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October 2007

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Tue, 2 Oct 2007 10:27:55 +0300
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Whereas Doug will use the copyright "It depends..." phrase, I can use 
the "How long is a piece of string...?" argument.

Look at this logically: you have assembly 1 which uses 5 V TTL 
technology in, say, a consumer alarm clock for use in an air-conditioned 
room, the PCB having conductor spacings of 0.8 mm. You have assembly 2 
for use in an aircraft navigation instruments, working at 10 V with 
conductor spacings of 0.08 mm, under condensing conditions in Changi 
airport. Which one should be cleaner? It is ridiculous to apply, say, 
1.56 µg/cm² eq. NaCl to both, n'est-ce pas?

Now, where did this magic figure of 1.56 µg/cm² eq. NaCl come from? It 
is a transliteration of Hobson's work at NAFI in the late 1960s, using a 
highly inaccurate method, later taken into MIL-P-28809. I identified 
over 30 potential sources of error, ONE of which. alone, could introduce 
a 48% error, depending on how the operator conducted the test (want to 
know more? see pp 249-257 of a well-known book).

Now, in 1968, when the test was developed, assemblies were very 
different from what they are today. The smallest component was 8 mm long 
and 3 mm diameter and had wires going through holes and 0.5 mm conductor 
spacing was truly state-of-the-art. Many PCBs were still single-sided 
and PTH-double-sided were almost the exception - and the most common 
base material for pro use was still G-10, if it wasn't XXXPC. Is a 
figure dreamt up by NAFI 39 years ago, with all the changes of 
technology, still valid today? I think not! No, that's wrong, I KNOW not!

So, what is the correct figure for you? Clean some circuits using a 
reasonable method and see whether they meet your requirements of 
reliability in the field, and/or with qualification testing with diverse 
test methods. When you are satisfied that they do, measure the residual 
ionic contamination on, say, 50 circuits and calculate the mean and 
standard deviation. That is your reference. If, in production, the ionic 
contamination levels fall within the 90 percentile of the SD curve, you 
know that everything (including the test method) is just fine and you 
can sleep on both ears at night. That is how long your piece of string is!

Brian

Chafin, Ken G. wrote:
> Does any IPC Standard recommend a minimal level of  PC Board, or PC
> Assembly ionic cleanliness in terms of material per square unit of board
> or assembly area?
> 
> E.g.   0.2 micrograms of sodium chloride equivalent per square
> centimeter.
> 
> In IPC-A-600D, section 19.1 a solvent resistively value for an
> IPC-TM-650 test is given (greater than 2 X 10 to power 6 ohm per
> centimeter is recommended).
> 
> Even if an appropriate response to this question is complex can the
> question itself be answered yes or no?
> 
> Thanks for any guidance. 
> 
> 
> 
> Kenneth Chafin
> 
> 
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