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February 2009

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Subject:
From:
John Anselmo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, John Anselmo <[log in to unmask]>
Date:
Mon, 2 Feb 2009 15:24:45 -0700
Content-Type:
text/plain
Parts/Attachments:
text/plain (126 lines)
The stencil is wiped after ever 2nd pass.
I have not had a chance to measure the stencil (I don't have the proper tool).
The flex is 8 to 10 mils thick and it has a 20 mil stiffener (I was wrong when I said it was 10 mils).
The panel is approximately 9" x 9".  The board itself is a 2.4" x 2.4" "L" shaped flex cable.
So far the paste is all from the same lot.

Your last question brings up an interesting point.
We only have a limited number of carriers, so the operators sometimes cool them just until they can be handled safely, them load up another panel.
Could this be causing some of the slump issues?  We do get bridging all the time, but the degree of bridging does sometimes change.

John

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Guy Ramsey
Sent: Monday, February 02, 2009 1:56 PM
To: [log in to unmask]
Subject: Re: [TN] Bridging on SMD Connector


Hey man, 

Are you getting a clean wipe? 
Is it possible that your stencil does not meet spec? 
How thick is the flex, how deep is the recess?
How small is the board? 
The carrier could shim the stencil off the board.

Is the paste all from the same lot? I have never had problems with 562 but
anything is possible. 
Is the carrier hot when you print? 





-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Anselmo
Sent: Monday, February 02, 2009 2:50 PM
To: [log in to unmask]
Subject: [TN] Bridging on SMD Connector

Hi all,

I am having some difficulties with bridging on an SMD connector and need
some help.  
I've searched the archives for similar problems but have come up empty so
far.  Below are the details:

We are placing two different JMD connectors, one 60 pin and one 80 pin.
Stencil:  5 mil stencil and the apertures are 1:1 with the pads (.020"
pitch, .012 pad width)
Paste:  Kester R562 (use a new jar every day)
Parts: Flex boards with .010" stiffeners supported by carriers (the flex
panels are placed in a Durastone carrier that is recessed the thickness of
the panel).
Printer:  MPM model AP25

We are measuring the paste parameters after printing with the CyberOptics
2000.  The data shows lots of variation in paste volume and thickness.
I am not sure how accurate the data is since the Cyberoptics does not verify
correct alignment of the panel before it takes the measurement, but I do
agree we have variation.
In general, it shows the paste thickness to be .007" to .009".

We see slumping of the paste sometimes before placement, mostly after
placement.
I've changed the parameters of the printer (pressure, print speed, the snap
off distance to ensure we are getting on-contact printing).
I've also had the operator rotate the stencil 90 deg to see if there was a
difference in how we applied the paste (parallel or perpendicular to the
length of the aperture).
We are using metal squeegees on the printer.
All of the above changes have resulted in no noticeable reduction in the
number of bridges.

We are currently running a flex panel with 12 parts per array and we get 50%
to 100% bridging per panel. Now you see why I am pulling my hair out.

The room parameters (throughout the day) are 70 to 74 deg F and 25% to 30%
RH

My questions are:
1) What else can I look at that effects bridging?
2) Can the reflow (hot air forced) oven be causing some of the problem?
3) What should the paste thickness be with a 5 mil stencil?  
4) Should we go to a .001" reduction in the aperture on the stencil?  .002"
reduction?
5) Is our panel carrier causing the problem?
6) Will rubber squeegees make a difference?

Frustrated in Montana,
John

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