|Ask the Experts|
November 5, 2006
We are encountering massive tombstone defects of a 1608 capacitor after switching to a thicker body component from a new supplier. The previous component is about 0.8mm and the new component is about 1.3 mm thick. Does component body affects tombstoning?
|Expert Panel Responses|
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.
Regional Sales Manager
OK International Inc.
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.
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