Hi,

Vie used 20x20 pads with 12 gap on boards with 1oz finish Cu 3 mils above
layer 2 planes. No tombstoning, and no reliability problems. Track one end
and direct to a topside plane at the other! Some of these designs have been
big(up to 14x16) with over 1000 0402's.

Assembler was good at preheating, and still managed it all with R0L0 flux.

Not exactly IPC, but it does work. Rework would definitely require "hawk"
vision and a steady hand because the toe fillets are tiny.

Best Regards

David Greig
______________________________
GigaDyne Ltd
Buchan House
Carnegie Campus
Dunfermline KY11 8PL
United Kingdom
t: +44 (0)1383 624 975
http:\\www.gigadyne.co.uk
______________________________

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Frank Kimmey
Sent: 2005-May-06 18:05
To: [log in to unmask]
Subject: [TN] 0402 issues

This question is for Assembly Process Engineering types.

Our designs are suffering from 0402 tombstoning.
The best feedback I am receiving from our Asian CM is this is caused by
footprints.
As I am a Designer and not a Process Engineer I have been tasked with asking
some experts.
So, here we go:

We are in the process of redesigning some PCBs that use 0402 chip parts (Rs
and Cs).
Our CM is requesting we deviate from our IPC-782 footprints.
At present IPC782 and 7351 both recommended .016" gap between lands.
Our CM is requesting reduction to .012"

Comments?


Also, we use flooded ground pours due to RF performance and there fore end
up with unequal land sizes (one copper defined pad and one soldermask
defined pad +.004 in X and Y size) depending on device connections.
What would be your concerns over this design style?
Do we need to equalize land sizes at the cost of time in design (literally
thousands of parts per board) or can this be relatively compensated for with
assembly process (paste, profile, fixturing, etc.)?


Any help is appreciated
Thanks,
FNK

Frank N Kimmey CID+
Principal PCB Designer
Powerwave Technologies Inc
Office 916-941-3159
Fax 916-941-3195
Cellular 916-804-2491

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