The increase in body thickness will also increase the wetted area of the component body. This can result in the tombstoning that you describe. You can try adjusting the volume of solder paste, application of SMT adhesive and verifying the placement accuracy of your placer.
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Ed Zamborsky is a Regional Sales & Technical Support Manager for Thermaltronics, located in New York. His position requires frequent customer visits throughout North America and the Caribbean and his position encompasses not only sales but the role of trainer and master applications engineer for all of Thermaltronics products. His expertise includes such specialties as hand soldering, convection and conduction reflow techniques, array rework, fluid dispensing equipment, and fume extraction. Ed has authored many articles and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead-Free Hand Soldering, High Thermal Demand Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.
In practice, solderability of lead-free surface finishes is generally proving to be good enough in normal processes. Although Pb-free alloys have lower inherent solderability compared to tin-lead solders, wetting performance under normal reflow conditions tends to be fast enough to produce sound joints. In fact, the reduction in solderability actually may have a beneficial effect on instances of soldering defects including tombstoning.
Modern lead-free fluxes are proving highly effective in safeguarding adequate wettability. As customers world-wide are now acquiring appreciable data on their new processes, lead-free pastes such as Multicore LF318 are performing satisfactorily, wetting well within the normal lead-free reflow window.
However, one aspect that has become clear is that lead-free pastes are not forgiving of component misalignments that leave one termination not contacting the printed solder paste. The Pb-free paste tends to reflow where it is printed, and will not wet out to reach a component that has been slightly misplaced. This can lead to an imbalance in the forces exerted at each end of a small passive, resulting in tombstoning. Tombstoning used to be the bane of many a process in the lead-alloy era, and was largely the result of small differences in wetting rates on the terminations at each end of the component. It was successfully solved by anti-tombstoning pastes designed to reduce wetting imbalances by extending the pasty phase of the solder. Because lead-free alloys display significantly higher surface tension than a lead-rich solder, during reflow, the initial energy required to lift one end of the component is higher than for tin-lead alloys. This, combined with the slower wetting speed of lead-free alloys, reduces the instances of tombstoning caused by the solderability imbalance mechanism.
If unacceptably high tombstoning is recorded in a lead-free process, this is far more likely to be related to component placement accuracy. Henkel has examined lead-free anti-tombstoning alloys, and concluded that there is no detectable benefit in such a technology. Processes that have used a lead-rich anti-tombstoning paste, such as Multicore® MP218, may continue to use this formula with lead-free component terminations and surface finish if tombstoning persists – provided, of course, that the use of a lead-rich solder is permitted. This will depend upon the intended market, location of manufacture, and any exemptions pending in the market territories, depending on the product category.
Douglass Dixon is the Chief Marketing Officer for 360 BC Group, a marketing agency with offices throughout the US. 360 BC specializes in consulting and implementing successful marketing programs that utilize the latest in marketing, sales and technology strategies. As an electronics veteran, Dixon has worked in the industry for over 30 years for companies like Henkel, Universal Instruments, Camelot Systems, and Raytheon. Dixon's electronics industry experience includes a broad skill set that includes engineering, field service, applications, product management and marketing communications expertise.