We have found very small solder deposits on the gold fingers of some PCI boards after assembly at our CM. In the IPC-A-610, it says that no contamination or solder is allowed in the critical contact area.
1. I'm looking for feedback on what others are doing to mask the gold fingers through the SMT process, or are you doing something else to prevent solder on gold fingers.
2. Is anyone determining acceptability of a solder deposit on a gold finger by the size of the solder deposit? (e.g. 5 mil or smaller is acceptable)
Forth, solder on gold fingers comes from: * Sloppy paste control. About 70 % of the time it's caused by poor cleaning of screen printer, staging area (table), conveyor, reflow oven chain or belt, and keeping boards separate from cleaning process * Operator training. After loading paste onto the stencil, are operators handling cards right after it without cleaning or removing their gloves?? * Paste process. Are you pasting anywhere near the gold fingers, are you leaching under the screen and contaminating the bottom side due to poor printer set up? Is your under-wipe frequency proper? Is paste ping from the squeegee blades onto the stencil? Could the paste near the edge be getting onto the conveyor? * Reflow process. Paste can splatter if your heating profile doesn't match your paste, if your board is full of moisture, or if you're not controlling your environment. Use a slow ramp to a decent dwell then jump up to reflow and back down within a 60 second window. Keep the ramp to under 2�/sec. * Inbound quality. Check the boards before starting the printing process to try to eliminate some possibilities
Finally, we believe the concern about solder on gold fingers is: the tin in solder can react with the gold to form fretting corrosion of the contacts.