Howard,
Yes, you need to focus on getting the lead to contact the solder paste.
Sometimes, an increased Z-stroke is too fast and the somewhat elastic solder
paste springs back. If your placement machine has dwell control (I know
some FUJIs, and Universal GSMs have this), then you may want increase the
dwell time at the bottom of the placement down stroke. This will allow the
solder paste to spread out when the part is placed.
Howard A. Cyker
Lucent Technologies
New Product Engineering
Email [log in to unmask]
From: Howard Watson [mailto:[log in to unmask]]
Sent: Tuesday, May 13, 2003 12:10 PM
To: Cyker, Howard A (Howie)
Cc: 'TechNet E-Mail Forum.'
Subject: RE: [TN] Coplanarity of PLCC's
Howard,
I agree with you that the overprint probably won't do anything - out of
desperation I was trying the Band-Aid approach thinking that the component
might sink into the solder as it reflowed, and more solder might help.
Assuming that the components are in spec, but just barely, then how do I
compensate this condition? It seems like the focus should be the downward
"Z" height to get the component more into the paste, correct?
Howard Watson
SMT Manufacturing Engineer
AMETEK/Dixson
"Cyker, Howard A (Howie)" <[log in to unmask]>
05/12/03 02:20 PM
To: "'TechNet E-Mail Forum.'" <[log in to unmask]>,
"[log in to unmask]" <[log in to unmask]>
cc:
Subject: RE: [TN] Coplanarity of PLCC's
Howard,
IF it turns out the lead is bent so that it is not contacting the solder
paste after it is placed, over-printing the paste probably will not help
"bridge the gap" as you say. The extra solder will not "jump" up to the
lead during reflow. The solder paste must be in contact with the lead
during reflow to initiate wetting up the lead. I assume that Motorola is
measuring their parts against the seating plane and not just measuring
relative coplanarity. I agree with Carrie, if you can see the bent lead, it
is probably not within the 4 mil coplanarity spec. When we were assembling
lots of PLCC's, we did occasionally see that the corner pins were the most
likely to be the ones that were bent, usually from tube feeders or taping
equipment.
Howard A. Cyker
Lucent Technologies
New Product Engineering
Email [log in to unmask]
-----Original Message-----
From: Morse, Carrie [mailto:[log in to unmask]]
Sent: Monday, May 12, 2003 3:45 PM
To: [log in to unmask]
Subject: Re: [TN] Coplanarity of PLCC's
Howard,
Are you sure that Moto is sure that the one corner that looks like it's bent
is really within .004"?
If you can see a bent lead, it is more than .004"! If they measured it on
the same piece of equipment
they measured it in the first place, there may be something wrong. Is it
possible their equipment is wrong?
I'd hate to see you running around trying to develop a new process when in
reality the input to your process
is not correct (ie: coplanarity of the comp.)
May I suggest that the next time you run into this problem you have a 3rd
party measure the coplanarity.
That way you can have objective evidence to provide back to Moto.
Also, is it possible that you may be damaging that one lead when retrieving
it from the tube or tape?
-Carrie
-----Original Message-----
From: Howard Watson [mailto:[log in to unmask]]
Sent: Monday, May 12, 2003 2:17 PM
To: [log in to unmask]
Subject: [TN] Coplanarity of PLCC's
Technetters,
I have been having a problem with a PLCC 68, manufactured by Motorola, where
about 1 out of 100 components have 1 corner lead bent upwards creating no
solder joint. It's usually pin 9 or 61. I use a DEK 265 Infinity, laser
cut 6 mil stencil, and no clean solder paste. The feedback from Motorola is
that the lead they measured from one defect was within their coplanarity
specification of .004". They suggested using a 7 mil stencil, which I tried
with no different result. I also tried increasing the Z distance, to push
the component into the paste more, but I don't think there was much of an
impact. It seems like it would be very difficult to push this many leads
further into the paste. Theoretically, with a 7 mil paste deposit, and if
the component went half way down, then I would only have .0035" of allowance
for coplanarity, which would not suffice for the tolerance of this
component, rig! ht?
My next plan is to overprint the solder paste 20% longer than the pad (.08"
x .025"), and use no reduction of the width (I normally use 10% reduction),
with the hope that the reflowed solder would "bridge" the gap. I am hoping
this will work, but does anyone have another suggestion? Do you think its
possible to push the component further into the paste? A step stencil is
impossible because of the density, and there is nothing on the PCB with a
finer pitch than this component. Thanks in advance for your help.
Howard Watson
SMT Manufacturing Engineer
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