Craig
This is quite interesting. There are a number of questions that spring
to mind.
1. Which makes of LPI mask are affected by w/s and latex solder masks?
It may be just one or two, whereas others may be fine.
2. What is the copper preparation to receive the LPI? As I presume it is
plated, it should not be brushed (destroys solderability). Is it clean,
is it microetched, is it oxidised? Strangely, smooth, clean,
non-oxidised copper is one of the worst metals to bond to organics (this
is why the electro-deposited copper is treated for cladding and before
stacking m/ls). A light oxidation will help (hydrogen bonding to CuO).
3. Is the LPI fully cured? If not, then all is clear. Very few PCB fab
guys do other than sticky tape and MeCl colour-wipe tests, but neither
necessarily reveals a tender LPI which is 9/10ths cured.
4. The composition of w/s solder stops is quite complex and they are
often a mixture of acrylic copolymers and natural w/s gums held in a
complex vehicle. The polymers require organic solvents and this solution
is part emulsified, part mixed in the gum/water solution, which thickens
it and renders the lot, after drying, water removable (it is not really
100% soluble). There are some other additives. Without systematic work
by the manufacturer, testing each component in turn, it would be very
difficult for a user to identify which component(s) contribute to the
problem.
5. It may be that there is even a chemical (as opposed to physical)
reaction between free radicals in the LPI and polymers in the stop,
causing a contributory cross-linking.
6. If the LPI is aqueous-developed, there is almost surely some
carbonates captured in the mass. They may leach out slightly into the
stop, before drying. Could this have some effect?
IOW, my guess (I haven't experienced it) is that you have a difficult
and complex compatibility problem which, if it is serious and cannot be
otherwise worked around, may require qualification tests of suitable
LPIs, stops and copper treatment.
FWIW
Brian
Craig Ropp wrote:
>
> Has anyone had any experience with Solder Stop (water soluble mask)
> affecting LPI adhesion around pad openings on Groundplanes. Typically
> adhesion issues almost always relate back to inadequate surface prep during
> the board manufacturing process. I had to "see it to believe it". We have
> reviewed every step of the process and cannot find any disconnects. We have
> only one Customer experiencing this problem. They have used the same Solder
> Stop for approx 5-6 yrs. At 1st it was believed that the peeling didn't
> occur until after Wave Solder. However, they had coated some bds and
> allowed them to set over the weekend and after washing them off found the
> peeling mask. We tape test all boards at our facility per IPC-TM-650
> procedure. There has been no evidence of peeling at all. After visiting
> with the Customer it seems that the time the Solder Stop is on the bds
> (i.e. longer than 2 days) determines whether or not peeling will occur. The
> peeling is always on pad openings which are on Copper groundplanes. We have
> no evidence of any peeling on pads which are surrounded by FR4. The peeling
> is usually limited to an area concentric to the mask opening and extends 5-
> 30 mils into the surrounding mask. We have run tests at one of our other
> facilities consisting of running numerous bds across the wave solder at
> different preheat and solder pot temps (we even matched the temperatures
> the Customer with the issue is using) with no evidence of peel. The
> customer is adding approx 1 cup of water to 1 Gal of Solder Stop to aid in
> flow. It was requested not to refer to the Material's manufacturer until we
> have additional data. Any suggestions or help you can provide will be
> greatly appreciated.
> Thanks,
> Craig
>
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