Hi Frank,
I have had a lot of board assemblies with over 1,000 of those little guys
per board.  I used the .012 air gap for my footprints with .020 pads with
no tombstoning.
I have found that those 0402's that do tombstone is due to pad to pad
imbalance due to the amount of traces going into the pads.  Those you have
to deal with the layout and designer to address.
Hope this helps,
Kerry






Frank Kimmey <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
05/06/2005 01:05 PM
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[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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