-2015]

3.7 X-ray Machine and Void Detection
The X-Ray machine shown in Figure 3.41 was used to determine the percentage of voids in the BGA solder joints. In the high-resolution X-ray source, electrons were accelerated from the cathode with speed close to that of light. The electrons, through a magnetic lens, are focused to a very small point on a metal target. On the impact on the target of an atom, electron loses energy through a series of collisions. A small part of the interaction produces X-ray, of which most of the heat were removed from the target material. In the x-ray source from the set parameters used in this investigation, electrons emitted from a fine wire were accelerated by up to 225 thousand volts (Charles Jr and Beck, 2007). The position of voids in the various solder joints can be visibly observed via the high resolution beam. The X-ray systems are also used to determine the volume of each void and its percentage in the joint. The percentage is used to characterise the joint regarding pass or fail category. To make sure same measurement conditions is applied to all samples, some of the parameters are kept constant. Name and value of these parameters are presented in Table 3.5 while the visualisation is given in Figure 3.42.
The visualisation, however, represented the result of the experiment on a test vehicle. Among these are (1) void edge threshold used to define the boundary of individual voids, (2) maximum total voiding is the upper limit of allowed void percentage in any solder bump and (3) maximum single voiding set the limit on the size of any single void in a solder bump. A solder bump will only ‘Pass’ if it satisfies these three parameter values.

Figure 3.41: X-Ray machine for BGA voids analysis examined

Table 3.5: X-Ray machine ‘ parameter setting for the lab experiment on BGA voids
Parameters Values
BGA Ball Size 13×13 matrix array, 0.76D, 1.5pd
Ball Edge Threshold 88
Maximum Compactness 2.50
Ball Diameter Auto
Tolerance 10%
Ball 16”4 (For four corners)

Figure 3.42: Sample of BGA solder bump X-ray visualisation.

3.8 Data Analysis
After conducting the experiments using the chosen DoE, and obtaining the respective results from the five critical experiments carried out and reported in five different chapters in this thesis, the data was analysed and compared with the expected results, most of which are from literature. After observations and analysis of the results, recommendations were made, and conclusions were drawn on each experimental test and are found in each of the respective chapters.

3.9 Chapter Summary
An overview of the experimental methodology, equipment and materials used in the study reported in this thesis has been presented. It includes a description of the test vehicles and elaborates the innovative equipment, materials and procedure employed in achieving the results presented and analysed in this work. The chapter also covers the manufacturing process of solder joints stencil printing, the reflow profile for the formation of solder joints, and the thermal ageing processes used in the study respectively. It also describes the metallographic preparation of test samples for the measurement of CSH and IMC, and for the microstructural analysis of solder joints after destructive test and micro-sectioning of test samples. The required optimal CSH for a reliable solder joint can be achieved through an optimal reflow parameter setting obtained from several trial ‘tests’. Several other results and the conclusions drawn from the data analysis carried out are presented in each of the experimental chapters. The effect of reflow profile parameter setting on the shear strength of solder joints in SMT chip resistors assembly is given in the next chapter.

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