Cavity Board SMT Assembly Challenges (Part 2)

Estimated reading time: 12 minutes

Figure 24: Cavity KOZ.

This formula is based on experiments with 200-μm cavity depth and can be used as a good starting point for cavity design. It may need additional experiments and adjustments for deeper cavities. For the KOZ outside the cavity (Co), as mentioned in the experiment results, the smearing was the only issue. To minimize the smearing, a KOZ of 0.5 mm and an additional 0.5 mm for HVM variation would be recommended. Note that if a welded technology is used for the stencil, a welding KOZ is also necessary, which will add 1.5mm.

Squeegee Experiment

The impact of the squeegee slit length (Figure 15), blade thickness, and the use of a soft polyurethane squeegee were evaluated. Nine boards—three from each PCB supplier—were printed with paste. The paste volume was measured in the SPI machine. Electroformed Stencil 2 was used with this study. Five different squeegee types were evaluated. Table 5 lists the experiment’s legs.

Table 5: Print study.

The chart in Figure 25 provides the experiment results of the different squeegees. Leg 1 with a 10-mm slit and a 0.2-mm blade showed the best print volume CV.

Figure 25: Squeegee-type print CV study.

Using a polyurethane squeegee with no slit (Leg 5) showed high solder print CV for the BGA outside of the cavity as well as the one inside the cavity. The chart in Figure 26 consists of the different squeegee blade legs and solder volume measured at U1 inside the cavity and U2 on the surface outside the cavity. The solder paste volume at Leg 5—the polyurethane squeegee—had some low paste points and had difficulty in printing the two levels at the same time without causing solder scooping and insufficient solder volume.

Figure 26: Squeegee-type print CV study.

Component Assembly in the Cavity

The assembly yield of a BGA SiP into a cavity was compared to a control BGA SiP outside the cavity, which was placed just 6 mm away from each other (Figure 27). BGA U1 was placed in the cavity while BGA U2 was outside of the cavity.

Figure 27: Assembled board.

The data was collected from multiple builds with Stencil 2 using a 0.2-mm slit squeegee blade and a 10-mm slit. After assembly, the boards were examined by X-ray for opens and shorts. Selected units went through failure analysis for cross-sections. To add HVM variability multiple board supplier were used. The results are summarized in Table 6.

Table 6: SMT assembly yield.

Failure Analysis

There were two surprises: the first one was that all defects came from one PCB supplier regardless of build time and shift although the same process was used at SMT to mount all boards. The second surprise was that the defects were open due to head-on-pillow (HoP) with a signature indicating excessive warpage. The SiP BGA that was selected had a stiffener to control its warpage during reflow to a minimum. The initial risk for the defect was presumed to be bridging due to the excessive paste and large paste volume variation at the edges and corner of the cavity lands. Figure 28 shows stretched joints at the package corners with classic HoP defects, which has been shown in many industry papers ^[3] as an indication of high warpage of the package. However, this was not the case in this experiment.

Figure 28: BGA head-on-pillow defect.

This defect, shown in Figure 28, is a result of localized warpage of the board in the cavity area, and not the BGA package. It was known that local warpage is a contributor to open HiP defects in SMT ^{[4 & 5]},but it has not previously been shown as being the only cause for this defect.

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