Jonathan,
There are basically a few things going on with the solder paste. A higher
activity paste will attack more aggresively and create more of a chemical
reaction, and thus more outgassing. This outgassing creates more turbulence
in the reflowing solder, and thus improves self alignmnet. The outgassing
also can become entrapped and cause voiding.
Some fluxes that are lower in activity create less voiding, but also there
is reflow speed (according to some solder paste chemists). Reflow speed
could be thought of as the time that the powder begins to reflow, until the
entire mass is liquidous (typically between 1/2 to 2 seconds for eutectic
SnPb).
Most no-clean formulas do not create too many voids (especially if your O2
ppm levels are under 200). I know of some slow reflowing OA formulas that I
can refer you to if you want to e-mail me off line.
I would get voiding to an acceptable level, and then work on
misplacement/self alignment. Smaller passives seem to perform well on
smaller pads. Additional solder paste will increase the overall surface
tension of the joint, and help pull the part into position.
Type 3 paste is generally better than type 4 if you want to reduce voiding.
Unfortunately the optimum parameters are up to you- at least once you are
sure your pad design is not too loose).
I assume you might be using a 96% metals by weight solder paste. My
experience has been with the 90% +/- 1% solder pastes, so while the
principles might still apply, one can only check and see with the
91.5Sn8.5Sb formula. Hope some of this helps..
Steve A
> -----Original Message-----
> From: Jonathan A Noquil [SMTP:[log in to unmask]]
> Sent: Monday,October 01,2001 11:14 PM
> To: [log in to unmask]
> Subject: [TN] Component skewing
>
> Hello Guys,
> I have been experiencing component skewing after reflow.
> However, when i used type 6 paste (no clean flux), there is no skewing
> or movement of the component but a lot of voids.
> When i used type 3 paste, the voids were reduced but there is much skewing
> of the components. Seems voiding and skewing were in contrast.
> Does anybody had experienced the same problem. Had they gotten the
> optimum parameter? (Less voids, no skewing?).
> My process is solder paste dispensing, then attach the component and
> reflow
> in an N2 Blanketed oven.
> Paste is leadfree (alloy : 91.5Sn8.5Sb).
>
> Appreciate any help
>
> God Bless
>
>
> thanks
>
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