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Defect of the Month

Bob Willis

Get the right stencil for the job.

This month we illustrate an example of what appears at first glance to be poor lead solderability. When examined, however, it is a combination of component and pad design.

FIGURE 1 shows the lead to be floating in the solder joint, suggesting poor wetting. When we examine the component lead and plastic body, however, the lead is not parallel, so it always sits off the pad surface, even if perfectly soldered. The lead sits in a cavity in the component body to maintain its position. But with the size of the pad used in the design and a full solder paste print, the component body will always lift.

 

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Bob Willis

Is poor stencil design the culprit?

This month we illustrate solder balling and incomplete reflow when reflowing through-hole components with pin-in-paste.

Solder balling pin-in-hole reflow (PIHR) close to the body of the connector suggests poor design of the stencil for this application (FIGURE 1). There is no standoff on the corner of this part, which may have permitted the paste to be displaced when the component was attached. There should always be free space around the paste deposit to permit placement and reflow, without contacting the paste.

 

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Bob Willis

A simple in-process test for determining component wettability.

This month we illustrate solderability testing using simulation. This is basically conducting simple on-the-shop-floor solderability testing to decide whether to use old components. I use paste printed onto glass slides using the paste and stencil from production. I use the same reflow profile for the board design and components in question.

These J-leaded component terminations wet successfully with the solder paste deposits on the glass plate. The paste has reflowed up, wetting the termination on the front face of the lead (FIGURE 1). However, look at the edge and the corners of the leads. If the solder paste has not added to the original tin plating, it calls into question the coating and component age. We have recently used this test to look at the compatibility of low-temperature solders on different terminations as well.

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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 ...

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