Hi Alan,
here's what I can come up after 18 months of running boards with 0402.
The design:
- I had both ENIG and white tin finishes, makes no difference. Didn't run
them on HASL, but I would stay away.
- pad design is critical. I had a board with 600+ 0402s, both resistors and
caps and no tombstoning with pads that are square, 20x20 mils, spaced at
22.5 mils. I had random tombstoning, about 5% of the quantity of the 0402 on
a board with different land pattern.
- did not see any difference due to glossy or matte solder mask
- I didn't see any tombstoning on so called bad thermal design, that is on
of the pads connected to a tiny track, while the other connected to trak of
the same size like the pad
The printing:
- critical. Make sure the registration is perfect.
- I did not see any difference between home plate apertures and normal
rectangles at 100% of the pad.
Placement:
- critical. Get the best out of your machines, slow them down if necessary,
use supports. You need to have the components equally seated on both pads.
By the way, normally is the caps that tombstone, since the torque due to the
surface tension is bigger because of the height.
Reflow:
- I use ramp to spike in a convection, 8 active zones, oven. I did not have
to play around with the profile, the standard one used for most of the
assemblies did a good job.
Solder paste:
- I didn't need any new paste. The regular one used for bigger parts was OK.
It's the normal no-clean CR36 from Multicore. Although is leaving a lot of
flux residue, this does not seem to have any influence of the yields.
Hope this helps,
Ioan
> -----Original Message-----
> From: Alan Kreplick [SMTP:[log in to unmask]]
> Sent: Tuesday, September 25, 2001 4:36 PM
> To: [log in to unmask]
> Subject: [TN] 0402s
>
> Hello Technetters:
>
> I know some of you are already doing 0201s, but I'm hoping you can give me
> your "lessons learned" about 0402s.
>
> Any information on stenciling, placement, reflow, and paste selection
> would
> be greatly appreciated.
>
>
> Thanks in advance for your responses,
>
>
> Al Kreplick
> Sr., Mfg. Eng.
> Teradyne, Inc.
> 978-370-1726
>
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