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Subject:	Re: Bridging on SMD Connector
From:	Ioan Tempea <[log in to unmask]>
Reply To:	TechNet E-Mail Forum <[log in to unmask]>, Ioan Tempea <[log in to unmask]>
Date:	Mon, 2 Feb 2009 16:08:31 -0500
Content-Type:	text/plain
Parts/Attachments:	text/plain (76 lines)

John,

7 to 9 mils deposits is huge for a 5mil stencils. You should be somewhere between 5.5 and 7! Check how the board is supported and make sure you have 0 snapoff (you could eventually try some negative snapoff, so that you push the board onto the stencil).

Besides that, for 0.020" pitch you should have 0.010" wide apertures, not more. If still bridging, you can try to reduce them at 0.009".

Regards,

Ioan Tempea, ing.
t : 450-967-7100 ext : 244
[log in to unmask]
www.digico.cc
P N'imprimer que si nécessaire - Print only if you must

-----Message d'origine-----
De : TechNet [mailto:[log in to unmask]] De la part de John Anselmo
Envoyé : 2 février 2009 14:50
À : [log in to unmask]
Objet : [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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