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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