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Incoming Circuit Boards - How Clean Is Clean?



Incoming Circuit Boards - How Clean Is Clean?
We're having cleanliness issues, our assembly processes are not introducing contaminants. Could it be the result of incoming PCB cleanliness?
Board Talk

Transcript


Phil
Welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting. The Assembly Brothers coming to you today from the ITM elegonte ballroom high atop Mount Realto.What is the question du jour?

Jim
The question comes from A.S.  

We're having cleanliness issues on our finished circuits. 

We feel that our assembly processes are not introducing contaminants. Could it be the result of incoming PCB cleanliness?

Phil

The answer to that is yes it is possible. Of course we're glad you have good competence in your internal process but just make sure youlook under every stone and every fixture.  

But addressing incoming, yes that could be a big problem. We don't know where your boards are being fabricated, but one aspect of it is of coursemost immediate is the final rinse. 

We've heard the circuit board depicted as a soup, a contamination soup. A lot of these contaminants are added during our assembly process, ionic and weak acids,non-ionic from the  handling, things along those lines.

But bear in mind that there's alot of chemical processes going on in the fab operation. Our friend Charlie Pitarys at Kyzen, you're only as good as your final rinse. And certainly from a fabrication standpoint you are only as good as your final rinse. 

So the question I would ask, short of auditing your fab facility, is what is the cleanlinessof the final rinse water that they're using before the boards are shipped off to you? 

A number of cases we run intothey are using tap water andthese are in places you wouldn't want to drink the tap water, let alone put it on your circuit boards. 

Jim
Phil, I put myself above circuit boards.

Phil
I try to treat circuit boards as I treat myself. That's probably not very good but let's put it this way. I don't know what the FDA recommends as far as cleanliness of drinking water.  

But I know for circuit boards, certainly from a cleanliness spec, in the vicinity of 200K ohms resistivity, which of course is an indication of the cleanliness of the water, is probably what you want as a spec.

Depending on how wonderful the water is coming in, wherever in the world you're using it, is the degree that you're going to have to filter it. Whether you could use a system with reverse osmosis and what level of purification and distillation, the deionization you're going to require. 

So again the spec that we understand is a good one for cleanliness of boards coming from the fab shop is again 200K ohms. The cleanliness of the boards coming out of your assembly process of course have to be a lot higher, a lot cleaner. Typically a minimum of two meg ohms. 

But from the fab shop that's what we're looking at. Of course the cleaner the better, but I would use that as a basic minimum.

You've been listening to board talk with Jim Hall and Phil Zarrow.

Whatever you do, don't solder like my brother.

Jim
And don't solder like my brother.

Comments

Additionally, IPC-5703 is a sort of Cleanliness 101 for board fabricators and can give insights into contaminants at various stages of the board fabrication process.
Doug Pauls, Collins Aerospace
As mentioned by Jim and Phil of course incoming PCB cleanliness can be a big player regarding reliability, especially if you are processing with NC flux as there is no final wash process to remove the process residues. My question to you A.S. is how you know that your assembly process isn't introducing any contaminants. In my 19 years in the cleanliness world I have yet to see a fully inert assembly process.
Eric Camden, Foresite
I would recommend that if AS wants to examine board cleanliness, that IPC-5704 be followed to determine what ionic materials are present and if they are at undesirable levels. The condition of the solder mask cure and surface energy should also be checked. Incompletely cured solder mask is a sponge for board fabrication fluids and is very hard to detect.
Doug Pauls, Rockwell Collins

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