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Defect of the Month

Bob Willis

Why “dye and pry” is a fast, workable solution.

This month we show examples of testing BGAs with a “dye and pry,” a simple and cost-effective way of looking at joint failure or their condition after some form of mechanical testing or abnormal assembly practice.

FIGURE 1 shows a sample BGA joint after dye-and-pry testing. Eighty percent of the separated surface is covered by the red dye. This clearly shows separation occurred before the dye was added.

 

 

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Bob Willis

When solder isn’t shaped correctly, the condition is known as head-in-pillow.

This month we show the ball surface on area array packages where no solder joint was formed. The joints were intermittent, but one of the surfaces – either the ball or the surface of the solder on the pad – was deformed. This is better known as head-in-pillow (HiP) or head-on-pillow (HoP), depending on the shape formed on the solder adjacent surface.

FIGURES 1 and 2 show examples of HiP/HoP. In Figure 1, the surface of the ball is shown after mechanically separating the device from the board. The indent of the solder from the pad on the board is visible.

 

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Bob Willis

Are you vacuuming the right way?

This month we see a solder paste print deposit with what appears to be migration of paste particles away from the main pad. If this is just a one-off, a careful wipe with acotton bud would avoid an unnecessary wash-off and reprint. Ensure the PCB surface finish can withstand a wash-off process; some surface finishes don’t like it. Wash-off can affect wetting and final solderability.

A few reasons for this defect, each of which could be the root cause:

  • Double printing or excess squeegee pressure
  • Solder paste contamination on the bottom of the stencil from a previous print
  • Misplaced or missing component
  • Vacuum hold-down of PCBs during printing with exposed vias under BGAs. Read more ...

Bob Willis

Measuring BGA joints can reveal process problems.

This month we show variation in the size of the solder joints on a section of a BGA. Measuring variation on solder ball size after reflow is useful. Even better is when measurements are taken automatically with an x-ray system, as this provides a good comparison tool between NPI and production builds.

Measuring NPI build, and saving the measurement data, provides a good point of reference when problems are seen on a build. It is also useful when moving between contractors or in the event of changes due to other process modifications.

 

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Bob Willis

While coatings are typically used on boards, some choose to coat components as well.

This month we show manual conformal coating on one component. One optical example is shown under normal lighting and then under UV light, to show the tracer added in coatings to allow easy manual or automatic inspection. This is not a defect. I asked if this was intended, however, as it was unusual.

Traditionally, coatings are used to protect circuit boards in humid environments and more so in condensing conditions to prevent corrosion. On some occasions design engineers also use coatings to provide that little stability.

 

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Bob WillisFlux becomes increasingly tenacious the longer it sits on the board.

This month our topic is not so much a defect as something to consider when running environmental tests before any destructive analysis on solder joints. The through-hole joints shown in FIGURES 1 and 2 were soldered with a high-temperature alloy as part of our trials on robotic laser and single point soldering. The amount of flux in high-temperature cored wire tends to be higher, hence more residues after soldering. If sample boards will be exposed to high-temperature storage, in this case 200°C for 1,000 hr., or temperature cycling, clean the residues first. It is much more difficult to clean after this level of aging, and mounting samples in epoxy for microsections is much more difficult.

 

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