but, but, but,...    We like playing with molten 
metal. Even better if there's open flame 
involved.  Why do you think we all got into this field anyway?

At 07:39 AM 5/5/2010, Paul Reid wrote:
>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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