I refer to IVF Research Publication 96846 - Cleanliness and Reliability:
Evaluation of Test Methods and the Impact on Contamination from Production
Processes on the Reliability of Printed Circuit Board Assemblies - Dr
Per-Erik Tegenhal 1996
He states:
"It was demonstrated that the accuracy of solvent extract conductivity
measurements, i.e. the most commonly used method for cleanliness
measurements, is very poor. In one case, it was found that less than one
percent of ionic contaminants present on a hot solder levelled PCB of FR-4
type was detected by a commercial tester. The main reason for the poor
accuracy is that flux residues are absorbed into the epoxy resin during
the soldering process. Therefore it is very difficult to clean FR-4 boards
but also to analyze the cleanliness of such boards. The use of equivalence
factors for various testers is not a feasible way to handle this problem."
If the 1% of ionic contaminants are removed from the surface of the board
then what is to stop the highly mobile chlorides from migrating out of the
"spongy" FR-4 substrate after cleaning in the direction of the newly
established concentration gradient and eventually reaching pre-clean
contamination levels? What effect does this have on reliability?
Logically then, taking chlorides for example, the reliability of the PCB
will NOT correlate with the concentration of the surface chloride levels,
but with the relative humidity and "wetting" of the PCB surface. A "wet"
surface would allow the chloride to migrate out of the substrate,
compromise the SIR and this may correlate more highly to PCB failures.
What levels of chlorides have been found in and around the solder pads of
HASL/FR-4 type boards?
In this situation. If the spot levels of chlorides in and around the
solder pads of HASL/FR-4 boards is high (how high I do not know), what
then is the relative contribution to failure rates by environmental
chloride contamination? Environmental contamination may only contribute a
fraction of the total ionic contamination found in an around the solder
pads.
Any comments.
TimC.