Hi There!
Before I start, I gotta' ask, isn't Compeq a PCB fabricator?...and your
customer is asking you how to implement their assembly processes? Like you
haven't got your own problems, huh? Oh well, I suppose that's all a part of
servicing your customers...
Anyways, I'll just go down the list with your questions. At my last
employer, I was responsible to convert us over to no-clean soldering. All we did
was pure surface mount, no wave soldering, so as far as that aspect of it,
there's others out on the list that have more experience with that.
One thing that you don't mention is if they're already in a no-clean
process or not. If they aren't and they're asking you how to implement no-clean,
I sure hope they know that there's *A LOT* more to implementing no-clean than
picking out a solder paste and throwing away the cleaners, A LOT MORE.
Selecting the solder paste is the easy part, it's everything else that a
no-clean process demands and requires that makes it quite a bit of work. Rework,
ICT, Receiving Inspection, Purchasing, Material Handling, all are affected in a
no-clean process. Another thing, if the existing assembly process isn't very
well controlled, switching to no-clean is going to be pretty painful. That's the
key, EVERYTHING about a no-clean process must be under good process control.
With that being said, let's address your specific questions;
> 1.What kind of no-clean solder paste is most compatible to Cu-106A?
I evaluated pastes from three different vendors, SCM Metals, Qualitek, and
Enthone. I don't want to say one is any better than the other, because they all
performed acceptably. I had my own personal likes and dislikes, but he decision
was made that included business factors and relationships (price) as well. The
criteria that we had for the paste we used was; 1). It had to print well down to
16-mil pitch. 2). Reflow performance had to be good, good wetting, nice shiny
joints, and that kinda' stuff. 3). The residue that remained had to remain clear
during multiple reflow cycles and non-tacky since we routed out a lot of our
products and didn't want to attract the router dust. 4). Of course the residue
remaining couldn't cause any electrical performance problems and had to pass
standard SIR testing. 5). The assembly also needed to pass temperature cycling
for 500-hours, under power without failing. 6). We wanted a vendor that could
provide a whole package of soldering materials to us, Paste, wire-core solder,
and touch-up flux. That way we would have to go through a bunch of experiments
finding compatible rework materials to use with the paste.
> 2.What will the max. storage time after open bag to assembly? Is there any
> concern to store in controlled environment ?
Now supposedly, according to stuff I've read in NEPCON proceedings and the like,
OSP coatings are better to, or at least equal to the storage life of HASL'ed or
Gold Plated PCB's. High humidity is something to be concerned about because it
will degrade the coating. That was somewhat of a concern to us because the
company has a facility in Puerto Rico. But as long as you keep the PCB's sealed
with a desiccant inside moisture-proof packaging, you should be good for at
least 6-8 months...you don't plan on keeping PCB's on the shelves any longer
than that do you? One thing that OSP coated boards have a problem with is
handling. The coating is pretty fragile. You can't slide and stack them against
each other without scratching the coating and exposing copper...then you DO have
a problem. One other thing, I've seen boards from A LOT of PCB fabricators that
have done the bed of nails testing AFTER the coating has been applied. You can
see where the probes marks have pierced the coating. Excuse me if I'm missing
something here, but ain't the whole point of OSP's is to seal the solderable
surfaces from the air? Maybe I'm wrong...but I don't think so.
> 3.Is it necessary to reflow in Nitrogen environment ?
It wasn't for me, but we weren't looking at any REAL LOW residue pastes. There's
some pastes that leave little, if any residues that are very low in activity
that benefit from an inert atmosphere. But there's a lot of no-clean paste
formulas that don't need nitrogen.
> 4.What is the max. hold time between first and second IR reflow ?
We would reflow one side one day, and do the second side the next day and not
have any problems. So if ya' want a figure, I'd say 12-18 hours is safe, but
that's with CU-106. The first time I used CU-56, you need to do the 2nd reflow
within 4-hours...the quicker the better.
> 5.How to rework bad paste printing like skip or misalignment ?
That's easy, get yourself an ultrasonic stencil cleaner and clean your misprints
in that too...it works great!
> 6.Any other concern using Cu-106 A?
Not that I can think of...I like the stuff! Nice flat pads, and it seems to cut
down on bridging with the fine-pitch components because the solder volume for
each joint is much more controlled. You don't have the plating thickness
variances like you do on a HASL'ed fab, and the solder paste seems to stay right
where it's been printed.
Good luck, and make sure your customer doesn't blame you guys for their
problems! (GRIN) B-)
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