Hi Bob,
While your CM is doing what is very common in the industry (adding SMT
rails for manufacturability), I would be concerned with the ESD
implications of sanding, as well as FOD on the assembled product.
Furthermore, We do not prefer tab routing (mouse bites) due to the
shock/impact to the board during de-paneling.

I would recommend you investigate a scored/routed rail that can be
de-paneled on a pizza cutter. If there are overhanging components on the
assembly, you may not have a choice. But, careful design of the mouse
bites, and the tooling thickness can still eliminate the need to sand
the edges after de-paneling

Regards,
Amol


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Landman
Sent: Saturday, February 18, 2012 2:23 PM
To: [log in to unmask]
Subject: [TN] Sanding board edges after reflow

Our contract assembler just sent us the following message for a short
prototype run on a modified design:

-------

"I asked the board house to add a 1/4" tab routed strip on the long side
of the board that P1 is on so that we can put solder paste and place all
of the top side SMT parts.  We need at least 4mm (0.160") for the
clamping rails on the machines.  C180, C181, D1, R19 and C27 are to
close to the edge of the board.
 
We will remove the strip and sand the tabs."

------------

It's a 6" x 8" 6 layer board (Sn63 soldered) with SMT parts (including a
256 ball BGA and a number of tiny QFN parts).  Apparently the EE who
designed the changes and the PCB layout person were very tight on space
so extended circuitry into the area reserved for the reflow oven's
grippers.

Should I not be concerned that sanding could cause cracks in the solder?
The EE told me, as did the assembler, there was nothing to worry about.

This is the first time this problem has occurred since we began making
products in 1980.

Should I pull the job (which is a 5 day turn) on Monday and have the
artwork fixed to provide gripping areas (which used to be there) ?  

Or is it safe to do what the assembler had told the board fabricator to
do?

Bob Landman

Sent from my iPhone

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