Thanks to all for the thoughtful and informed replies.
This issue of ionic cleanliness comes up in some form approx. every 3--4 years.
It is usually prompted by a process change we make or an issue with and offshore supplier.
Thanks again for helping me understand the complexity of the issue and IPC's equivocal views regarding it.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: Tuesday, October 02, 2007 3:28 AM
To: [log in to unmask]
Subject: Re: [TN] Ionic Cleanliness
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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