I think that the solder float test more accurately replicates a wave solder than oven testing does. The problem is that you have very limited acuity using a solder pot and microsectioning to establish reliability. You don't know which of the samples failed so you have to section all samples; you don't know where they failed so the change of randomly hitting the right PTHs is reduced and you don't know in which cycle the circuit failed so there is no ability to identify early failures. Another point to make is the solder floats to failure can produce artifact failure modes that may not be represented in boards that fail in assembly or the field.
I agree that weak circuits fail early in both methods but the failure modes observed in assembly or the end use environment may not be replicated with high temperature cycles to failure. In high temperature testing, like solder float, surface finish is a significant factor in test results. Solder in the hole fills barrel cracks masking changes in resistance. Nickel can rupture at high temperature reducing cycles to failure. High temperature testing should be done after a reliability expectation is established with testing at 150°C. Once 150°C testing is understood, testing at a high temperature can be advantageous. High temperature testing reduces time to results but reduces the test range also. In high temperature testing the difference between good product and bad product may be one thermal cycle while tested at a lower temperature the difference between good and bad may be 100 cycles, allowing a better ranking of relative robustness. We call high temperature testing "survivability" testing and we usually set acceptance at 10 cycles and the end of test is 50 cycles max.
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 Bev Christian
Sent: May 5, 2010 8:52 AM
To: [log in to unmask]
Subject: Re: [TN] Why not Pb-free?
Paul, Loa,
Wouldn't Loa be less off the mark if the boards were for wave soldering only?
Other than that, Paul has a lot of good stuff to say, Loa.
I have a question for you, Loa. Why would you want to solder float 20x?!
The most REFLOW cycles I can see for a double sided SMT only board is in the following scenario.
Circuit pack made - 2 heat cycles
Testing reveals faulty components/solder joints for two separate components on opposite sides of the board, under RF cans.
Take off can - 3rd heat cycle
Take off component - 4th heat cycle
Dress the pad - 5th heat cycle
Replace the component - 6th heat cycle
Replace the can - 7th heat cycle
Take off can - 8th heat cycle
Take off component - 9th heat cycle
Dress the pad - 10th heat cycle
Replace the component - 11th heat cycle
Replace the can - 12th heat cycle
Bev
RIM
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Reid
Sent: Wednesday, May 05, 2010 8:38 AM
To: [log in to unmask]
Subject: Re: [TN] Why not Pb-free?
Hi Loa,
Solder float is not that effective a reliability method as compared to a thermal cycling oven, HATS or IST testing. Frequently thermal cycle type test results refute solder float type test results. Solder float testing can give false positive results. It appears that (I know I will be brought to task for this statement) the temperature ramp rate is not as significant to degrading a PWB as time at temperature. The solder float is typically just a few seconds to temperature while the other thermal cycling methods take minutes to get to temperature. It is my opinion that, if you take a thermal profile, the area of under the curve above Tg better reflects reliability affect of the stress that is applied to the copper structure and the degradation of the dielectric from thermal excursions associated with assembly and rework. The thermal profile of a solder float is a straight line up and a straight line down; "no time at temp". Reflow ovens, thermal cycling ovens and thermal cycling test methods all have "time at temp".
That being said you can measure the PWB's expected life in the end use environment by testing representative coupons and determining cycles to failure (10% increase in resistance) at three temperatures that are below Tg. We call this method "Accelerated Testing". Armed with a 50% failure rate at each temperature you can extrapolate the PWBs field life at the end use environment. Typically we test at 150°C, 160°C and 170°C if the material's Tg is 170°C or higher. Then we can calculate mean cycles to failure at say 70°C. Let's say your application is a computer that is turned on in the morning and off a night that would be one thermal cycle. If the "acceleration" test demonstrates that at 70°C the board is robust for 1,000 cycles that is equivalent to 1,000 days before failure (2.7 years). By comparing tin/lead preconditioned coupon results to lead-free preconditioned coupon results you can demonstrate that the life of the PWB is greater than your expected field life even with lead-free assembly. With this method you can understand implicitly the affect of lead free.
Mike Freda (Sun, now Oracle) and Jason Furlong (PWB Inc.) wrote a couple of white papers on this accelerated test method. Email me if there is interest in the papers but we better go off line if you want copies so we don't overwhelm TechNet.
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 ALLEN, LOA
Sent: May 4, 2010 7:09 PM
To: [log in to unmask]
Subject: Re: [TN] Why not Pb-free?
Werner,
I am not trying to disprove supplier quality - we have a couple of good fab shops & I don't want to get on their bad side. This exercise is to compare a number of solder floats at 220°C & at 260°C to prove to management the reduction in reliability.
I hope to find a large quantity of previously accepted PWBs with several coupons that we can cut up into 20 floats.
Float 2ea 5x, 7x, 9x, 11x, & 13x at 220°C & 2ea 5x, 7x, 9x, 11x, & 13x at 260°C.
Then x-section & evaluate the barrels.
The sole purpose is to demonstrate to management how the increase in temperature will affect the PWB - it is not meant to be a material/process qual or reflection on supplier quality.
Do you think this is overkill? Use less samples?
Thanks again,
Loa
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Tuesday, May 04, 2010 3:02 PM
To: [log in to unmask]
Subject: Re: [TN] Why not Pb-free?
Hi Loa,
Please do not always blame the PCB FAB house-many of the problem PCBs I see were never properly specified for Pb-free assembly needs.
Werner
-----Original Message-----
From: ALLEN, LOA <[log in to unmask]>
To: [log in to unmask]
Sent: Tue, May 4, 2010 5:38 pm
Subject: Re: [TN] Why not Pb-free?
Paul,
Thank you very much . . . this type of information is what I am looking for. I
remember you & Werner commenting on this earlier & now wish I had paid more
attention.
Since we do test the coupons before the PWB lot is accepted it shouldn't be too
difficult to provide actual samples of failures & number of "preconditioning"
attempts then compare to the 6X for assembly & rework to determine percent of
reduction in reliability .
This forum is awesome . . . gotta go fire up the solderpot.
Thanks again,
Loa
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Reid
Sent: Tuesday, May 04, 2010 11:08 AM
To: [log in to unmask]
Subject: Re: [TN] Why not Pb-free?
Werner Engelmaier has pointed out, on a number of occasions that PWB reliability
directly and profoundly affected by lead-free assembly and rework and we at PWB
Inc. concur.
We have found that in a perfectly fabricated PWB, reliability is reduced by 50%
in a lead-free application. Tin lead processing reduces reliability by 25% or
less. By that statement we mean that if the reliability of a coupon, expressed
and thermal cycles to failure, tested "As Received" is your "entitlement" the
reliability after 6X260°C preconditioning to simulate assembly and rework will
be half of the "entitlement" value. PWB failures can occur in assembly and the
end use environment.
That 50% rule of thumb for PWB reliability in a lead free application has not
changed since the adoption of RoHS. The report I completed today, for example,
suggested the reliability of the product tested after lead-free simulation, was
reduced by 70%. Since a 70% reduction in reliability is significantly higher
than we expect, there is probably a fabrication discrepancy contributing to the
early failure. Lead free assembly is at the limit of most epoxy systems.
It would be prudent to optimize your PWB fabrication requirements, material and
design (if possible) for a lead free application.
The copper failure mode tends to shift from barrel cracks to corner cracks. The
corner cracks tend to be horizontal rather than at a45° angle. Material is
degraded and is expressed has cohesive failure, adhesive delamination and
crazing. Since QC type requirements, like copper thickness, registration do not
anticipate reliability in a lead free environment many companies are using
reliability testing as a condition of lot acceptance. If you do reliability
testing you must test for both copper and material reliability or you could get
a false positive result.
Of course you must predicate all of the above comments with "It depends..."
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 LA
Sent: May 4, 2010 11:56 AM
To: [log in to unmask]
Subject: [TN] Why not Pb-free?
Until now we have not considered going to Pb-free soldering processes for any
of our products. But in trying to keep up with the latest technologies it is
becoming harder & harder to find Sn/Pb solder surface finishes, meaning we
are spending huge amounts of resources reworking components prior to
assembly.
Some management are wanting to convert the bulk of our products to Pb-free
& are tiring of my usual arguments of - Tin whiskers, unknown reliability
models, cost of running two separate processes, PWB development for higher
reflow temps, review of current component database to verify compatibility
with higher reflow temps, teaching old dogs new tricks.
Equipment is high end telecommunications mostly rack-mount office
environment.
Questions - Are my arguments still valid? Are there any recent studies
concluded indicating Sn/Pb is superior?
Thank you in advance . . .
NOTE: This email is not the opinion or desires of the sender; it is solely the
desires of the sender's company.
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