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It’s easy to presume that the most challenging aspect of electronics assembly is the logistics of gathering together all the required components. Then, it’s simply a matter of soldering them in designated positions on the PCB. What could go wrong with that?
It’s actually not as simple as might have been imagined. With today’s profusion of component package configurations, termination styles, design rules—as well as different soldering technologies, solderable finishes, solder alloys, and fluxes—there is plenty of scope for error.
What is a good solder joint? The objective is always to get it right the first time, but how can the integrity of solder joints be demonstrated and qualified? How can they be tested not only for purposes of process characterisation, optimisation, monitoring, and control but also for ensuring their long-term reliability?
A webinar organised by the Europe Chapter of SMTA provided meaningful illustration and abundant practical advice, presented by Bob Willis, an expert in soldering, assembly technologies, and failure analysis.
His series of illustrations of what a good joint should look like began with traditional through-hole solder joints using tin-lead and lead-free solders, produced using wave and selective soldering, where solder had filled the hole and wetted-out the top-side pad. All of the examples met the requirements of IPC Level 3, as did his examples of reflow-soldered joints on small chip capacitors, plus some J-lead and gull-wing leaded parts, and some LGA and QFN leadless packages. He did remark, however, that although all the examples met international reference standards such as IPC-9701A, IPC 600H, and IPC 610H, individual organisations might have company-specific inspection criteria.
Turning to area-array terminations, in the example of a plastic BGA, he showed optical and X-ray images that demonstrated the formation of joints between package terminations and surface-mount pads. Equivalent images of ceramic BGAs with high melting point balls showed interfaces between non-melted balls and fused paste. Likewise, the solder columns on column-grid-array packages did not melt when the paste reflowed. The internal solder joints in package-on-package assemblies were typically formed by fusing ball-on-ball.
How were solder joints tested? Where could the reference information be found? Although Willis did not have time to describe every test in detail, he undertook to point out where to get the information and to illustrate as many tests as possible.
He always preferred to start with practical mechanical testing, especially when advising companies with limited resources, typically smaller design-led companies with manufacturing capability but without comprehensive test facilities. “If you can do simple tests which give you a comparison between A, B, and C, that’s still valuable information.”
He emphasised the fundamental importance of traceability of PCBs, components, materials, and processes, as well as gathering all the information related to that build, particularly if the boards had been made by a contract manufacturer on the other side of the world.
What were the appropriate tests for assessing a product and confirming its integrity? What environment was it expected to work in? How long was it expected to last? If it failed a test, how could the product be improved? Deciding what sort of testing to do might be guided by an IPC standard or guided by the supplier, or by the person who was going to buy the product, “or you may have your own ideas.”
One of the first things to ensure when building a product was to keep reference samples: optically inspected, maybe electrically tested, and X-ray analysed to prove the quality of the build, but otherwise held as reference information with which other assemblies could be compared after testing. And it was critically important to define and record all of the details, requirements, relevant specifications, and results of any testing done either in-house or by subcontract.
Performance test methods and qualification requirements for surface mount solder attachments were detailed in IPC-9701A, and IPC-SM-785 provided guidelines for accelerated reliability testing of surface-mount solder attachments.
Willis discussed thermal cycling in some detail, using IPC-9701 as a reference. Table 3.1 of the specification showed typical environmental conditions for different product categories from consumer, computer, and telecom through industrial and automotive to avionics and military. Thermal cycles were designed to mimic worst-case operating conditions for each category and defined temperature ranges, heating and cooling rates, and hold times.
As an example of a test board for demonstration, he showed a composite coupon he had used in work with the National Physical Laboratory. One test pattern represented a BGA footprint with a chain of interconnections that could be continuously monitored for electrical resistance during cycling to detect solder joint failures. Defective joints typically went open-circuit during the heating phase and closed again as they cooled. It was important that test coupons accurately represented the materials and processes under investigation.
If monitoring facilities were limited, it was possible to subject a product to some thermal cycling, then do comparative testing with X-ray and microsectioning to observe what degradation of the joints may have taken place. Willis also referred to IEC 62137, IEC 60068 and Mil-Std-883 for temperature cycling, and listed numerous other MIL, IEC, JEDEC, ASTM, and JEITA standards related to testing solder joints.
Moving on to discuss microsectioning, he recommended learning the basic manual skills of cutting, mounting, grinding, and polishing before relying on automatic equipment. IPC-9241 provided guidelines for microsection preparation, and there was additional information in IPC-TM-650 part 188.8.131.52. Choosing where to cut the sample for microsectioning was critical to maximise the information obtained. For example, he suggested cutting area-array samples on the diagonal rather than square.
Throughout the webinar, Willis emphasised the importance of meticulous grinding and polishing technique and the use of appropriate etching solutions to reveal fine metallographic details of defects, boundaries, and layer thicknesses. He showed various examples of microsections and referred to his defect guide videos on YouTube for further practical advice on their preparation and interpretation.
Dye penetrant analysis, commonly known as “dye and pry,” was a quick practical way of destructively assessing solder joint integrity and confirming assembly process parameters. Willis explained the procedure. Residual dye left behind at the solder interface indicated failures, such as pad cratering and solder joint fractures from thermal, mechanical, or drop shock.
Pull-testing of through-hole joints could identify failure modes, such as separation of the pin from the solder, separation of the soldered joint from the barrel of the hole, or breakage of the pin. Shear-testing was a quick way of comparing solder materials; Willis showed examples from investigations at the National Physical Laboratory and discussed the metallurgy of solder joints from different alloys on various PCB solderable finishes. Bend-testing was appropriate for assemblies that would experience bending in use, and he showed an example of an automatic bend-test machine. Vibration testing was expensive and normally carried out on the finished product rather than the PCB assembly. Scanning acoustic microscopy was an elegant technique for the assessment of delamination.
In addition to the test methods he had discussed, Willis had been very successful in using video simulation to understand failures and demonstrate how they occurred. He showed several examples from his extensive library.
This webinar provided an excellent introduction to solder joint characterisation and reliability assessment and demonstrated the value of straightforward, practical testing. “You don’t want to know if the product is good or bad; you want to know how good or how bad the product is and how it failed. Just being told it failed the test is of no value. You need to know what’s gone wrong.”
Many thanks to SMTA and Bob Willis for putting together this webinar.