Immersion white tin wave soldering problem

What you are most likely dealing with: Insufficient Pure Sn layer ( multiple causes ) Because elevated temperatures accelerate the diffusion process, the thickness of the Pure Sn layer will reduce during every thermal cycle... ( bakeing PCBs, SMT Reflow, etc... ) Normally reduces the Pure Sn thickness by 0.1-0.15 microns per cycle Min. should be: ( providing proper thickness for 3 thermal cycles ) 0.65 measured by SERA 1.00 measured by XRF Your Fab house should be able to achieve 1.0 measured by SERA in optimum conditions If you still have some bare fabs you might want to send one out for testing. Although humidity will not speed up diffusion, it will give a boost to oxidation.

Other things to look at that can cause a solderability problem on White Sn: Prolonged periods of time before cleaning off Fluxes. Insufficient rinse at FAB house, leaving chemicals from the coating bath on the PCB surface.

To determine which one might be the culprit use common sense... i.e. chemical attack will normally reduce solderability around the edges of the pads due to the collection of chemicals being easier where the solder mask provides a nice and cozy dwelling... Excessive thermal cycles will attack areas that retain heat for longer periods of time. Use oven profiling experience to determine these areas on your board.

As far as fixing the boards you have already produced through SMT: Use a high activation flux to wet the surface. Use the Air Knife to assist flux penetration into the PTHs. Use Chip Wave ( Turbulent ) to increase hole fill Increase Wave contact time and depth in solder to increase hole fill.

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Heat cycles deplete the pure tin layer. Each solder cycle reduces the tin thickness by ~0.1 micron (4 uin). So, if you start with a thin imm tin coating you'll have problems after multiple heat cycles. Steve Wentz says, "In almost every case I've seen, when the first pass or two solders fine and then problems arise, it is an insufficient thickness of white tin that was on the board."

There are at least two major imm tin technologies out there. Each has different plating thickness. Your board fabricator should be able to satisfy you that proper controls are maintained to ensure adequate plating thickness.

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