After placement, the board is soldered and electrical tests are performed on the components. The daisy chain measurement results are shown in Figure 2 with a blue dash-dot line. The theoretical tolerance positions of the solder mask are also shown in the figure.
The three theoretical zones are:
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. A zone that is always good, a safe zone, corresponding to the copper pad or soldering area
. A transition zone, which is the tolerance zone of the solder mask; but when placement occurs here, soldering will depend on the position of the solder mask.
. A zone that always fails, which is beyond the tolerance limits of the solder mask; connections in this zone are not soldered.
Measurements on several boards show that the position (tolerance) of the solder mask is not always consistent and may shift in all directions, with deviations varying between -30 micrometers and +35 micrometers. The maximum allowable placement offset (Figure 3) is equal to the size of the copper pad (safe zone). The solder mask must not be on the copper pad and must not be thicker than the copper trace. In the latter case, if the solder mask offset reaches its maximum and the solder ball is placed on the edge of the copper pad, the solder ball will not make contact with the copper foil (Figure 4).
The use of the transition zone, or the area where the solder mask can move, can increase the allowable placement offset, but this depends on the position of the solder mask and the minimum solder mask opening (Figure 5). In Figure 6, the placement accuracy is given as a function of all tolerances.
For CSPs with local fiducials and copper foil-defined landing pads, four conclusions are drawn. First, when the solder ball contacts the copper pad area, it will always solder during reflow. This is considered as:
Placement accuracy < (minimum copper pad diameter)/2
Assuming the solder mask is not thicker than the copper pad. This result occurs under ideal conditions; no further research is needed to determine a practical limit. At this stage, a 10% safety margin can be recommended, which means a placement offset of +-40% off the pad can be recommended.
Second, in the transition zone, not all solder balls will solder during reflow, depending on the position of the solder mask.
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Third, when the solder mask shifts in the positive direction, the allowable offset in the positive direction increases and decreases in the negative direction.
Finally, when the solder mask shifts in the negative direction, the allowable offset in the negative direction increases and decreases in the positive direction.
Flip chip experiments
For flip chip dip coating, the flux thickness in the flux unit is 70 micrometers, which is 50% of the flip chip solder ball height. The same experiments as for the CSP were performed. Flip chips were placed with five negative and five positive offsets. For each offset step, 15 components were placed on three different solder mask-defined pad layouts:
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. Solder mask opening = solder ball diameter
. Solder mask opening = 85% of solder ball diameter
. Solder mask opening = 70% of solder ball diameter
After placement, the boards were soldered and the components were measured using the daisy chain measurement method. Several boards were used, and the position of the solder mask layer varied on all boards due to solder mask tolerances. After analysis, the following conclusions were drawn:
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. The size of the fiducials introduces additional deviation. This deviation can increase, compensate for, or reduce the given offset. Therefore, for certain deviations in the positive/negative direction, both soldered/unsoldered flip chips were found.
. For the combination with a solder mask opening = 70% of the solder ball diameter, some solder balls sat on the solder mask and did not contact the copper foil. During reflow, these solder balls did not solder.
For flip chip placement, local solder mask-defined fiducials were used. In Figure 7, the theoretical tolerance limits and the experimental results (blue dash-dot line) are shown.
The two visible theoretical zones are a solder mask opening zone and a zone that always fails (beyond the solder mask opening). When placement occurs here, the connections are not soldered. Figure 8 shows the solder ball positions relative to the solder mask-defined landing pads and the placement area.
