{I think this is the typical blister pattern we used to see during "IR Reflow". The blister appear spontaneously way from PTHs, ground planes and the like. I imagine that aggressive HASL could produce this also.}
RDS: I agree, but I believe Pradeep when he states that they would have seen that in their inspections during assembly.
{I am not so big on the water uptake theory. Baking does not seem to change the propensity for delamination.}
RDS: That is not at all true. Baking at the right time/temp and assembling/reflowing within a few minutes up to four hours afterwards has been proven to greatly reduce or prevent delamination, measling, z-axis expansion, and CAF. Refer to IPC-1601. It is very difficult to get any of those issues without the presence of internal moisture, even at lead-free reflow temps.
{I attended a lecture where the presenter suggested the aged or highly cured PWBs can physically absorb more moisture than boards that are not aged because the internal voids in the epoxy expand as more and stronger cross linking occurs. This is counter intuitive. Baking makes those voids larger.}
RDS:I have never heard that. I would imagine 99% of the resin crosslinking has completed within a few hours after cure. However, the whole idea of baking is the get the moisture OUT of any voids, abscesses, or pores just prior to processing.
{You can remove moisture in FR4 in hours but it takes days or months to reabsorb the same amount of moisture. Again, counter intuitive, but objectively demonstrable.}
RDS: Absolutely, positively not true. I have undertaken literally hundreds of bake studies using a precision scale to determine the rate of moisture removal and the rate of adsorption by using a condensing humidity chamber to first load the board to its saturated moisture level (the maximum amount of moisture a given PWB can hold), then baked the board and weighed the board every 30 minutes to determine its precise rate of moisture removal and the length of time needed to get to an "acceptable" moisture content (target:20% of saturation) at a given bake temperature (typically 105 deg. C.). Then, we remove the board from the oven, note the humidity level in the room, and weigh the PWB every half hour again to determine the rate of moisture re-adsorption to 80% of that level. The re-adsorption curve up to 80% of saturated weight resembles (remarkably so) the inverse of the evacuation curve, but yes, over a slightly longer time after that. However, I have never seen a standard PWB that did not re-absorb at least 80% of its standard moisture content from being exposed to "regular" room humidity within 8 hours. It depends on a number of factors, including the number of layers, the pre-preg type, the number of holes, cutouts, etc. in the board, and the amount of copper sheets in the layer stack-up, amongst other things. Read IPC-1601.
{Polyimide absorbs moisture at a greater rate than FR4.}
RDS: Yes, it typically does. But there are certain FR-4 PWB design/material combinations that suck up moisture from the air much faster than certain other polyimide rigid-flex or flex circuit design/material combinations. Again, it depends. (Ka-ching).
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Reid
Sent: Tuesday, March 15, 2011 12:22 PM
To: [log in to unmask]
Subject: Re: [TN] Delam after thermal cycling
I think this is the typical blister pattern we used to see during "IR Reflow". The blister appear spontaneously way from PTHs, ground planes and the like. I imagine that aggressive HASL could produce this also.
I am not so big on the water uptake theory. Baking does not seem to change the propensity for delamination.
I attended a lecture where the presenter suggested the aged or highly cured PWBs can physically absorb more moisture than boards that are not aged because the internal voids in the epoxy expand as more and stronger cross linking occurs. This is counter intuitive. Baking makes those voids larger.
You can remove moisture in FR4 in hours but it takes days or months to reabsorb the same amount of moisture. Again, counter intuitive, but objectively demonstrable.
Polyimide absorbs moisture at a greater rate than FR4.
Sincerely,
Paul Reid
Program Coordinator
PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1
613 596 4244 ext. 229
Skype paul_reid_pwb
[log in to unmask] <mailto:[log in to unmask]>
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Honsowetz, Eric
Sent: March 15, 2011 12:54 PM
To: [log in to unmask]
Subject: Re: [TN] Delam after thermal cycling
Good Afternoon...the circular patter suggests local drops of something...have you ever seen delaminating in that pattern before?
Eric
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pradeep M
Sent: Monday, March 14, 2011 9:52 AM
To: [log in to unmask]
Subject: Re: [TN] Delam after thermal cycling
Dear Paul,
Answers to few of your queries.
This board did not have any sequential lamination. Yes this board is
manually assembled.
We also suspect moisture absorption as a possible cause. More because this
board does not have solder mask and the assembly location of the customer is
just off the sea coast and is very humid. They bake the boards for 2 hours
before they take up for assembly. But they take very long time to complete
the assembly as they do it in stages. When they take up for part assembly in
between, the boards are not cured.
Regards,
Pradeep
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Reid
Sent: Monday, March 14, 2011 4:56 PM
To: [log in to unmask]
Subject: Re: [TN] Delam after thermal cycling
Hi Pardeep,
You wrote that the thermal excursion was an isotherm at 60°C then another
five at 13°C. That would be a very benign thermal cycle. Even if the thermal
excursion were 60°C and 130°C that would be a very mild thermal excursion. I
expect the soldering temperature would exceed 230°C in a tin lead
application. The thermal excursion may be more representative if the five
cycles were closer to 230°C with an isotherm just long enough to make sure
the PWB reached 230°C.
Are these plated through hole boards? Was there any sequential lamination?
HASL and tin lead fusing may degrade dielectric materials but the manual
assembly may be the most variable thermal process in this case and the most
damaging.
Delamination is insidious. Using macroscopic examination delamination tends
to be visible on outer layers only. Delamination tends to be more prevalent
in the central zone of the board. Internal delamination tends to go
unnoticed.
The increases in assembly temperatures in response to "RoHS" forced us to
develop better methods to find material damage in our test coupons. Because
we find material damage electronically, then follow up with microscopic
examination to confirm or refute the presents of delamination, we are
becoming experienced with material damage in PWBs. What we observe is the
random nature of delamination makes it hard to find using microsectioning
exclusively, unless the delamination is wide spread across the test samples.
Typically we see delamination in just a few coupons out of a test set which
presents a challenge to the random sample selection, and limited sample size
imposed by microsectioning.
We find the solder float stress test is not as representative as it was when
boards were assembled using wave solder. The solder float test can give
false positive results. It appears the time at temperature is just too low
in the solder float test. The solder float method has difficulty detecting
compromised PWBs. TM 650 2.6.27 Thermal Stress, Convection Reflow Assembly,
may be the better test even though it simulates automated lead free assembly
it is the better, more informative, test. I would have a lab run that test
modifying the thermal profile down to tin/lead temperatures for an objective
evaluation of your boards.
If the boards pass 2.6.27 and solder float then you may have an objective
case that the vagrancies of hand assembly is too variable and aggressive for
that product design, fabricated with that material.
Sincerely,
Paul Reid
Program Coordinator
PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1
613 596 4244 ext. 229
Skype paul_reid_pwb
[log in to unmask] <mailto:[log in to unmask]>
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pradeep M
Sent: March 14, 2011 5:25 AM
To: [log in to unmask]
Subject: [TN] Delam after thermal cycling
Importance: High
Dear technetters,
We are from a PCB mfg organization. One of our customers has made an
observation on one of the boards that we had supplied to them earlier. This
is a 6 layer card which we had manufactured in July 2007. This board does
not have solder mask and has normal HASL as the surface finish. The customer
had taken this up for manual assembly with hand soldering after some time.
Typically with this customer, the whole assembly usually takes nothing less
than 6-8 months for completion. They had completed the assembly and had
subjected the assembled board for thermal cycling ( a procedure which they
follow). Thermal cycling is being done at 60 deg C for 2.5 Hrs then at 13
degC for 2.5 Hrs for five cycles. After this the assembled board has been
coated with conformal coating. Some more functional tests were conducted
and this card is integrated together with other boards. Then a hot and cold
storage test as part of test and evaluation (T&E) of the package was done.
Storage test (passive ) is at the same extremes but active at 40 deg C. The
customer had opened the package for correcting an anomaly observed, when
they found some 'delamination' on the board. The customer has sent a
photograph and wants us to respond with our comments. Customer says the
board was looking fine after assembly as well as after the first thermal
cycling.
We are quite surprised on how a delamination can happen at max of 60 deg C.
We do a microsection analysis of every processed panel and also does a
thermal stress as per IPC TM 650 prior to the dispatch. We have asked for
the actual board to ascertain the delamination, but this might take some
time. The conformal coating material removed by peel off manually by the
customer. We had used Nelco 4000-6 170 Tg FR4 material.
Can anybody provide us some inputs. The photograph may be shared directly.
Rgds
Pradeep
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