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Defect of the Month

Bob Willis

Tricks to eliminate exposed copper.

This month we look at solder pad coverage. Some quality engineers still want to see solder coverage, regardless of the PCB surface finish. If they see the original surface coating, they become concerned, regardless of what is stated in IPC standards.

In the case of NiAu (FIGURE 1), it is most likely the solder paste will reflow and wet the pad toe area right up to the solder mask. In the case of an organic surface protectant (FIGURE 2), the solder paste will reflow successfully, wetting the termination and pad, forming a reliable joint. However, the solder may not reflow and wet any farther than the original print area on the OSP coating on the pad.

 

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Are parts falling incorrectly from the feeder?

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Bob WillisSolder paste escaping gaps in underfill can cause electrical failures.

Welcome to our latest Defect of the Month. This month we illustrate what can happen with area array packages that have been underfilled during rework.

During any rework, it is important to avoid overheating components on the opposite side of the board or adjacent to the parts being reworked. If solder joints surrounded by underfill undergo reflow, or are close to reflow temperatures, the solder will expand/extrude, then flow under pressure through any openings (FIGURE 1). (We have captured solder reflow in underfill during reflow simulation in x-ray investigations for a customer at Dage.) Voids in the underfill or gaps caused by expansion of the component can result in joints becoming intermittent. Such joints can fail at any time during the product’s life.

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Martin WickhamMeasure changes in package dimensions during NPI.

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Martin WickhamImproper reflow profiles make cleaning harder, if not impossible.

FIGURE 1a and 1b show QFN packages that have been reflow-soldered to boards, then mechanically removed in order to examine the flux residues under the body of the component and terminations. Many debates have been held on effective cleaning under this low-standoff package. It is clear in these examples that flux residues remain.

Experience shows that, with the correct paste reflow profile, flux residues can be cleaned. Still, flux residues must also be cleaned with the correct chemistry and cleaning process, else the results shown in Figures 1a and 1b are to be expected. The images show a combination of unsuccessful cleaning, practical solubility and white residues were not soluble in the cleaning process and remain on the parts and the board.

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Martin WickhamAssessments are needed for new parts and alloys to ensure reliability.

Solder joint failure on QFNs may occur for several reasons. These include:

  • Differential thermal expansion of board and component.
  • Flexure or drop testing substrates.
  • Conformal coating/encapsulate expansion.

The rate at which solder joints have been found to fail is due to thermal expansion of the solder alloy, joint height, temperature range, size of package, and size of die in package. These reasons for failure also relate to the product design and substrate thickness. To confirm product reliability for a specific environment, engineers need to undertake reliability assessments on any new component types and alloy combinations. The SEM images (FIGURE 1) were taken after 1000 cycles between -55° and 125°C with no apparent visual damage. Microsections did detect some level of cracking in selected joints. It’s fair to say many of these packages are used today, but when the package size increases, often the basic reliability questions are not being asked.

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