Hi L. A. Quoc,
Assuming pad design is not bad and component solderability is
not bad, placement accuracy is the biggest determinant of tombstones
with solder paste alignment a distant second. You've got to be AT LEAST
0.03mm of accuracy, which will still give some tombstones. I've seen
the newer machines get as accurate as 0.005mm and the tombstones dropped
to such a low DPM they were no longer noticeable. You can predict the
direction of the tombstone based on the direction of misplacement. Or
vice versa, predict the direction of misplacement (since it is subtle)
by the direction of tombstones.
Remember there is a normal distribution of placement accuracy,
so looking at just one or two placements won't tell you the net shift.
You'll need to look at several placements (at minimum, through a scope;
preferably with automated measuring equipment) to determine the net
shift of the distribution. (This is basic SPC). You'll also find both
placement and printing accuracy are worse the farther from the center of
the board. So larger boards are more of a risk that small boards.
One of several causes for tombstones is that one of the solder
deposits becomes liquid before the deposit on the other side of the chip
becomes liquid. Without any surface tension to hold the opposite side
down, the chip will be pulled by the liquid solder on one side.
Anti-tombstoning solder paste is an alloy that has a pasty range so that
the transition time between solid to liquid is longer. This increases
the chance that both solder deposits will be at least partially liquid
before one of the two becomes fully liquid. Your lead-free solder paste
already has this quality.
By the way, I've read several studies of the wonderful qualities
of anti-tombstoning solder paste. I've also heard of a few,
"unpublished", studies where the anti-tombstoning solder paste was no
better than normal solder paste in a DOE. I had a psychology professor
once describe this effect for paranormal phenomenon. The studies
disproving the phenomenon are far more numerous than studies proving the
phenomenon, but they are never published because disproving something is
sooo boring. So most of the "published" studies concerning paranormal
phenomenon indicate it is possible.
For me, the jury is still out on how well anti-tombstoning
solder paste works. I do think the theory of them is correct so I would
imagine it will help; but I doubt it will "prevent" tombstones.
Ryan
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Le Ai Quoc
Sent: Thursday, October 14, 2004 8:41 PM
To: [log in to unmask]
Subject: Re: [TN] Aperture for 0603mm chip
Thank you Ioan,
The gap between the pads is 0.3mm (if it is 0.023" (0.6mm), the chip
whose
length is 0.6mm could not get good wetting in all cases). The size of
pad is
0.3x0.3mm. We are using Pb-free solder paste for that PCBA. With the pad
design like this, each electrode of part will sit on pad about 0.15mm in
ideal case. How big is the misalignment which is allowed for this part?
Printing result is registered well, no offsets at placement (I checked
with
microscope) and we are doing the aperture 100% as pad size.
What is the ideal time for pre-heating at reflow? Now we don't get any
case
of solder ball at these chips with current profile.
I heard that there are some aperture styles which can prevent this
problem.
Also, I have never heard about anti-Manhattan/tombstoning solder paste,
what
is that?
Thanks and best regards,
L. A. Quoc
----- Original Message -----
From: "Tempea, Ioan" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, October 14, 2004 7:54 PM
Subject: Re: [TN] Aperture for 0603mm chip
> Quoc,
>
> I don't see you mentioning land pattern design. If the gap between the
pads
> is bigger than 0.023", you start increasing your probability of
tombstoning.
> Then, in case your soldering paste (if no-clean) does not create
mid-chip
> balls, you could print 1:1, no homeplating, although we reliably do
> homeplating on 0603s that have a good pad
> design.
>
> After this you just check your process step by step: the printing
shall be
> well registered, no offsets at placement and yes, the reflow recipe
has an
> influence, if everything else is OK. You will need longer preheating,
to
> make sure that the paste on both pads melts at the same time when
hitting
> the spike. But a good reflow recipe will not correct poor printing or
> insertion.
>
> Also there are so called anti-tombstoning pastes out there, that are
worth
> trying.
>
> Good luck,
> Ioan
>
> > -----Original Message-----
> > From: TechNet [SMTP:[log in to unmask]] On Behalf Of Le Ai Quoc
> > Sent: Wednesday, October 13, 2004 11:32 PM
> > To: [log in to unmask]
> > Subject: [TN] Aperture for 0603mm chip
> >
> > Hi Technetters,
> > Do you know the opening style (aperture) for stencil (metal mask)
applied
> > for printing solder paste on resistance, capacitor chip 0603mmm to
prevent
> > "Tombstone/Manhattan" after reflow. Also, pls. advise about stencil
> > thickness.
> > Last but not least, is the reflow profile important for this defect?
Any
> > suggestion?
> > Thanks in advance,
> > Le Ai Quoc
> >
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