7 Novel scientific results
possible with insignificant error in terms of the quality of soldering. The temperature difference between the surface and the midplane depends mainly on the materials’ thermal diffusivity, while the time required to reach the melting point depends on the volumetric heat capacity of the materials. These parameters have a wide spread over the analysed range.
• I have concluded that in the case of the novel, biodegradable polylactic acid (PLA) based substrates make an exception to the previous statement over 1 mm thickness. In these cases, the resulting temperature difference between the surface and the midplane of the substrate can reach up to 30 °C, which can significantly increase the failure phenomena associated with shrinkage or warpage. Therefore, the use of explicit modelling is not recommended in this case.
Related publications: [L1], [R1], [R2]
Thesis II:
I have characterised the response time differences of welded hot spot thermocouples attributed to their constructional alterations with finite element simulation method;
where the differences can be significant with their order of ~100 ms in the electronics soldering technology applications.
• I have shown that the length of the uninsulated wire around the thermocouple hot spot significantly affects the thermocouple measurement response time in the order of ~100 ms. The uninsulated wire around the thermocouple hot spot results in a faster response as the uninsulated wire length increases. For the design under investigation, ~3-4 mm of uninsulated wire length was found to be critical, with a significant change at this value. Variations due to different construction can be critical when measuring the soldering profile of symmetrical lead-out components.
• I have shown that larger hot spot diameters, larger insulation thicknesses and different types of high thermal mass insulation can slow down the heating of the thermocouple and the resulting time response over a time range of ~100 ms. The results have been validated by measurements for two thermocouples of different construction but identical material compositions, with a relative error of ~5-15%.
This effect is lower for reflow soldering methods where heat transfer is very intense. The thermocouples used to validate the working space in classical vapour phase soldering furnaces (or other intense heat transfer processes with heat transfer coefficients ~400 W/m2K) give consistent measurement results, despite the geometric variations around the hot spot.
Related publications: [L2], [R3]
Thesis III:
I have shown that in the case of solder beading failure under a surface mounted electrolytic capacitor with a baseplate, the reduction of solder paste volume alone is insufficient to eliminate the issue. Modification of component design by increasing the spacer height (from 85 µm to 110 µm) and decreasing the lead width (from 850 µm to 780 µm), the failure rate was significantly reduced (from 13700 ppm to 500 ppm). A suggestion of a minimum distance between the substrate and component (400 µm, taller than the solder paste height with considered expansion during reflow soldering) with a modified baseplate design effectively eliminated the issue.
• I have shown that the design of the baseplate surrounding the component leads (which is found on all commercially available surface mount electrolytic capacitors) mounted on the bottom of the component has a significant effect on the process of solder bead formation. The occurrence of solder bead formation can be significantly reduced by increasing the distance between the baseplate and the substrate surface and by increasing the clearance around the component lead, as these allow a more uniform escape of gases and vapours from the solder paste during soldering and reduce the chance of physical contact between the solder paste and the component, which can lead to solder paste separation (illustrated with X-Ray investigation using a modified heating profile to reveal the behaviour of the solder paste during the reflow process). It was shown that reducing the amount of solder paste reduces the occurrence of failure, but insufficient to eliminate the issue. The amount of reduction is limited by the mechanical reliability of the solder joint.
• I have shown that a part selected with the foregoing properties in mind can significantly reduce the occurrence of defects and thus reduce manufacturing defect costs: components on average having a 110 µm spacer and 780 µm component lead width resulted in 500 ppm solder beading occurrence rate, while components in average having an 85 µm spacer and 850 µm component lead width resulted in 13700 ppm occurrence rate.
• An alternative design of the base plate is recommended and can be designed to resolve the solder beading issue by providing a high distance between the substrate and components’ baseplate. The minimum distance should be larger than the solder paste height while considering the expansion during reflow soldering.
Supplying 400 µm distance under the baseplate resulted in no excessive flux outlow, nor solder beads.
Related publications: [L3], [R4]
Thesis IV:
I have shown that in the case of passive, two-terminal, chip-sized surface mounted components, electromigration in the solder joints may become more pronounced due to the asymmetry of the solder meniscus, which causes local and two-sided microstructural asymmetry of the copper- and the intermetallic compound layer thickness.
• I have experimentally shown that the solder joints of Sn3Ag0.5Cu (SAC305) passive chip-sized components 0402 and 0603 mounted on high-tolerance hydrocarbon-ceramic substrates change their microstructure and electromigration occurs under 2 A and 2.5 A DC current loads, respectively. I investigated the microstructure of soldered joints after different loading times with cross-sectional grinding, using optical microscopy and scanning electron microscopy (SEM) on DC-loaded and isothermally aged samples. The thickness of the intermetallic layer increased with increasing loading time. Under current loading, the thickness of the intermetallic layer increased asymmetrically: it was larger on the anode side and smaller on the cathode side of the parts in the direction of the current, whereas there was no difference between the two sides for the reference isothermally aged
samples. The studies showed that the electromigration-induced void growth did not develop even at 4000 hours of current loading. In the solder joint, the high current density causes copper dissolution, which leads to the transport of material from the soldering surface to the component metallisation-solder interface in the direction of the electron current and the formation of a large Cu6Sn5 intermetallic alloy, which results in a decrease in the reliability of the solder joint.
• In the latter test configuration, I investigated the shear strength of the soldered joints. Isothermal ageing and current loading reduced shear strength by about 22%
and 24%, respectively, after 4000 hours of loading. In contrast to the isothermally aged specimens, SEM examination of the fractured surface of the 4000-hour- loaded specimens showed a Cu6Sn5 intermetallic alloy with a brittle fracture surface over a large surface area, which was thickened at the interface of the component metallisation and the solder matrix due to electromigration-induced material transport.
Related publications: [L4], [L5], [L6], [R5], [R6], [R7]