Dear All,
the german FED/VdL-Projectgroup "Design" did a lot of investigation
concerning the behaviour of laminates in lead-free soldering
processes (SAC & SC solders) within the past 2 1/2 years. The first
goal was to find out how big the risk will be - especially for
delamination - if a material is only specified with FR4 and as
expected, the results showed a very big risc. The samples were made
from 6 european PCB-manufacturers with their standard materials and
were tested 6 times with the 260° qualification reflow profile from J-
STD-020.
During the tests we saw, that drying only had little or no influence
to improve the behaviour concerning delamination. Thus a second step
was started with 4 PCB-Manufacturers, all producing the same Design
(8-Layer board, no planes, halogen-free Material Tg=150°C, Td=325,
STII=215). All boards were produced with the same material, cores and
prepregs from the same production batches.
The results are partly confusing, especially as far as moisture is
concerned. Boards from one PCB-Manufacturer delaminated after the
first 260°C and others were still ok after 10 times 260° and these
boards showed no differences between dried and undried samples.
Samples from other Manufacturers a maximum of 2 more thermal loads
before delamination than the undried samples, but as only 4 samples
per testrun were used, that may be accidentally.
Even boards of the robust ones which were loaded with moisture till
saturation for 10 days in DI-water or in a climate chamber 40°C/90%RH
withstood 8 times 260° reflow without delamination.
There are a lot more details but what i want to express is, that
there are a lot more influencing variables than moisture and you
should not expect to much from drying, if the material has not been
adequately processed by the PCB-Manufacturer. The moisture content of
the board may be a contributing factor for reliability but obviously
there are other variables which influence seems to be much higher and
are not very well known.
If anyone is interested - the is an english presentation about the
first step of the project "Report about a project of the
FED/VdL(ZVEI)-Projectgroup Design concerning the behaviour of
"Standard-Laminates" in the lead-free soldering processes with SAC/SC-
solders" which i can provide.
Best regards
Rainer Taube
Master IPC-Trainer IPC-A-610
TAUBE ELECTRONIC GmbH
Nostitzstraße 30
10965 Berlin
Tel.: 030 6959 2511
Fax: 030 694 2338
e-mail: [log in to unmask]
http://www.taube-electronic.de
For further investigation
Am 14 Jul 2010 um 10:57 hat Paul Reid geschrieben:
> Hi Joe,
>
> You are correct. Time and temperature to get the water out; time,
> temperature and relative humidity to reabsorb water back in. It
> appears water leaves faster than it reabsorbs.
>
> I met a person, (whose name I do not remember) at the fall IPC meeting
> two years ago who made an interesting observation. He connected the
> out flow of the oven to some sort of spectroscopy. He found when
> baking boards (at 125°C I believe), that up to 4 hours water was being
> driven off. After four hours water and CO2 were being emitted. Where
> does the carbon come from? Epoxy! Degrading epoxy should liberate
> water and CO2. At any rate, when baking gets silly, like two weeks at
> 150°C, and the sample is still losing weight, it may not be just water
> being liberated.
>
> I agree with you that most materials can handle a 105°C bake.
> Dielectric materials, however, degrade over time, increased
> temperatures accelerates the degradation. Some low Tg materials may be
> aged by long bakes even at 105°C.
>
> It is reasonable to assume that reliability testing can enunciate the
> degree of degradation, or performance improvement, based on baking
> reflected as changes thermal cycles to failure. If baking is playing a
> significant influence in reliability we would observe the change in
> cycles to failure. In practice baking rarely makes a difference in
> reliability results. I mean by that it does not usually extend nor
> does it reduce the cycles to failure. And by failure I mean failure of
> copper interconnect or material degradation. Baking is generally
> benign in reliability testing. Coupons that delaminate in testing will
> still delaminate with our without the bake.
>
>
> I understand that aged material may reabsorb more water than it
> liberated. It appears as the material ages the strength of cross links
> increase until they break. As the material ages small voids and pores
> expand and there is more intermolecular volume which allows more water
> to be absorbed than before. These internal voids expand until the
> material fails cohesively. That explains why, on TMA, testing T260 or
> T288, we see the material shrink in size before it delaminates
> catastrophically.
>
>
>
> 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 Kane, Joseph E (US
> SSA) Sent: July 14, 2010 9:38 AM To: [log in to unmask] Subject: Re: [TN]
> BAKING OF PWB QUESTION
>
> Two separate questions: (1) how long and what temperature to bake, and
> (2) how long after bake to process the boards? In other words, how
> fast does moisture leave during bake, and how fast does it crawl back
> in under ambient conditions?
>
> Both questions depend on the factors that Bev and Dewey mentioned, and
> you can get answers easily enough if you have an analytical balance
> and can weigh some samples before, during, and after bake. There's a
> new IPC-TM-650 test method coming out very soon, but the details are
> fairly obvious.
>
> If you don't feel like weighing, there may be guidance available from
> your laminate supplier, and there are a few published papers out
> there, including one that I did for IPC Midwest last fall.
>
> Rough rule of thumb, we bake flex after one hour of exposure at
> ambient, but rigid boards can go much longer. A one hour limit after
> bake seems like overkill for most rigid boards; think how those same
> boards are handled at the fabricator, or what you'd do with the same
> boards if you received them dry and opened the dry pack to start
> processing them.
>
> As for bake, we follow Paul's prescription exactly for most boards. I
> don't necessarily agree that longer bakes will be detrimental to the
> dielectric properties, our designs can withstand continuous service
> above 105C, and I suspect so can many laminates. But solderability
> degradation is a very valid concern for some surface finishes like
> immersion silver, OSP over copper, or immersion tin.
>
> -Joe
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Reid
> Sent: Tuesday, July 13, 2010 4:55 PM
> To: [log in to unmask]
> Subject: Re: [TN] BAKING OF PWB QUESTION
>
> My rule of thumb has been 105°C for 4 hours max.
>
> Consider
> - Time and temperature degrades the dielectric.
> - Time and temperature degrades the surface finish.
> - Lead/free applications have a greater need for baking as compared to
> tin/lead applications.
>
>
> 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 Roberts, Jon
> (SA-1) Sent: July 13, 2010 11:35 AM To: [log in to unmask] Subject: [TN]
> BAKING OF PWB QUESTION
>
> Is there a rule of thumb or any best manufacturing practices of how
> long after baking does the PWB have to be processed? Jon
>
>
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