Hi Victor,
we always have used rectangular apertures without reduction for capacitors
and with reductions 19% (Width and length reduced 10%) for resistors and
36% (Width and length reduced 20%) for R0805. The apertures are centered
on the pad.
We have balls on the two type of components.
So it seems I have to test new aperture shapes.
Best regards,
CANTAGALLO Luigi
Technology Engineer
Victor Hernandez <[log in to unmask]>
Envoyé par : TechNet <[log in to unmask]>
22/02/2012 12:43
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Re: [TN] R: [TN] Microballs
A picture is worth one thousand words.
Victor,
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of SALA GABRIELE
Sent: Wednesday, February 22, 2012 2:07 AM
To: [log in to unmask]
Subject: [TN] R: [TN] Microballs
Hi Luigi,
on top to Reuven suggestions, I would had also, paste volume could be
the
origin of the problem.
This kind of defect is commonly called Solder Beading effect, often due to
excessive paste volume under the chips.
On IPC-7525, they suggest how to prevent or avoid this by designing
dedicated stencil aperture shapes and the propose few examples like :
Home
Plate Aperture, Bow Tie Aperture, Oblog Aperture, etc.
3.2.3.5 Chip Components -Resistors and Capacitors Several aperture
geometries are effective in reducing the occurrence of solder balls. All
these designs are aimed at reducing excess solder paste trapped under the
chip component. The most popular designs are shown in Figure 3-9 (same
aperture design as the MELF diode) the C shaped aperture. This aperture
design reduces the amount of solder paste under the Chip Component but
maintains land coverage on the peripheral of the lands.
Have you already in use stencils with aperture shape like above ?
Suggestions like above have been experimented with success in plenty of
cases.
Best Regards
Ciao
Gabriele
-----Messaggio originale-----
Da: TechNet [mailto:[log in to unmask]] Per conto di Reuven Rokah
Inviato: mercoledì 22 febbraio 2012 7.53
A: [log in to unmask]
Oggetto: Re: [TN] Microballs
Hi Luigi,
You have to check:
1. Size of pads (too long under the components body).
2. Size of opening in stencil
3. P&P accuracy
4. Solder paste temperature (should be room temperature)
5. Oxidation of component / PCB [ads / solder paste particles
Reuven
On Wed, Feb 22, 2012 at 8:33 AM,
<[log in to unmask]>wrote:
> Hello everybody.
>
> Any idea about random presence of microballs (diameter 0.1 to 0.5 mm)
> beneath some chips (R and C 0805, 1206,1210, 1812) ?
> We use a SnPb process with a vapor phase oven.
> We use same types of PCB, same process on same SMT line in a clean room
> from years and the balls are randomly present from some months now.
> They could be present in one lot of PCB and not the following; beneath a
> component on one PCB and not on the same component on other PCB; we have
> balls one day and not the day after; the balls disappear for weeks and
> re-appear after; ...
> I have checked different lots of solder paste, origin of PCB's, all
> parameters of printing, P&P and the cleaning (co-solvent station). The
> thermal profiles are same.
> As it's hi-rel production I cannot change easely products and process.
> I suspect a process parameter but which ?
> I ask myself if there is not a ghost in the line .. :-)
>
> Best regards.
>
> CANTAGALLO Luigi
> Technology Engineer
>
>
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