Printed Circuit Design & Fab Online Magazine - Current Issue

Defect of the Month

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

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.

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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.

Read more: What Solder Ball Size Variation Can Tell Us