Hi Gabi,
        Acceptable tombstoning defects for 0603's should be in the single
digit PPM's.  That's by component, not solder joint.  0402 are ten times
that.  We have found 0603's are particularly difficult to tombstone, so I
you are having difficulty, solderability of the component could certainly be
suspect.
        When we have had difficulty with 0603's, it has been because the
pads were spaced too far apart, allowing molten solder on one side to pull
the component off the other pad.  Once one side of the component is off the
other pad, its an open, and it doesn't matter if the component goes one step
farther and tombstones.
        With 0402's, we have found that they are VERY sensitive to placement
alignment.  We try to keep the centroid of the component better than 0.002"
of where it should be.  (Not exactly easy with the Fuji CP6)  Sensitivity is
not so much side to side or even skewing, but end to end makes the biggest
difference.  In other words, if the component is on one pad more than the
other, it is more prone to tombstone.  Further, the side that is more on the
pad will also be the side that ends up in the air.  This means, you can
predict the direction the placement machine must be adjusted if there is a
trend in the direction of tombstoning.
        The screen print must be centered as well, for similar reasons, but
I see you already check that.

If you beat this problem, let us know how you did it.

Thanks

Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]



> -----Original Message-----
> From: Bogdan Gabi [SMTP:[log in to unmask]]
> Sent: Monday, February 05, 2001 7:58 AM
> To:   [log in to unmask]
> Subject:      [TN] Tombstoning
>
> Hi, Technetters
>
> As we moved to smaller ceramic multilayer capacitors and
> resistors,(0603)we encounter an increase of tombstoning .
>
> We checked our process-printing, reflow, layout, and did not find any
> other clues but the component solderablity- so it seems.
>
> Due to very populated boards, the defect is seen quite often per board,
> but usually only one component /board.
>
> Could you please tell me what is the acceptable number of this kind of
> defect, and how you record it, per board or per solder joints?
>
> Thank you,
>
> Gaby
>

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