Hi Ahne,
PCB card size is 111x101mm with tolerance about +/-0.15mm. The PCB card has
80 small boards connected together via their tabs. Chip 0603mm are on all of
these small boards.
For each small board on card, it has the fiducial mark and we use each this
fiducial mark as reference point for each small board, therefore, with
tolerance +/-0.15mm, it should be no problem because Placer (AX-5) will base
on the fiducial mark and do the offsets.
Worrying that temperature at two ends of part is different during melting of
solder paste, we checked temperature there and the difference of time from
melting point (218oC) is about 1~1.5sec but I have to admit that it is very
difficult to get the correct result during this trial because 0603mm is too
small that we can put the thermocouples (tip twisted by hand) at two ends of
chip very correctly.
We also check the misalignment after part placement on PCBA covered
double-sided tape by 3D Mitutoyo machine, the max misaligment is about
0.07mm.
Printing is nothing to complain about.
Reflow applies nitrogen with setting 1000ppm for Pb-free paste.
Thanks and regards,
L. A. Quoc
----- Original Message -----
From: "Ahne Oosterhof" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, October 15, 2004 11:28 PM
Subject: Re: [TN] Aperture for 0603mm chip
> One assumption is missing: Assuming the circuit board is accurate.
> It is very possible to accurately place the component next to the pad
(maybe
> not quite next to). My question is: how accurate do you assume the circuit
> board (say 12x14 inches) is, using the fiducials as reference points.
>
> Have fun,
> Ahne.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Ryan Grant
> Sent: Friday, October 15, 2004 8:36
> To: [log in to unmask]
> Subject: Re: [TN] Aperture for 0603mm chip
>
> 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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