I assume the automated inspection machines (which brand are they, by the
way?) are finding your printing defects, some of which you described.
When they spit out the defect list, a skilled operator should be able to
rework (efficiently) any small, isolated defects prior to sending the
PWB to pick and place. Defects such as a single pad with insufficient
solder, one or two paste bridges due to slumping, etc., are examples of
this. Application of a small amount of solder paste by pin transfer with
a small dental pick and/or running the pick between two slumped deposits
to separate them can be done. For wholesale misregistration or more than
two or three defects per board, they should be cleaned and re-printed.
I understand why the requirement is there that does not allow wiping or
cleaning of the PWB, as dry wiping and attempting to reprint results in
pwbs with solder paste particles (called solder fines) embedded into
vias, in spaces between the pads and the solder mask, and other places
you do not want solder to be. However, that does not mean that the
misprinted PWBs should be handled as scrap. If you have a stencil
washer, the misprinted PWBs should be able to be placed in that and
cleaned, followed by a pass through the assembly wash machine, baked for
1/2 hour to remove all surface moisture, inspected under at least 10X
magnification, and put back into the cue at the beginning of the
process. This cleaning without any wiping will remove all solder paste
on almost any PWB.
However, if the stencil registration is grossly off, the squeegee will
press solder paste fines in the areas we just discussed, and once
impressed it is nearly impossible to remove them. It should be noted
that small fines attached along the sides of pads or trapped under
components is acceptable as a process indicator condition, even for
Class 3 product per J-STD-001. Of course, this is dependent on the
internal requirements of the OEM as determined by the end-use
requirements.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
Sent: Wednesday, January 21, 2009 9:57 AM
To: [log in to unmask]
Subject: [TN] Automated Solder Paste Inspection - Best Practices Request
Technetters,
I sent this out last week during a period of nasty weather across the
country, and of course, got limited responses. I'm trying it again, as
this is quite important.
Please help me if you can.
Thanks!
Leland
-----Original Message-----
From: Leland Woodall
Sent: Thursday, January 15, 2009 2:40 PM
To: 'TechNet E-Mail Forum'
Subject: Automated Solder Paste Inspection - Best Practices Request
Everyone,
We have automated solder paste inspection machines immediately following
each of our screen printers. These all check for volume, height, and
pad coverage. We run only lead-free products, and also have a
requirement that disallows any wiping or cleaning of a misprinted board.
I'm looking for folks running a similar operation that can give me some
recommendations on how they manage this process. How many bridges do
you allow? How do you handle insufficients or inadequate pad coverage?
Insufficient paste volume bothers me, as I'm always afraid our
post-reflow AOI process will not detect the condition. I also hate to
scrap a board that has only a couple of pads with low or no solder.
Any suggestions on how we can minimize scrap but ensure we have no
escapes? Do we write down the effected pads, mark the board, place the
parts, and capture it after the oven? How could one reliably manage
this?
Your comments are sincerely appreciated.
Stay warm!
Leland Woodall
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