The Changing Face of Chip Component Damage
First, we asked for samples to be sent back to us, and once we received them, the issue became clear. There was of course the missing component, however there was also evidence of a well wetted solder fillet with a distinctive imprint of where a component had been, and we were able to establish that it was a multilayer chip capacitor.
We have seen this type of capacitor failure before, and it was found to be due to handling issues. This component is fitted to side two (underside of the board), and sits proud of the board. It is therefore easily damaged, so we use foam filled trays to carry and store these types of boards, and our inspectors are well scripted on the watch for damage.
Missing multilayer chip capacitors from a board is a common problem, they tend to sit proud of the board and therefore are more prone to damage.
Most damage occurs when sliding onto a bench or hitting tall components when sliding into a shelf or tray. The sudden impact would be like tapping with a small hammer.
Ceramic is brittle, and in the tin lead days, this type of impact would fracture the ceramic body, but in this lead-free era, the capacitor comes out of the solder fillet.
The soft terminal capacitor
Before lead-free multilayer chip capacitors, damage was caused by either board deflection, causing internal cracking under the termination, or a fractured ceramic body due to physical impacts.
Board deflections were usually caused by some sort of board bending, like depaneling. The compliant nature of tin lead solders absorbed a lot of this energy and whilst it was a problem, it was well understood and could be contained by good handling practice.
The introduction of lead-free solders changed this condition; the SAC type solders are less compliant and quite hard, moving the damage to the next weakest area, the ceramic body. This shift in damage location led to an increase in the flex damage to capacitors, and a need by the component manufacturers to address this increase in damaged capacitors.
The problem was complex, the component manufacturers had to address the change from soft compliant tin lead solders, to less forgiving lead-free solders.
The solution was to add a flexible conductive epoxy layer to the termination.
The termination construction:
- Then the conductive compliant epoxy
- And finally a coating of nickel then tin
With this new terminal finish, when the board deflects, rather than cracks developing in the ceramic body damaging the capacitor elements, the stresses are absorbed by the flexible epoxy layer.
Of course this also makes the component easier to knock off the board, through the soft compliant epoxy layer, leaving a visible solder cradle, rather than the fractured ceramic body.
We managed to demonstrate this weakness to the client, and how we protect against the dangers of hitting the capacitor body. They also had to update their own handling procedures in light of this new damage possibility.