Bridging at Reflow Print E-mail
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Written by Robert Dervaes   
Monday, 30 April 2012 12:20

What causes it, and can it be eliminated?

One area of the surface mount assembly process that frequently causes defects and subsequent rework to fix the problem is bridging on gullwing devices during reflow. The subsequent rework costs money, time and solder joint reliability, all of which have a direct impact on quality, whether it is functional failure of the assembly, cost overruns or missed shipments. Eliminating rework is the goal, but is it really possible? In the case of bridging on gullwing devices at reflow, the answer typically is “yes.”

When moving through the reflow oven, the ideal scenario is to have all of the printed circuit board pads and component leads at the same temperature when entering liquidus. This promotes uniform wetting of the molten solder between all the surfaces. However, component substitutions and incorrect PCB land patterns can disrupt uniform wetting of the solder by creating temperature variations between the PCB pads and component leads. Molten solder seeks the highest temperature, and temperature differences will change how, and where, the molten solder wets.

In the case of fine-pitch gullwing components, a foot (Figure 1) that is substantially (<75%) shorter than the PCB pad creates a sizeable difference in their thermal masses. The shorter foot has a lower thermal mass and typically more surface area exposed to the heat than the PCB pad. As a result, the gullwing lead will heat faster and stay at a higher temperature throughout reflow. Since molten solder moves toward higher temperatures, the bulk of the molten solder will wet to the foot and up to the shoulder of the gullwing lead. With a stencil aperture size based on the PCB pad and a short enough foot (foot <75% of the land pad length), there is too much solder volume at reflow for this component. The gullwing lead has a limit as to how much solder volume will wet to the surface. When looking at fine-pitch components, any excess flows off the PCB land pad and bridges to adjacent pads. The solution is to reduce the volume of solder paste deposited at print.

In extreme cases, the solder paste volume reduction is more than 50%. In the majority of cases, the volume reduction is 25% to 40%. In all cases, the amount of solder paste volume reduction depends on a size comparison of the foot and the PCB land pad, as well as the solder paste stencil thickness.

It is important to review the gullwing component specification to determine where the gullwing foot touches the PCB land pad. Ideally, the foot should be centered on the PCB land pad. However, component substitutions frequently mean the foot is no longer centered. The solder paste brick should be centered about the foot to maintain symmetric wetting of the solder. This is especially important when using Pb-free solder because the wetting is not as strong as that of leaded solder.

The relationship between the foot and land pad sizes is highly repeatable, and solutions can be applied to any leaded or Pb-free assembly containing HASL, ENIG, immersion silver or immersion tin surface finishes. When using Pb-free solder and an organic surface protectant (OSP) finish, caution must be exercised. The reduction in wetting ability of Pb-free solders, coupled with aggressive reductions in solder paste volume, can create scenarios where the solder does not wet the entire PCB land. OSP has a limited shelf life and eventually will break down, permitting oxidation of the underlying copper. It is vital that solder wet the entire surface when using OSP. There are instances where this solution cannot be applied to PCBs with an OSP surface finish.

Bridging at reflow with gullwing components is very common, but can be completely eliminated by comparing the size of the gullwing foot and PCB pad and reducing the printed solder paste volume accordingly.

Ed.: Join our newest columnist Bob Dervaes at PCBChat.com for a moderated chat on surface mount assembly.

Robert Dervaes is vice president of technology at FCT Assembly (fctassembly.com) This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Monday, 30 April 2012 15:27
 

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