Electronic Design

Design Tip: Lead-Free DFM For PC Boards

OEMs should understand that the packaging, thermal characteristics, and footprints of eutectic and lead-free components can be different, and these differences can be the basis for potentially costly manufacturing errors. Lead-free components are packaged to withstand higher reflow temperatures, and they may or may not have different footprints.

Component footprints, package types, and associated component spec sheets should be studied carefully during the pc-board layout process. This design-for-manufacturing (DFM) step can easily be over-looked. As a result, the OEM may have to re-spin a first article pc board at a cost ranging from few hundred to, in some extreme cases, a few thousand dollars.

Eutectic and lead-free components should be properly and clearly specified on the assembly drawing, leaving no doubts, ambiguities, misunderstandings, or room for faulty assumptions. Detailed instructions are especially important for hybrid pc boards.

Instructions should include how engineering change orders are to be addressed. Also, these change orders should be clearly listed in the assembly drawing. A poorly documented assembly drawing can produce melted solder balls in a eutectic ball-grid array (BGA) that's run with a lead-free thermal profile or, in the worst scenario, completely melted ICs.

Technicians and operators need to be trained to inspect and rework the lead-free products. Operators who aren't trained properly may think that the solder paste's appearance, which is dull and less shiny, means that there s insufficient solder.

Component placement also can introduce new and costly mistakes, especially if the pc board is a hybrid board populated with eutectic and lead-free components. Keep in mind that eutectic and lead-free components require different thermal profiles. This becomes even more significant when there s rework.

For example, if a lead-free BGA needs to be depopulated from the pc board, the designer would need a different nozzle with more heat on the periphery of the BGA to pull it. that's because it requires a higher temperature to properly melt the solder, which is necessary for de-soldering.

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