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February 1999

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Mon, 8 Feb 1999 13:57:51 -0800
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Dear Andy,

Many thanks for your reply. You are exactly correct that we are using
no-clean solder paste. It is Heraeus K180.  I'm still figuring out a way to
remove the resistor without destroying the evidence (dendrites, if any).
Any ideas would be appreciated.

Regards
KK Chin
Artesyn Technologies
47173 Benicia Street,
Fremont, CA 94538





Andy Mackie <[log in to unmask]> on 02/08/99 01:38:35 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to [log in to unmask]

To:   [log in to unmask]
cc:    (bcc: KK Chin)
Subject:  Re: [TN] Voltage Across 1206 SMD




KK: There's a myriad of possibilities here, and I'm sure the electronics
engineers will have some thoughts on what is happening with the component.
I'm assuming that you're using a no-clean flux....Electrical problems with
solder paste flux are not very frequent, but if there are signs of
electromigration (dendrites), under the components, or haziness or
discoloration, flux residue issues are usually indicated.

The solvents used in solder paste fluxes are usually assumed to be driven
off during reflow, but may remain entrapped if either a./ the reflow
profile spike is too low in temperature, or b./ the residues have a
significantly reduced surface area (such as when they are trapped under
BGA's or other components). Although the solvents used in no-clean fluxes
are usually non-hygroscopic (that is, they don't pick up moisture), some
are problematical, if they are not volatilized suring the reflow process.
If you have a 'capillary ball' which touches one of the component
terminations, this will increase the voltage field gradient by reducing the
gap, and this can be another factor causing electrical difficulties.

I hope this helps.

Andy





KK Chin <[log in to unmask]> on 02/08/99 03:29:54 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to KK Chin <[log in to unmask]>

To:   [log in to unmask]
cc:    (bcc: Andy Mackie)
Subject:  [TN] Voltage Across 1206 SMD




Dear Technetters,

We see some failures in one of our products due to resistance drop across a
1206 resistor on a PWB assembly. The products failed after hours of
operation in the burn-in environment and is suspected to be the effects of
SIR drop or electromigration. There's 135Vdc across the resistor during
operation. The designer thought it's ok because the resistor itself has a
max. operating voltage of 200Vdc. The land pattern is of the IPC-SM-782
standard (1.2mm gap in between). The solderpaste is also claimed to be
compliant with IPC-SF-818 and Bellcore TR-78 in terms of SIR and EM.

My understand is that SIR and EM are tested with bias voltage of 50Vdc and
10Vdc respectively. Are these standard tests really applicable to this high
voltage case?

Thanks!

KK Chin
Artesyn Technologies
47173 Benicia Street,
Fremont, CA 94538

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