CSP and flip chip assembly using viscous flux (2)
SMT Technical literature
After mounting, the board is soldered and the components are electrically tested. The daisy chain measurement results are shown in Figure 2 by a dashed line. The theoretical tolerance position of the solder mask is also shown in the figure.
The three theoretical areas are:
A consistently good, safe area that corresponds to a copper foil pad or soldered area
A transition zone, which is the tolerance zone for solder masking; however, when placed here, the soldering will depend on the location of the solder mask.
An area that is always in error, it is beyond the tolerance of the solder mask; the connections in this area are not soldered.
Measurements on several plates show that the position (tolerance) of the solder mask is not always consistent and may move in all directions with a deviation between -30 microns and +35 microns. The maximum allowable placement offset (Figure 3) is equal to the size of the copper pad (safe area). Solder masks cannot be on copper pads and cannot be thicker than copper traces. In the latter case, if the solder resist offset is maximized and the solder balls are mounted on the edges of the copper pads, the solder balls will not touch the copper foil (Figure 4).
The use of a transition zone or a region where the solder resist can move can increase the allowable placement offset, but this depends on the location of the solder mask and the smallest solder mask opening (Figure 5). In Figure 6, the accuracy of the placement is given as a function of all tolerances.
Four conclusions were drawn for the CSP and copper foil defined landing points with local reference points. First, when the solder ball touches the copper pad pad area, it is always soldered during reflow. Seen as:
Mounting accuracy < (minimum copper foil pad diameter)/2
It is assumed that the solder mask is not thicker than the copper foil pad. This result is in the ideal curtain fat road (5) 恍枰徊 难 坷 坷 坷 坷 坷 坷 ㄒ桓鍪 ㄒ桓鍪 ㄒ桓鍪 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A mounting offset of +-40% offset pad can be recommended.
Second, in the transition zone, not all solder balls will be soldered during reflow, depending on the location of the solder mask.
Third, when the solder resist is shifted in the positive direction, the allowable offset in the positive direction increases and decreases in the negative direction.
Finally, when the solder resist is offset in the negative direction, the allowable offset in the negative direction increases and decreases in the positive direction.
Flip chip test
For flip chip dip coating, the flux thickness in the flux cell is 70 microns, 50% flip chip solder ball height. The same test as CSP was carried out. The flip chip is mounted in five negative and five forward offsets. For each offset step, 15 components are placed on three different solder mask defined pad layouts:
Solder Mask Opening = Tin Ball Diameter
Solder mask opening = 85% solder ball diameter
Solder Mask Opening = 70% Tin Ball Diameter
After placement, the plates were soldered and the components were measured using a daisy chain measurement method. Several plates were used, and the position of the solder mask on all the plates was different due to the tolerance of the solder resist layer. After the analysis, I came to the conclusion:
The size of the reference point introduces additional deviations. This deviation can increase, compensate or reduce the given offset. Therefore, for a certain deviation in the forward/negative direction, a flip chip with/without soldering was found.
For a combination of solder mask opening = 70% solder ball diameter, some solder balls are placed on the solder mask without contact with the copper foil. These solder balls were not soldered during reflow.
For flip chip placement, a reference point defined by a local solder mask is used. In Figure 7, the theoretical tolerance limits and the results of the test (blue dotted lines) are shown.
The two visible theoretical regions are a solder mask open area and an always faulty area (beyond the solder mask opening). When mounted here, the joints are not welded. Figure 8 shows the position of the solder balls with respect to the landing point and the placement area defined by the solder mask.