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Solder mask discoloration

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Solder mask discoloration | 17 August, 2005

We have encountered what appears to be mask discoloration after wave soldering.

The areas of concern coincide with the areas that were exposed selectively by fixtures during wave soldering, dominated by one date code of PCB with a slightly different green (lighter) solder resist. Other data code (same PCB vendor) with darker solder resist waved at the same time have less/no discoloration.

We have not seen this issue before this build. There has not been any PCB vendor or (known) process change.

Process details:

Reflow (x2 times) Wave solder: preheat (120/130C), solder temp 260C, with fixture Flux: no clean, halide free Top side max PCB temp 110C Wash&Rinse cycle 8min at 130F in DI with saponifier, Dry 1 min at 100F

Following link shows PCB images with white milky masks after wave soldering AND WASH. It does not seem to be surface residues (IPA has no effect).

PCB vendor is currently investigating and preliminary results show that post baking the finished assembies for 1hr @ 90C eliminated the discoloration.

Has anyone seen this before? Any idea on what causes the mask discoloration and its potential impact on the assembly?

Any comment is greatly appreciated.



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Solder mask discoloration | 17 August, 2005

I usually see this with mask that is not fully cured from the PCB shop. This probably agrees with the baking statement that you made.

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Solder mask discoloration | 17 August, 2005

Peter are you having fun with solder masks erwat?

We saw this years ago with LPI solder masks. It shows occasionally with other masks.

We agree with Russ. If nothing [eg, alcohol, flux thinner, acetonitrile, or water] dissolves the haze, then it is time to run a scrap board through the wave, wash it, make sure it has the white haze, then turn the hair drier on and aim it at a spot on the PWB surface. If what you are seeing is underdeveloped solder mask, the white stuff is the fumed silica used as a thickening agent in the some masks.

If the mask is not completely cured/cross-linked, then the flux and its solvents will dissolve away the mask, exposing the whitish thickener or skeleton of the mask. Heat up the material and the white residue will sink into the body of the mask and disappear.

If this the case, check the UV lamp output to ensure you are or your PWB supplier is completely curing the mask.

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