That one's easy:
a) to reduce the thermal shock on hitting the wave
b) to dry off the flux solvent to prevent/reduce sputtering and ball
formation
c) to start activating the flux, so that the deoxidation process has
started before the board hits the wave
d) so that the flux, being more viscous when dried, does not wash off
during the first millisecond it is in the wave
e) to ensure better rise in PTHs
f) because, without it, it does not work at all well.
Sufficient reasons? :-)
Brian
Eric Laroche wrote:
> If this is like baseball, I'm batting ,500!!
>
> I am not a chemist, I'm truly sorry if the explanation is not all that
> accurate. But I do manufacture boards, and my chips do not crack.
>
> Can someone then tell me why we bother having a preheat section in the
> wave?
>
> Cheers!
>
> Eric
>
> -----Message d'origine-----
> De : [log in to unmask] [mailto:[log in to unmask]]
> Envoyé : 17 novembre, 2003 22:05
> À : [log in to unmask]; [log in to unmask]
> Objet : Re: [TN] Not that our process is out of control or anything, but
> --
>
> In a message dated 11/17/2003 16:52:11, [log in to unmask]
> writes:
>
>>It is the preheat it the wave that is controlled (the infamous 2C/sec)
>>to prevent thermal shock.
>
> ---This is correct.
>
>>Once the chip has achieved a specified temp, all the materials have
>
> expanded
> equally and >after this point the rapid thermal change of hitting the
> wave
> does not affect the chip.
> ---Sorry, but there is no such condition; different materials will
> always
> have a differentiasl thermal expansion, unless they have identical CTEs.
>
>
>>It is the different expanding properties of the two materials that
>
> cause the
> chip
>
>>to crack.
>
> ---Quite correct.
>
>>Once they have both expanded to their maximum state, the temperature
>
> curve
> is irrelevant.
> ---Sorry again, but there is no such state; as long as you raise the
> temperature materials will expand, unless they have a negaqtive CTE.
>
> Regards,
> Werner Engelmaier
> Engelmaier Associates, L.C.
> Electronic Packaging, Interconnection and Reliability Consulting
> 7 Jasmine Run
> Ormond Beach, FL 32174 USA
> Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
> E-mail: [log in to unmask], Website: www.engelmaier.com
>
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