Rudy Sedlak wrote:



I do love it when someone asks a question that we are real knowledgeable on.
Thank you.

Tarnish on Copper after stripping is endemic to resist stripping.  It is made
much worse by buildup of Copper in the stripping chemistry.  In other words,
most strippers will not tarnish when new, but begin to tarnish as they are
used, and Copper builds in the sump.

The problem is cureable by using a  resist stripping chemistry with the
proper anti-tarnish in it.  Most resist stripping chemistry has 250 ppm of
Copper when the stripper is dumped.  It is possible to get resist strippers
that will not build up more than 30 ppm of Copper for the same number of
panels stripped, and average less than 2 false defects per 18" X 24" panel.
This has a nice added benefit of keeping the Copper in the rinse waters so
low that they usually do not have to be treated, and can legally be put right
down the drain.

Many stripper chemistry vendors have resolved this issue by simply going to
high volume feed and bleed, thus turning the sump over often, and keeping the
dissolved Copper levels low.  This is great for the chemistry vendor, but
expensive for the PCB fabricator, and not required with the proper
anti-tarnish chemistry.

As you probably have surmised, we are one of the vendors who can supply
resist strippers with the proper anti-tarnish properties.

Rudy Sedlak


Rudy, the response you gave is somewhat surmised. It is true that many
strippers do have a significant copper metal build-up in solution - depending
on the chemistry used to strip the dryfilm. But just analyzing the strip
chemistry after it has stopped stripping gives very little insight as to how
the process was being operated.

If the process chemistry capability is not suited for the application, or if
the process has been set up wrong from the beginning - alot can happen -
tarnish of the copper - shorts due to an intermetallic deposit of tin across
the surface of the panel itself....and so on and so on........

An example would be of a manufacturer of printed circuit boards who wishes to
keep a process speed of 8fpm - etching thin core material up to 1/2 oz copper
thickness.
There is no anti-tarn in the world that will reduce the tarnishing of the
copper surface if it is being processed at 3 to 4fpm. Eventuallly what will
happen is the solution will start etching copper and tin. Final outcome -
dull panels which are being shorted out when being processed through the
etcher and tin stripped.

Feed and bleed has been a highly successfull process control based on the
simple fact that certain board manufacturers wish to keep a steady operating
speed of their SES (strip-etch-strip) lines. This ensures constant
replenishment of fresh solution and enabling a given strip processing speed.

I have personally witnessed many board manufacturers simply installing the
wrong chemistry for their applications - why? Vendor was perdomiantly not
knowlageable enuff to understand the customers requirements and the effect
hiis chemistry would have not only on the printed circuits being processed -
but also the effect on the equipment.

Please, do not make it that feed and bleed is a bad thing for the PCB
manufactrurer - it isn't - it has kept many SES lines in optimum processing
parameters. From all the time I have spent in the field - I have as yet to
see a resist stripper on the market that can be utilized that can hold all
given criteria of a perfect world - ie....line speed.....no metal etching (
tin or copper ).............

No use having a resist stripper that will not dissolve some quantity of
metals - or tarnish your boards ( and I'm not saying that all do or don't)
operating on a  batch operation (no feed and bleed) - when you as the process
engineer are asked by the production manager to maintain a line speed of 4 to
8 fpm because there are 1000 to 1200 panels that must be processed in a given
amount of time. Either the chemistry will be changed every day in batch mode
- with a loss in processing speed or you implement a feed and bleed system.

Your input sir, is always appreciated and noted.........and I thank
you....but at times we must face reality and meet certain manufactruing
criteria that leaves the perfect world in behind in the  laboratory.

ps: there is chemistry presently on the market that will maintain line speed
and dissolve under 250 ppm copper and have virtually minor attack on tin or
tin-lead and not tarnish the copper finish.

Regards

Richard

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or
847-509-9700 ext.5365
##############################################################