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
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)
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
220.127.116.11 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
18.104.22.168 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
22.214.171.124 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).
126.96.36.199 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
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
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.
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