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

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************