Nicolas-
Without seeing your problem firsthand or knowing what you've already done, I
don't mean any offense if you've already done the stuff below- just offering
up in case you didn't.
Perhaps your apertures are too small already, and you are having a release
problem with your paste from the apertures- I say this because of your
mention of insufficients w/bridging. Check your aspect ratio, if it's
really close to "1", you're probably having a late, partial, or no release.
Late will slump across neighboring pads, & you get a bridge, with an
insufficient nearby. Inspect your stencil without wiping after a few
prints. Inspect your qfp after every print & prior to reflow. Is your
placement right on, or is there some variation?
Bent leads and coplanarity issues can give you bridging and insufficients,
so check your raw materials. Finally, one thing you may want to examine is
the flatness of the pcb in that area. You may have a poor "gasket" around
your IC as a result of silkscreening over a heavy trace nearby.
We used to do spc at my old job, but not here, so I can't recall the
numbers, but know that you can improve anything significantly if you are
able and willing to address the root causes. You'll probably have to do
some serious babysitting for an hour or two on this one.
good luck! Bill C.
-----Original Message-----
From: Nicolas Ortiz [mailto:[log in to unmask]]
Sent: Tuesday, February 06, 2001 12:10 PM
To: [log in to unmask]
Subject: Re: [TN] QFP144 solder bridging
Thanks to all for your support. I already reviewed most of all the
potential root causes (stencil, reflow profile, humidity, temperature,
etc.), but I never reached the "zero defects". My optimum process output
is 38 dpmo's.
I think, when I issued the first question, I did not asked the right
questions:
1. Does somebody has the "perfect" process, where you don't have solder
bridges or insufficient solder?
2. What are your dpmo's, ppm's or % defective?
3. Do you have more or less than 38 dpmo's?
Thanks to all for your support again.
[log in to unmask]
-3com.com To: [log in to unmask],
[log in to unmask]
02/06/01 cc:
08:26 AM Subject: RE: [TN] QFP144 solder
bridging
Nicolas,
If the paste looks good after printing and has not bridged or smeared after
the parts are placed, the paste may be 'hot slumping' inside the reflow
oven. You can test this by printing a board and then placing it into an
oven at 100 deg C. for about 10 minutes. If the paste moves significantly,
you have bad solder paste.
Jim Kittel
-----Original Message-----
From: Nicolas Ortiz [mailto:[log in to unmask]]
Sent: Tuesday, February 06, 2001 7:45 AM
To: [log in to unmask]
Subject: Re: [TN] QFP144 solder bridging
Stencil is OK. No changes in design.
Trevor
Goddard To: [log in to unmask]
<tgoddard@XLT cc:
EK.COM> Subject: Re: [TN] QFP144
solder
bridging
Sent by:
TechNet
<[log in to unmask]
ORG>
02/06/01
06:03 AM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
Trevor
Goddard
Nicolas
I have a couple of questions for you.
Is this a new board or a new Rev of an existing product?
Did you review the stencil design or did you actually have the stencil
measured?
I got caught one time when I was given a new Rev of an existing product.
We
had a couple of QFPs .20mm and a .50mm pitch that were shorting out like
crazy, which was never a problem on the previous Rev. I questioned the guys
that ordered the stencil and the layout guys about what specs the stencil
was ordered to and if there had been a footprint changed. The answers to my
questions was that nothing had changed, but I could still not fix the
problem with process changes. Finally we were able to get the stencil
measured and the pad sizes measured and found out that the apertures on
some
components were 100% of the pad size and another was a tiny bit bigger than
pad size.
My guess is, if nothing has changed in your manufacturing process i.e..
screen printing parameters, reflow profile and your placement is good than
something may have changed in the design side.
Trevor
-----Original Message-----
From: Nicolas Ortiz [mailto:[log in to unmask]]
Sent: Monday, February 05, 2001 3:45 PM
To: [log in to unmask]
Subject: [TN] QFP144 solder bridging
Currently we are experiencing solder bridging on a 0.5mm pitch, LQFP, 144
leads. I already reviewed the stencil design, screen printer parameters and
reflow profile. Everything is OK. We have HASL boards. Our current process
outputs 38 dpmo's. My concern is if I reduce more the apertures, I have the
risk to have solder open or insufficient solder, instead of bridging. Does
anybody has some input? What dpmo's do you have?
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