The failure is short at test.the moment we apply 10 v-the capacitance value
reads as normal value dropping to zero instaneously .
since we are using voltmeter for DOE, the leakage current does not read out
( as infinity).
We are not sure -what you mean by wet or dry type? We are testing as dry ?
we did test s on some of them while wet with flux- no significant
difference.Normally, the fabs are cleaned and dried after wave. incubation
time is provided for self-healing to occur.
We observed that one side(end) of cap is having flowlines-roughness while
other end side (cross-section) is smooth. All other surfaces-top, bottom,
left and right are smooth.There are no pinholes.
No surface cracks or distorsions observed on any surface as such.
We have not observed this problem on any other tantulam cap from the same
vendor.
The vendor has indicated to look on thermal and mechanical shock; moisture
at cleaning etc.
Another factor under observation is being thick board we are running wave at
below 2 ftm (1.8 to 2 ftm) and because of two waves -- we are having peak of
220 degc at first wave -cool down to 170 degc and second peak of 250 degc at
Lamda wave for 12 to 13 sec.Due to thich fab, it is difficult to achieve
reduce time on lamda.It is this area which we are not fully sure ?
Gluecure is at 150 degc for 88 sec with peak temp as 176 degc. Though this
process does not affect as indicated in our DOE, we lowered temp further to
125 degc for 120 sec with peak of 150 degc.
----------
From: [log in to unmask]
To: Dhawan, Ashok CA-WINPO2
Subject: RE: Bowing off Tantulam Caps
Date: Monday, June 09, 1997 5:13PM
Can you give some additional information?
Are these wet or dry tantalums?
When you say they are failing, what is failing? Are they shorts? If so,
what sort of currents do you see? Have you let the items sit unbiased for a
time for "healing" to occur? (Tantalums sometimes need time to recover.)
ls it the capacitance value that is failing?
We haven't had the same problem that l am aware of but will check the
database.
----------
From: Dhawan, Ashok CA-WINPO2
To: IPC TechNet
Subject: Bowing off Tantulam Caps
Date: Monday, June 09, 1997 8:48AM
We are facing acute problem on blowing off caps . Specifically, these caps
are tantulam caps with high capacitance range of 220 MICROFARADS , rated at
10 voltage and tolerance in capacitance value is 20%.We have tried more than
one vendor: one vendor was worse than the present one--however failures are
always there.
These caps are processed ( assembled) alongwith other tantulum caps and
usually go through two heat cycles: first one on gluecure and second one
at wave. The standards (temp profiles) followed at reflow and wave are as
per process guidelines .
These caps when tested after wave pass 100% at 5v test. Fails 20 to 25% for
voltages above 7v . Dwell between wave and ATE does not matter much. Tried
with and without wash -- it does not make significant difference.
It is expected that the cap should pass 100% at rated voltage. Clearly there
is deterioration after assembly process. we ran DOE and observed caps
failing after wave. however our wave process does not seem to out of
control.
any one having experience on such a behaviour of Tantulum Caps -- we will
appreciate your technical advise on this problem as we are running out of
ideas and thinking to change process to bypass wave solder operation
altogether.But that is not easy to do in short time.
Please send your advise on this mail or email directly to me.
[log in to unmask]
Tel: 204-257-9199
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