The current installation method for tall, stacked capacitors is very cumbersome because the lead form is very sharp and prone to solder cracking due to thermal cycling. An astringent installation process was developed to obtain the best chance of a successful solder joint with a proper heel fillet so the chance of cracking is minimized.
Because of the component weight, a spacer needs to be bonded between the component belly and the PWB (printed wiring board). The gap under the component has to be carefully measured and a special tight tolerance spacer has to be chosen to minimize bondline [0.002 in. (≈50.8 μm)], since the CTE (coefficient of thermal expansion) of the bond adds additional stress on the component leads. Installation and rework become especially difficult when adjacent components also are tall. In addition, inspection becomes difficult.
The Special Lead Adapter is installed without the stacked cap. The two lead adapters (one per lead side) are temporarily tied together to create a fixed member. The lead assembly is positioned onto PWB pads and soldered. Since the tie attachment is hollowed out in the middle, the assembler has more available access to solder down the leads. Inspection becomes very simple once the temporary attachment is removed. The stacked cap is slid in between the installed lead adapters upside-down.
This work was done by Stuart Kai of Northrup Grumman Systems Corp. for Goddard Space Flight Center. GSC-16781-1