Another potential source of "residue"  on copper surfaces during dry film stripping is absorption of oils - often attributable to antifoam and/or excess use of antifoam.
Because copper  is very oleophilic, minimal contamination may be problematic (including fingerprints) and can contribute to a variety of post strip processing problems; incomplete final etching ("copper spots") being one of the more common defects.  The methods suggested by Rudy would detect this problem.

As a severe (and unusual) example of oil contamination, we once encountered a customer employing compressed air agitation to dislodge accumulated dry film particulate in the corners of the dry film stripper spray sump.  The air was introduced when the line was idle  to improve continuous dry film particulate filtration.   The customer had been employing the process for quite some time when random copper spots began to be observed.  If the stripper was replaced, the problem disappeared; only to reoccur in 1-2 days.  To make a long story short, it was determined (after eliminating anti-foam, checking for tin in the bath and on panels, addressing copper oxidation, examining rinse water quality, dry film leaching in the alkaline etch, etc.) a leaking seal in the air compressor was contaminating the stripper sump with minute volumes of oil, but enough to contaminate the copper surface and inhibit alkaline etching.

Regards,
Ted

Rudy Sedlak wrote:

In a message dated 4.25.02 12:23:23 PM Pacific Daylight Time, [log in to unmask] writes:
 
 

Does anyone know of a test to look for film/stripper residue after copper
stripping.

Assume you mean after stripping film from Copper?

Stripper residue is a tough one, and not real likely to be an issue.

Tarnish could be another issue that is very difficult to test for except by sight.   Depending on stripper, this could be significant....the clue is, if the problem gets worse as the stripper chemistry gets older, it is quite possibly tarnish...

Film residue is the most likely residue.  One way to test that is a good test, but, can make your test board need rework is to drop on an immersion plating solution.  Two different ones are common.  Some people use immersion Tin to test if the surface is clean.  A drop should plate uniformly, and adherently.  Another test is to drop on Silver Nitrate solution...usually use the stuff that is used for titration in the lab.  The surface should essentially immediately turn black, and it should be adherent.

Another residue problem that can occur, and especially in old stripper chemistry, is immersion Tin residues....this does not relate to the test above.  Rather, your stripper chemistry can pickup Tin from the oxide on the plating on outerlayers, and redeposit this on the Copper next to it.   The potential for this problem is magnified tremendously if you strip in a tank, rather than exclusively in sprays, and can be strongly affected by the condition of the Tin plating solution.  If your Tin electroplating solution looks very milky, this is disperse Tin oxide, which co-deposits with the Tin metal, then easily dissolves in certain kinds of stripper chemistry,  and redeposits, sometimes even is sprays, on the adjacent Copper.

The film residues have become an especial problem in recent years, as the new generation of photoresists, which stick VERY well, require specially formulated types of stripper chemistry.

I probably have told you more than you want to know, but, in case I haven't, you can call me to discuss it.

Rudy Sedlak
RD Chemical Company 650-962-8004