Cavity Board SMT Assembly Challenges (Part 2)

Estimated reading time: 12 minutes

Stencil Technology Experiment

In the evaluation of solder printing into the cavity, five different stencils were used from four different suppliers. Three different stencil technologies were used (electroformed, machined, and welded). The stencils are listed in Table 3.

Table 3: Print study.

Each experiment leg consisted of nine cavity boards—three from each PCB supplier. The boards were printed, and the printed solder paste volume deposited was measured with SPI equipment.

The goal of this study was to determine the effect of the stencil on the print volume and quality. The print volume coefficient of variation (CV) was used for comparison and assessment of each leg category. The solder print CV% is defined as the standard deviation of the paste volume divided by the average paste volume. The goal for a good printing process is CV of less than 15%.

Figure 18 presents the results of CV% from each stencil category. It shows both BGA locations—in the cavity U1 and outside the cavity U2. As expected, the BGA outside the cavity showed better CV% than in the cavity, however, four stencils provided adequate print quality for HVM process meeting the 15% target. The welded stencil showed comparable solder volume mean to the electroform stencils but with more paste variation.

Figure 18: Stencil technology print CV study.

However, there were no wet paste defects with the welded stencil, and the paste volume variation was not random. It showed excessive paste at the BGA edges with some tall solder joints. This was probably the result of a geometry mismatch—the stencil cavity depth was on the short side while the boards had a deeper cavity (Table 4). Another cause for the welded stencil excessive solder was the reduced KOZ for the spot-welding and the bumps near the welded spots. Figure 19 provides the paste volume distribution on the BGA in the cavity. It indicated that all the high solder volume areas were at the corners and along the short side of the component.

Figure 19: Paste volume distribution (welded stencil).

The stencil aperture in this study was a single size. In a case of a non-randomize pattern of paste similar to the one seen on the welded stencil, optimization of the aperture size will be beneficial. By reducing the aperture size at the corners, the required volume of paste would be more evenly distributed.

The print CV % data was analyzed in Figure 20 by PCB supplier, and it indicated the impact of the cavity shape and cavity size tolerance on the print. PCB Supplier C had consistently higher CV than others due to smaller XY dimension of the cavity. Printing at the edges was harder for both electroformed stencils (Stencils 1 and 2).

Figure 20: Board supplier impact on print.

The electroformed stencil had tapered walls which required appropriate clearance. The machined stencil had straight outside walls and provided better CV% for Supplier C. Table 4 contains X&Y measurements data from all the stencils outside packet and the board inside packet. The cavity depth for the boards in Table 4 was measured by cross-section near the cavity wall. As mentioned, Stencil 5—the welded stencil—had depth mismatch in the cavity (Table 4). This resulted in high CV value on all board suppliers (Figure 20).

In summary, this experiment showed that there is an impact to the stencil and board tolerance in addition to the shape, which is difficult to quantify due to the shape complexity of the board and the stencil. The mismatch was tolerated by the process, and 4 out of the 5 stencils were meeting the CV% target.

Table 4: Dimensional analysis board/stencil.

Keep-out Zone (KOZ) Study

The test vehicle used in the study had a fixed KOZ from the BGA aperture to the cavity inner wall (Ci). It was 3 mm, and it was designed based on the need to have sufficient space to apply corner glue or underfill (Figure 21).

Figure 21: Cavity KOZ.

To find how close a cavity could be from the printed aperture (in cases were no underfill or corner glue are needed), a black, anodized aluminum fixture was fabricated with 200-μm cavity depth. A 3D stencil was fabricated to apply solder at 0.5-mm pitch. The spacing at the cavity level, Ci from the cavity wall to the first row of pads, started at 0.75 mm. The spacing from the cavity wall on the surface layer Co was evaluated by pads array of 6 x 6 with a distance of 100, 200, 250, and 300 μm from the edge. Figure 21 illustrates the KOZ on the fixture, Co, and Ci.

The KOZ experiment consisted of eight runs of four aluminum fixtures. The fixtures were printed, and paste was applied to the BGA in the cavity and the surrounding 6 x 6 arrays on the surface level with different distances from the cavity walls. The solder volume was inspected by SPI machines, and the print was evaluated for wet bridging.

Figure 22: Print on cavity fixture.

The picture in Figure 22 shows a printed coupon and a close-up view of the cavity wall. The results from this experiment indicated that the cavity level experienced excessive solder as the pads are closer to the cavity walls. Some locations had wet bridging. All wet bridging occurred on the first four rows away from the wall, which translates to ~2 mm (Figure 23a).

Figure 23: Wet bridging.

On the surface level, the print was good on all of the 6 x 6 pads array but started to smear on the first row after multiple print cycles due to improper under cleaning of the stencil near the cavity wall (dead area from the cleaner) as shown in Figure 23b.

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