| | I am currently in the middle of a company wide war and I'm looking for data (AMMO). Here are the problems: | | | | 1) I am looking for anyone who has done or seen any reports on Thermal Shock to smt parts and/or via holes caused by Soldering Irons at rework. I am also very intrested in getting my hands on a complete report on fracturing of inner layer connections within vias due to Thermal Shock at Rework. I have a test department that is touching up testpoints (in our case the Vias are being used as TPs) so they have a nice solder dome at ICT. I need the data to show them the errors of their ways. | | | | 2) I am also looking for a complete report on reliability of smt connectors verses through hole connectors. I need such data as pull tests, shear strength, and harsh conditions. I am also looking for the aprox. lbs/pin on both TH and SMT pins. The VP of Manufacturing is requesting that all connectors go back to through hole. HELP!!!! | | | | | | Best Regards, | | | | | | Andrew William Dalrymple | | PCB Design Specialist | | adalrymple@intelectinc.om | | | Andrew | | Dave's answer is fine. However, I would like to add that in nearly every paper on reliability aspects, it states that a retouched joint is a potential weakness. I don't know whether it is still the case, but for ESA space-borne applications, it stated that solder joints, made under binocular microscopes, had to be right first time. Subsequent retouching was forbidden. For PTHs (i.e. vias), it is easy to see why. The coefficient of linear expansion (CLE) of FR-4, above the Tg, is about 20 times that of copper. As the Tg is usually in the 130�C region, soldering takes an excursion of, say, 100� above it. If the copper in the through hole is a good quality ductile deposit, then it is stressed but not to rupture point. However, if the soldering iron is thermostatted to a typical 350�C, the excursion can rise to 220�C and there is every chance that the copper in the through-hole barrel will crack at about 2/3 of the way up, impeding the rise of solder. On cooling, there may be electrical contact, but this is hairy and decidedly unreliable. IMHO, test points and vias are two different components of the PCB. If you must combine them, use an elongated pad with an offset hole so that the test point contacts the copper part and not the hole itself (filled or not), but it is better to place the test point as a specific land if real estate permits. FYI, I prefer to make the TP land a unique shape e.g. a rhombus (a square at 45�) to make it instantly recognisable for what it is: this helps service engineers in the field who don't have an ATE in their tool kit. Many of the better CAD systems will add your test points automatically, during routing, optimising the real estate loss. | | As for connectors, Dave is 10 000% correct in his assertions. SM connectors are of doubtful reliability, compared to PTH ones. Even if the connector body is bolted down to the PCB, the act of inserting a mating part into it will transmit a strain to the solder joint in both the longitudinal and Z axes, possibly even sideways if the alignment is poor, because the contact is always floating in the body to some extent. I have come across many failures due to this, where the solder joint has sprung away from the PCB. The equivalent in PTH connectors is rare because the surface area of the solder joint is n times greater; it is often better supported mechanically by forming or splaying the solder pins and, in many cases, the strain is relieved by the right angle bend in the contact design. As a speculative guess from experience, I would say that you could expect 100 times as many faults occurring with SM connector solder joints as with PTH ones. FYI, my company had an interconnection board with a row of connectors on it. Initially, this was made as a single-sided board. We started getting failures within a year because the solder joint was just the meniscus between the pin and the pad - not dissimilar to the conditions of an SM connector. We decided to PTH these boards, even though the conductor pattern was still on one side. Since that day, not a single failure was recorded. In this particular case, the strain was essentially longitudinal to the hole. | | Hope this helps you in your pitched battle. | | Brian |
Andrew, These gentlemen's responses are spot on. When a board is heated, the slow to expand materials (i.e. FR4 and ceramic) cause all your headaches. Pads delaminate after only 2-3 attempts with an iron or hot air nozzle. Contemplating the reciprocal, when the job is done right the first time with an iron, we have a board with at least 33% damage to it (recalling brian's earlier response, this is certainly the main reason that ESA space-borne Applications will not allow subsequent touch ups after the first benchtop soldering attempt). A vast number of bench technicians change the industry standard soldering technique (heat the lead, and reflow the solder to the pad) to install ceramic chip components because they don't have a choice. Think of how different the soldering process is comparing production vs. benchtop. Production utilizes solder paste while the benchtop uses wire. In production, PCB's are ramped and carefully profiled while we thermally shock these same PCB's on the benchtop. Reflow temps never exceed 480F in your oven, however, we typically reflow 63/37 solder around 700F on the benchtop. and finally, almost every oven has a post cooling phase following reflow...where is the post cooling on the benchtop? Most rework companies sell pad replacement kits, proof positive that the "science" of their process is damaging. Is there a production company that sells them?
I have included several articles to aid in your battle, some of them can be found in the smtnet library on this website.
"Paradigm shift in SMD Rework" SMT Magazine 10/97
"Temperature Boundary Conditions" SMT Magazine 7/97
"Preheating and Post-Cooling for Rework" SMT Magazine 3/97
"Reflow Soldering and Profiling" SMT Magazine 3/98
"Ending A Double Standard: Uniform Thermal Profiling Between High Volume PCBA Production and Hand Soldering/Rework" SMTNET library
Good Luck Andrew, the truth is on your side!
John
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I am currently in the middle of a company wide war and I'm...
- Oct 01, 1999
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