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From [log in to unmask] Mon Nov 18 10: |
52:46 1996 |
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It has been shown by Hank Sanftleben (IPC Review, Vol. 37, No. 2, pp. 30-32)
that circuit boards manufactured with an aqueous developable liquid
photoimageable solder mask and hot air solder levelled contain high levels
of sodium (3-6 microgram per square centimetre) and chloride (3-5 microgram
per square centimetre). Multiply with 6.45 and you will get it in microgram
per square inch. The sodium originated from the aqueous developer and the
chloride from the flux used for HASL. According to Sanftleben, these
contaminants "were forcibly introduced into the solder mask and could not be
removed by normal surface cleaning". We have performed investigations that
indicate that halides from fluxes used for HASL are absorbed also into the
epoxy resin in FR-4 laminates.
Due to the fact that contaminants are absorbed into the polymer resin in the
laminate and the solder mask, it is extremely difficult to clean PCBs, but
also to measure the cleanliness level since cleanliness measurements are
based on a cleaning process. In one study we measured the cleanliness of a
hot air solder levelled PCB (without solder mask) using a commercial test
equipment and obtained a contamination level of 0.06 microgram per square
centimetre. Then we improved the cleaning efficiency by putting the PCB to
be tested in a plastic bag, filled the bag with cleaning solution and sealed
the bag. The PCB was then cleaned for four hours at 50 degrees Celsius using
ultrasonic agitation. Using this method we obtained a contamination level of
5.6 microgram per square centimetre, i.e. about one hundred times higher
than using the commercial tester. This means that the PCB manufacturer only
removes the contamination on the uppermost surface of the PCBs after which
contamination may diffuse up to the surface causing the ionic readings to
intensify over time. We have in several cases seen this happen.
If you want to have more information of our study, please contact me
directly.
Per-Erik Tegehall
IVF (The Swedish Institute of Production Engineering Research)
Tel: +46-31-7066148
Fax: +46-31-276130
E-mail: [log in to unmask]
------------------------
Nov 15, Scott wrote
I am having some problems with some PCB's that were manufactured approx. 1
year ago. According to the manufacturer of the printed circuit boards,
ionic readings prior to shipping read from 1.2 to 3.0 micrograms per square
inch. The boards were having a problem going through the assembly line, so
we decided to do additional ionic testing to see if it was a contamination
issue. The readings nearly quadrapled from the original readings reported
by the manufacturer. We had also noticed a high level of sodium and
chlorine when conducting our testing. My questions are:
1. What would cause the ionic readings to intensify over time?
2. What could be the possible origins of sodium and chlorine? Could they
be
introduced in any of PCB manufacturing processes?
Any comments are greatly appreciated.
Scott
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