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From: krw on 27 Mar 2010 15:04
On Sat, 27 Mar 2010 11:31:16 -0700, Joerg <invalid(a)invalid.invalid> wrote:

>krw(a)att.bizzzzzzzzzzzz wrote:
>> On Sat, 27 Mar 2010 10:50:32 -0700, Joerg <invalid(a)invalid.invalid> wrote:
>>
>>> krw(a)att.bizzzzzzzzzzzz wrote:
>>>> On Sat, 27 Mar 2010 08:54:26 -0700, Joerg <invalid(a)invalid.invalid> wrote:
>>>>
>>> [...]
>>>
>>>>> Did you guys get the transformer issue licked?
>>>> Not completely. We received a small number of samples of the new transformer
>>>> but without manufacturing quantities we don't _know_ that it's solved. Only
>>>> something like 5% were failing in reflow so it's pretty hard to test three
>>>> samples for the problem. We have a very good manual test for the failure now
>>>> and an in-circuit test that tests ten out of fifteen of them. When we get
>>>> production quantities we can remove the manual test and will then rely on ICT
>>>> to flag any regression. Completely solved, no. We have a very good handle on
>>>> it and it's no longer impacting deliveries, so it's 95% of the way there.
>>>>
>>> If there is time you could cook two transformers. One from the old batch
>>> and one of the new samples. Ratchet up the oven temp and see which one
>>> is going at what temp. At least that'll tell you whetehr they really
>>> used wire with better coating.
>>
>> The "new" transformer only has the high temp wire on 1/2 of the primary, since
>> that's what is exposed. The transformers have a thermal time constant of
>> something like 8.5 minutes and are only baked for 3 minutes (about 10 seconds
>> at 250C, according to the profile). It's going to be real hard to balance
>> time and temperature to measure insulation melting with any confidence.
>
>
>Then the only option would be to send them through the reflow oven "dry"
>several times, together. Ratchet up the temp profile by 10C at a time
>and see when the old one fails and when (or whether) the new one fails.
>Of course if you have the SMT assy contracted out that isn't really
>going to work.

With only a normal 5% failure, I'm not sure that would tell us anything
useful. We do our own assembly.

After taking a few of the failed transformers apart, as was suggested by
someone here, I found that the failures were all on the bottom of the
transformer (horizontal core). The ones with a small number of turns shorted
(the ones with a nominal DCR) appeared to be shorted in the corners where the
winding changed direction. Perhaps the windings/insulation was damaged (piled
up when changing direction?) or perhaps that's the hottest area in reflow.
With the higher temp wire (200C vs. 130C.) it should be better. The first
production batch should tell us pretty quickly. IIRC, a few hundred are due
in a couple of weeks and 1500 or so a couple of weeks after that.
From: Joerg on 27 Mar 2010 15:25
krw(a)att.bizzzzzzzzzzzz wrote:
> On Sat, 27 Mar 2010 11:31:16 -0700, Joerg <invalid(a)invalid.invalid> wrote:
>
>> krw(a)att.bizzzzzzzzzzzz wrote:
>>> On Sat, 27 Mar 2010 10:50:32 -0700, Joerg <invalid(a)invalid.invalid> wrote:
>>>
>>>> krw(a)att.bizzzzzzzzzzzz wrote:
>>>>> On Sat, 27 Mar 2010 08:54:26 -0700, Joerg <invalid(a)invalid.invalid> wrote:
>>>>>
>>>> [...]
>>>>
>>>>>> Did you guys get the transformer issue licked?
>>>>> Not completely. We received a small number of samples of the new transformer
>>>>> but without manufacturing quantities we don't _know_ that it's solved. Only
>>>>> something like 5% were failing in reflow so it's pretty hard to test three
>>>>> samples for the problem. We have a very good manual test for the failure now
>>>>> and an in-circuit test that tests ten out of fifteen of them. When we get
>>>>> production quantities we can remove the manual test and will then rely on ICT
>>>>> to flag any regression. Completely solved, no. We have a very good handle on
>>>>> it and it's no longer impacting deliveries, so it's 95% of the way there.
>>>>>
>>>> If there is time you could cook two transformers. One from the old batch
>>>> and one of the new samples. Ratchet up the oven temp and see which one
>>>> is going at what temp. At least that'll tell you whetehr they really
>>>> used wire with better coating.
>>> The "new" transformer only has the high temp wire on 1/2 of the primary, since
>>> that's what is exposed. The transformers have a thermal time constant of
>>> something like 8.5 minutes and are only baked for 3 minutes (about 10 seconds
>>> at 250C, according to the profile). It's going to be real hard to balance
>>> time and temperature to measure insulation melting with any confidence.
>>
>> Then the only option would be to send them through the reflow oven "dry"
>> several times, together. Ratchet up the temp profile by 10C at a time
>> and see when the old one fails and when (or whether) the new one fails.
>> Of course if you have the SMT assy contracted out that isn't really
>> going to work.
>
> With only a normal 5% failure, I'm not sure that would tell us anything
> useful. We do our own assembly.
>

It would be useful. At some point the old xfmr will fail. Then if you
keep ratcheting up the temp profile until the new one fails you'd know
how much more the new xfmr can take. It would reduce the wait time and risk.


> After taking a few of the failed transformers apart, as was suggested by
> someone here, I found that the failures were all on the bottom of the
> transformer (horizontal core). The ones with a small number of turns shorted
> (the ones with a nominal DCR) appeared to be shorted in the corners where the
> winding changed direction. Perhaps the windings/insulation was damaged (piled
> up when changing direction?) or perhaps that's the hottest area in reflow.
> With the higher temp wire (200C vs. 130C.) it should be better. The first
> production batch should tell us pretty quickly. IIRC, a few hundred are due
> in a couple of weeks and 1500 or so a couple of weeks after that.


Sharp bends often compromise an insulation just by stretching so that
would be the area to go first.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Spehro Pefhany on 28 Mar 2010 01:05
On Sat, 27 Mar 2010 10:50:32 -0700, the renowned Joerg
<invalid(a)invalid.invalid> wrote:

>krw(a)att.bizzzzzzzzzzzz wrote:
>> On Sat, 27 Mar 2010 08:54:26 -0700, Joerg <invalid(a)invalid.invalid> wrote:
>>
>
>[...]
>
>>> Did you guys get the transformer issue licked?
>>
>> Not completely. We received a small number of samples of the new transformer
>> but without manufacturing quantities we don't _know_ that it's solved. Only
>> something like 5% were failing in reflow so it's pretty hard to test three
>> samples for the problem. We have a very good manual test for the failure now
>> and an in-circuit test that tests ten out of fifteen of them. When we get
>> production quantities we can remove the manual test and will then rely on ICT
>> to flag any regression. Completely solved, no. We have a very good handle on
>> it and it's no longer impacting deliveries, so it's 95% of the way there.
>>
>
>If there is time you could cook two transformers. One from the old batch
>and one of the new samples. Ratchet up the oven temp and see which one
>is going at what temp. At least that'll tell you whetehr they really
>used wire with better coating.

The samples are not usually the problem. Quality has a tendency to
grow fainter with time in some cases.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff(a)interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
From: Hammy on 28 Mar 2010 09:50
On Sun, 28 Mar 2010 07:59:27 -0500, John Fields
<jfields(a)austininstruments.com> wrote:

snip

Well John I have no desire to argue anymore. I have better things to
do with my time as I'm sure you do.

I'll leave it at this you do what works for you; I'll do what works
for me.
From: John Fields on 28 Mar 2010 10:07
On Sun, 28 Mar 2010 09:50:43 -0400, Hammy <spam(a)spam.com> wrote:

>On Sun, 28 Mar 2010 07:59:27 -0500, John Fields
><jfields(a)austininstruments.com> wrote:
>
>snip
>
>Well John I have no desire to argue anymore. I have better things to
>do with my time as I'm sure you do.
>
>I'll leave it at this you do what works for you; I'll do what works
>for me.

---
That works for me. :-)

JF
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