Gaby
Some things to check:
1) thermal profile - ramp up rate.  Your comment re the conductor area
makes this suspicious.  Also consider orientation in the oven - does one
side reflow before the other?
2) is the oven running smoothly?  (chain or conveyor vibration)
3) pads may be a factor, too much solder can do this.  As a crude
experiment you could try removing some of the paste from the pads before
placement.
4) I am assuming you verify the accuracy of the placement before reflow,
but if not it's also a possible problem.  Also look at the accuracy of
the screening onto the pads - if your paste is offset to the left and
the part to the right...

** standard disclaimer, opinions are my own, etc....  **

regards,

Graham Collins
Process Engineer,
Litton Systems Limited, Atlantic Division
(902) 873-2000 ext 215


> -----Original Message-----
> From: Gabriela Bogdan [SMTP:[log in to unmask]]
> Sent: Tuesday, October 21, 1997 6:47 PM
> To:   [log in to unmask]
> Subject:      [TECHNET] Tombstoning
>
> Hello, Technetters!
> I need infomation about Tombstoning of Surface mount components during
> Reflow.
> Does it occure on leadless components in general, disregarding size
> and
> number of connections?
> If so, what are the rules for avoiding it?-Design of pads?
> We encounter a problem which seems to be tombstoning with components
> featuring 3+3 terminations .The corresponding pads on one side are
> connected
> to large conductor areas. The others are not. After reflow many
> components are tipped on one side or misplaced.
> Any suggestions?
> Thank you,
> Gaby
>
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