Hi John,
1)Is the flex stiffener thermally bonded or on with only a pressure
sensitive adhesive- if it is PSA it will move in reflow
2) is the stiffener FR-4 or kapton - if kapton you probably do not have
enough support for the connector- even a .010 FR-4 is pushing it
3) recommend a .004 stencil and a 5-10 % reduction in the aperture

Good luck.
Steve Kelly

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Anselmo
Sent: February-02-09 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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