COVER STORY MILITARY & DEFENCE SUPPLEMENT
A-PACKAGE SOLUTION for military & aerospace applications
COMPLETE SYSTEM-IN-
Steve Munns, Mil-Aero Marketing Manager at Linear Technology Corporation takes a more detailed look at the comparative performance of LGA and BGA packaging and discusses the merits of gold or tin-lead (SnPb) alloy and lead-free (Pb-free) component finishes
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 has been shown to resist the penetration of tin whiskers for many years.
ach year the military and aerospace sector demands higher speed systems with better accuracy at lower cost. Increasingly stringent size, weight and power constraints, together with the demanding power supply requirements of large FPGAs, have led to a decisive move towards POL (point of load) power supply architectures. One solution to address these needs, which has enjoyed widespread market acceptance, is Linear Technology’s µModule technology, providing a complete system-in-a-package solution that simplifies design and minimises external components. The original packaging technology selected for µModule switching regulators was LGA (land grid array) and this has served the broad marketplace well. However, some applications
E S6 JULY/AUGUST 2015 | ELECTRONICS
subject to very harsh environments prefer BGA (ball grid array) interconnect and the company has developed packaging to satisfy these requirements.
COMPONENT TERMINAL FINISHES The military and aerospace market in Europe 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
“Increasing the volume of
solder in a joint and the standoff to the PCB improves interconnect
INTERCONNECT CONSIDERATIONS Given the mix of needs already 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
reliability by making the joint more compliant ...”
of using gold plated components is gold
embrittlement and this is
especially true of large BTCs (bottom termination
components) subjected to harsh / ELECTRONICS
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