Component terminal finishes
The European military and aerospace market presents quite a fragmented demand picture when considering component finishes with some companies having adopted Pb-free finishes entirely whilst others have policies to completely avoid them, more common is a mix dictated by the specific needs of individual projects.
A major factor is the continued exclusion of military and aerospace equipment from the RoHS II (Restriction of Hazardous Substances) directive that permits the indefinite use of SnPb component finishes. Of foremost concern is the impact on reliability from the formation of tin whiskers on pure tin plating, leading to potential equipment failures from short-circuits of adjacent fine-pitch conductors. The addition of lead (Pb) to the tin plating remains the industry-standard approach for mitigation of whisker formation.
Weighed against the use of SnPb finishes are the practicalities of component availability, lack of distribution inventory and extended delivery times. By mandating use of SnPb finish companies are also sometimes foregoing access to a wider range of new, Pb-free only components. Whilst this can be overcome by third-party companies offering strip and re-plating or re-balling processes the additional heat cycles and difficultly in re-testing together with the associated costs make this a less desirable approach.
When Pb-free components are used another form of tin whisker mitigation frequently employed is the use of polymer conformal coatings such as Parylene or Arathane which have been shown to resist the penetration of tin whiskers for many years.
Given the mix of needs discussed in the previous section, Linear Technology µModule products in LGA packages offer a universal solution as gold plated pads have been in long-standing use within military and aerospace systems and they have the advantage of also being RoHS compliant.
However, one concern of using gold plated components is gold embrittlement and this is especially true of large BTCs (bottom termination components) subjected to harsh environmental conditions. During reflow gold is dissolved into the solder joint creating weak interfaces in the crystalline structure, the CTE (coefficient of thermal expansion) differences between the gold-tin IMC (intermetallic compound) platelets and the surrounding volume of solder can result in fracturing of the solder joint and an eventual open-circuit when the assembly is repeatedly temperature cycled. A long-standing industry rule of thumb recommends not exceeding a threshold of ~3 wt% gold within the joint, and Linear Technology LGA µModule products meet this requirement for both SnPb and SAC305 paste. Some companies conduct their own trials on BTCs with representative PCBs and environmental conditions using special daisy-chain interconnect samples for this purpose.
Increasing the volume of solder in a joint and the standoff to the PCB improves interconnect reliability by making the joint more compliant. Tests in the military and aerospace industry have concluded that BGA packages are generally preferred over LGA where harsh environmental conditions are likely to be encountered, particularly in airborne systems. An added benefit is that cleaning becomes easier thereby reducing concerns over contaminants.
For these reasons µModule regulators are now offered in BGA packaging in addition to LGA. The downside of the BGA package is slightly reduced thermal efficiency of around 0.5ºC/Watt and an overall increase in component height of 0.6mm. Linear Technology BGA µModule regulators are offered as standard in SAC305 Pb-free configurations and on specific products according to demand with tin-lead Sn63Pb37 balls.