The increasing complexity of electronic components and assemblies has introduced new challenges to reliability engineers. Of these new challenges, solder fatigue is a primary concern for assemblies undergoing thermal excursions caused by fluctuations in environmental temperature, power dissipation or a combination of the two. Factors which arise due to package-specific construction, mounting conditions, electronic housing and configuration within the printed circuit boards can all influence the thermo-mechanical fatigue of solder interconnects.
To better understand solder joint reliability from the system level, both local and global factors should be considered in order to quantify the effect on solder fatigue. In fact, there are five general system-level characteristics that should be taken into consideration in any solder joint reliability analysis.
1. Effect of Glass Style on Solder Fatigue
In many printed circuit boards (PCBs), the glass style plays a significant role on solder joint reliability. Often times, electronic design teams do not take into consideration the glass style for reliability analysis. Glass style, layer count and resin material determine the final coefficient of thermal expansion in the plane x-y directions. Each glass style has different layer thickness and spacing in the warp and fill directions of the woven composite. Paying closer attention to the glass style and laminate construction can result in an optimized coefficient of thermal expansion which can increase the thermo-mechanical reliability of the solder interconnects. Figure 1 illustrates two 1.6 mm PCBs with different glass style and layer count. Due to the large difference in volume fraction of resin to glass, difference in the in-plane coefficient of thermal expansion between the two laminates was measured to vary by as much as 5 ppm/°C.
2. Effects of Coatings and Pottings
Coatings and pottings used in electronic assemblies for environmental protection of sensitive components in harsh use environments can often result in deleterious effects on solder joint fatigue. These effects originate from the polymer’s coefficient of thermal expansion and elastic modulus interaction with temperature specifically around the glass transition temperature. As encapsulants expand around the electronic components, load transfer occurs through solder joints. Complex loading conditions arise with rapid expansion around the glass transition temperature and reduction in the elastic modulus as material softens. Modeling this interaction has been proven to be challenging especially in situations where the polymer expansion is constrained between the electronic housing and packages. This can generate large bending and axial stress that can reduce solder joint fatigue.
3. Board Mounting and Housing
Large board strains are generating around mounting point during thermal expansion of PCBs. Components which are placed within the vicinity of mounting points can be influenced by the excessive board strain and be susceptible to early failures during thermomechanical fatigue conditions. Housing shape and materials contribute to the thermal managements of the electronic system in addition to mechanical load transfer. During temperature fluctuation, the expansion and contraction of the housing can increase loading on solder joints by increasing bending of the PCB where the housing is in direct contact with the PCB or through displacement of standoffs.
4. Solder Alloy Selection for Use Environment
With the current advancements in Pb-free solder alloy metallurgy, manufacturers of solder alloys offer a wide range of alloys for different application and manufacturing processes. The intended use of the electronic system should be considered in order to identify dominant failure mechanisms solder joints experience prior to the solder alloy selection step of the design stage. For systems susceptible to mechanical shock and drop impact, such as military vehicles or consumer handheld electronics, low silver (Ag) solder alloy should be considered. Tin-silver-copper (SnAgCu or SAC) alloys offer better shock protection by enabling the formation of fewer brittle intermetallic and lower elastic modulus which increase the elastic compliance of solder interconnects. For power electronics which operate at high temperatures, a number of special doped, composite and transient liquid phase sintered solder alloys are available and have been shown to offer superior thermos-mechanical fatigue performance compared to conventional SAC alloys.
5. Influence of Components Configuration
Component placement in PCBs with respect to high power dissipating devices and connectors can affect solder fatigue in the form of over-constrained thermal expansion mismatch. Over-constraining between the package and board can amplify load transfer to solder joints. This increase in load transfer usually shifts the loading mode to a preferential direction. To illustrate this condition, simulation of single sided and double sided (mirrored) ball grid array (BGA) package is shown in Figure 2. The displacement magnitude of the package and board is shown at each temperature extreme. It can be seen that displacement continuity is present in the mirrored package and not in the single-sided package which is allowed to bend and release some of the shear stresses acting on corner solder balls. The over-constrained condition of the mirror configuration prevents localize warpage of the PCB during low and high temperature extremes, resulting in larger load transfer. This load transfer has been shown to reduce fatigue life of mirrored components by factor of 2-3X compared with similarly constructed single sided configurations.
For any solder joint reliability analysis that you complete, it’s essential that you keep these considerations top-of-mind when designing and testing your PCBs. To learn more about how coatings and pottings specifically can do more to help ensure reliability with your solder joints, download our Guarantee Reliability with Coating and Potting webinar.