Is it possible to eliminate insufficients on leadless packages?
The majority of electronics today utilize, to some extent, surface mount technology components. Without SMT, the reduction of component sizes, and product sizes, would be extremely difficult. However, reducing the component sizes increases the complexity of putting them on printed circuit boards. The smaller the size, the more difficult it can be to print solder paste onto the PCB, precisely place the component, and send it through the reflow oven with no problems.
Myriad SMT defects (bridging, solder balls, voids, etc.) are possible. In some cases there is only one assembly build, and one has to try to get it “right” the first, and only, time. Leadless packages are prone to insufficient solder volume. This SMT defect is costly, but can be repaired at rework. Is this a defect one has to live with, or is it possible to eliminate this altogether?
The short answer is “yes.” The longer version involves a comparison of the leadless package termination size and PCB land. The required ratio of the two is very consistent, and adjustments to the solder paste stencil can prevent insufficients at reflow.
For sufficient solder paste volume, the optimum PCB land length is ~110% of the leadless termination length, based on SMT assembly (Figure 1). However, inspection and rework of leadless components is extremely difficult when almost 100% of the land is underneath the component. For this reason, the majority of leadless land pad designs will lengthen the PCB land.
Compared to a gullwing style component, leadless packages tend to obstruct more of the convection reflow oven air flow and IR. Unless the PCB has extremely heavy copper weights, the leadless termination and PCB land pad temperatures will increase fairly uniformly, and be close together at liquidus. This will produce a uniform wetting of the solder across the surfaces. Increasing the land pad length beyond 110% of the termination length increases the surface area that the solder has to cover. This will limit the formation of acceptable solder fillets, if solder paste volume is not increased at print.
The volume increase is based on the size difference of the leadless termination and PCB land and is applied to the stencil. Referring to Figure 2, the volume increase is calculated as
Volume increase (%) =
Additional solder paste volume should always be printed to the “toe” side for leadless components. (Avoid extending the solder paste brick further underneath the leadless package, due to the bridging potential, and also avoid increasing the stencil aperture width.) Extending paste up to 0.040" beyond the SMT land is not a problem with leaded or lead-free solders. (Both coalesce and pull back onto the SMT land pad extremely well.) However, it is extremely rare that an overprint gets even close to 0.040" with leadless components. (An overprint to this extent is typically reserved for paste-in-hole applications.) The majority of the time the extension is somewhere between 0.005" and 0.010".
In addition to increasing the stencil aperture size, stencil foil thickness is very important. The majority of leadless components require a 0.005" foil thickness. If foil thickness has to be reduced to accommodate other SMT components, the stencil aperture volume for the leadless components should be increased accordingly. Solder volumes are critical, and it does not take much of a reduction before yield problems start.
Robert Dervaes is vice president of technology at FCT Assembly (fctassembly.com); firstname.lastname@example.org.