September, 2019


www.us- tech.com


Page 79


Reducing Voiding Under Large, Thermally Challenged Devices: A Materials-Based Approach


By Mark Currie, Ph.D., Director, Business Development Manager Solder Materials, Neil Poole, Ph.D., Chemistry Fellow, Henkel Corporation T


he integration of large devices, such as MOSFETs, QFNs, CSPs and BGAs, into high-reliability


applications has resulted in an in - creased interest in the requirement for void reduction in order to meet more demanding reliability standards. For array devices, the perception


is that voiding represents a reliability concern. For bottom-terminated com- ponents and large devices with bottom thermal planes, voiding size, frequen- cy, and location underneath the com- ponent are concerns, due to the impact on thermal conductivity. Elevated operating tempera-


tures have the potential to impact function and/or induce heat-related reliability issues. For example, industry peers have noted that an 18°F (10°C) increase in operating temperature can decrease the compo- nent lifetime by 50 percent, due to the size of the thermal mass. Various industry task force


groups are engaged in the analysis of acceptable void levels for both ther- mal and mechanical reliability. But, with no definitive conclusions as of yet, the theory that voiding reduces reliability is driving development of low-voiding solder materials. While their creation can be


caused by several factors, voids — both in terms of size and quantity — observed underneath larger compo- nents are often the result of solder


paste flux characteristics and the way the flux behaves during reflow. However, formulating solder pastes


manage the full spectrum of compo- nent types, PCB assembly densities and pad finishes, while also providing


where the primary failure mecha- nism is internal, due to poor heat conduction. It is the need to minimize the


internal temperature and, thus, the lifetime of components that is driving the desire for low voiding, especially on the thermal pads, and not classic solder joint reliability. With this in mind, it is not surprising that there is no significant data linking voiding to lifecycle performance. It has been well-established that


High-density assembly and large thermal mass components can increase hot spots on PCB assemblies.


solely for large area ground planes is impractical since all assemblies will contain a variety of component sizes and types. Therefore, in order for electron-


ics assemblers to satisfy all require- ments and meet the demanding qual- ity standards of modern-day EMS board designs, which span technolo- gies found across multiple end mar- ket applications, adaptable, high- performance solder materials are essential. While there are no current void-


ing standards for components, other than BGA/CSP, there is an urgent necessity for EMS companies to pre- pare for every market eventuality, which means the inclusion of all com- ponents. Selecting a material that can


market-leading process and reliability performance with a low cost of owner- ship, is the path forward.


Evaluating Assembly Reliability Historically, assembly reliabili-


ty has been driven by interconnect reliability. This has resulted in a plethora of accelerated reliability test protocols, including thermal cycling, power cycling, thermal aging, vibration, and mechanical shock testing — all aimed at acceler- ating solder joint fatigue. This approach has served the industry well for components that include chip capacitors and resistors, BGAs, CSPs, QFPs, SOICs, etc. One must question, however, if this is the appropriate way to test components


different component devices can be linked directly to lifecycle perform- ance, especially when thermal cycling is considered. Thermal-centric compo- nents, such as QFNs, often fail early in comparison to the more traditional QFP, CC and BGA components. It is typically not the interconnect solution causing the failure but, rather, the design of the package. QFNs appear to have applica-


tion field reliability based on compo- nent design, and not interconnect solution. However, many in the industry suspect that reduced void- ing in the interconnect material (under the QFN post-reflow assem- bly) could extend lifecycle. Without technical guidance to


prove voiding’s impact on reliability (either negligible or significant), low- voiding materials offer a safeguard solution.


Continued on page 81


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