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Underfill Punches In High Reliability

With OEMs increasingly moving to smaller, more portable products, PCBs are continuing to shrink and the real estate is begging for components to be increasingly smaller, like the micro CSP packaged device seen in this picture.

Packaging like this must meet such growing requirements as greater chip functionality, reduced size, tough mechanical and high reliability demands, plus greater stress relaxation for thermal stress and extremely short solder joints.

Smaller packaging introduces greater design and assembly challenges on PCB-level reliability due to the decreasing pitch between balls in a fine pitch micro BGA, for instance. Currently, that pitch is at 0.3 millimeters (mm) and rapidly moving to 0.25 mm. With the pitch decreasing, ball size is reduced.

Consequently, the standoff height between the PCB and chip package is decreased. The bottom line is the shorter standoff height reduces PCB-level reliability for a package. Consequently, ultra fine pitch micro CSPs and micro BGAs have difficulty meeting mechanical shock and substrate flexing tests for portable electronics applications.

This is where underfill comes in to satisfactorily deal with those emerging challenges at PCB assembly.

Click here to check out our article in EE Times and Embedded Systems Design Magazine to get more details on underfill and what it’s doing to increase reliability for your products.

In the meantime, here are some quick hints to assure you are taking the right route toward high reliability with underfill:

  • Understand there are different types of underfill and get assurances the right one is being used on your PCB project.
  • Make sure your CM’s design and assembly engineering are working together to avoid problematic areas.
  • The PCB designer must have a solid understanding of underfill specifications, plus a proven record for designing it in.
  • At design , micro BGA and micro CSP pads should be created based on the chip manufacturer’s documentation.
  • Selecting the proper PCB surface face is another crucial design element. Today, 98 percent of the boards using underfill use electro-less nickel immersion gold or ENIG due to its flat surface and excellent solderability without oxidation problems.
  • If proper steps at design and assembly aren’t taken, improper use of underfill can lead to intermittent or latent problems in the field.

The industry is now dealing with ultra-fine pitch ball diameters of these miniscule packages, plus the fabrication land pad diameter is considerably small ñ most often, less than nine mils, but in some cases less than four mils. So underfill is critical to compensate for the differences in thermal expansion rates of two unlike materials. Those are solder joint and FR4 laminate materials. If the PCB designer doesn’t factor in underfill in these particular board designs, the end product’s life expectancy is significantly reduced brought about by catastrophic interconnect cracking in the field.