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Stencil Design, Not What It Used To Be

Before launching into this tip of the month (TOTM), we want to tell you about our Open House scheduled for April 30. Put it on your calendar and we’ll be letting you know more about it in the coming weeks.

Meanwhile, let’s talk about stencil printing. It’s the process used for printing solder paste onto PCBs. Today, with PCBs heavily loaded with mixed-signal technology and finer pitch devices, stencil design and its application at the assembly level are taking on new dimensions. Look for the April issue of PCD&F/Circuits Assembly Magazine for an in-depth look at stencil design. But for now, here are some tips to follow when your mixed-signal designs are ready to go to your contract manufacturer (CM) or EMS provider.

  • First and foremost, assure that the PCB layout engineer has properly defined the stencil for your application. Check and double-check.
  • Keep in mind the analog portion requires a thicker stencil, like a 6 to 8 mil stencil with a ratio of 1.1:1, meaning 10 percent more paste is deposited compared to pad size.
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  • The digital portion of a PCB uses a 4 to 5 mil thick stencil.
  • The term ‘solder brick’ describes how efficiently paste is dispensed on the SM pad in an analog section of the PCB.
  • Get a brief tutorial on step stencils from your CM. Understand their importance, especially for analog and column grid arrays (CGAs).
  • Thinner 1 to 2 mil stencils used for flip chip devices and 01005, 0201 packages demand extra careful paste dispensing.
  • Last, but not least, check with your CM to make sure they’re not taking the so-called middle ground that leads to major assembly issues and lost time-to-market.

An inexperienced CM may select a ‘middle ground’ 5 or 6-mil thick stencil for a mixed-signal PCB design and thus eliminate the two different and critical cycles of paste deposition. As a result, defects are created at assembly. For example, if you have finer pitch devices such as QFNs or micro BGAs, there is the possibility of shorts between the balls.