Does anyone have information, opinions or preferably data regarding micro etching with persulfate vs sulfuric acid/peroxide chemistries? We are currently using a sulfuric acid/peroxide based microetch at the HAL line, and are considering alternatives. I have seen some information which suggests a persulfate based chemistry is better at removing solder mask residues. Any feedback would be appreciated.

Jim Kenny

I believe the advantage Persulfate has over peroxide sulfuric in removing residues is it's undercutting ability due to the nature of the etch. At a previous job, I had used the Persulfate to do what you suggest. I also have learned that it is better to spend the resources trying to fix the residue problem than band-aid the problem with a more aggressive etch. What I found was that unless you want to etch huge amounts of Copper to undercut the mask residue, the fix will work when the residues are mild, but fail when they get bad.

The above is right on the money. Look into developer/rinsing issues. Some of these soldermasks leave residues in the developer/rinse that are essentially impossible to remove chemically, and will eventually clog nozzles, etc.

Another approach that one of customers found, quite incidentally, was that a mildly alkaline (pH = 9-10) cleaner did a marvelous job of cleaning up the surface before microetch, did not attack the mask, and allowed a minimal microetch to give a thoroughly clean, and wettable surface.

Rudy Sedlak
RD Chemical Company
phone 415-962-8004
fax 415-962-0370

As a HASL service provider we see the entire gambit of solder mask residues. We use a persulfate based etchant at a rate of 40-60 microinches. The trade off is that it is much more difficult to rinse than peroxide/sulfuric chemistries and can cause non-wets if not entirely removed before entering the HAL machine.

Michael Holan
Process Engineer
Radian Electronics Div.
HYPERLINK mailto:michael_holan@radian.com michael_holan@radian.com

As a micro-etch (either type) becomes saturated with copper, there is a tendency for it to redeposit copper sulfate crystals, especially when it hits a cold rinse. These tiny copper sulfate crystals will cause dewetting in HASL. It is common practice to follow the micro-etch rinse with a sulfuric acid dip of 5-10% and rinse. This redissolves the copper sulfate. Another fix is to change (persulfate) or regenerate (peroxide-sulfuric) the micro-etch more frequently, to prevent copper buildup in the solution. Also, maintain enough acid in the etch (see supplier) to keep the maximum amount of copper in solution.

Patty