Vacuum is a nasty environment. We have no pressure, we have outgassing, and (least we not forget) we have the simple removal of air. Materials outgas, which is to say, liquids and volatile solids boil off. This outgassing can then coat itself onto nearby parts. Ever buy something wrapped in plastic, and the item smells like plastic for the entire day after you unwrapped it? That’s outgassing. Now imagine plasticky vapor depositing itself on, oh, your eyes, and bonding there. Because in space, the outgassing stuff just may hit parts of the satellite as it evaporates away.
That is bad. It can coat detectors or solar cells. If any outgassing materials happen to be conductive (unlikely but possible), you can mess up circuits. Plus, it’s not tidy, letting stuff swirl away like that. However, in a vacuum chamber, you can have the bulk of the outgassing occur safely on the ground, sparing your components once in orbit.
Vacuum testing also ensures your soldered links are stable, and aren’t going to break due to air pockets, air conducting where there should be solder, and similar mishaps. That’s part of the no pressure and removal of air issues. Maybe there’s a part or two that is fragile and can’t take the drop to zero pressure. Maybe there are air bubbles in a component due to poor manufacturing, which will cause it to break when it hits vacuum, as shown in Figure 5-1. (As the external pressure lowers, the balloon expands due to internal pressure.)
Figure 5-1. A ...