CSP and Flip Chip Assembly with No-Clean Flux (Part 3)

Table 1 contains the calculated safety margins and placement accuracies for the flip chips used in the tests, with three different solder mask thicknesses.

?For flip chips with solder mask-defined fiducials and solder mask-defined landing pads, the conclusions are:
. There is a strong relationship between solder ball diameter, minimum solder mask opening, and solder mask height. All parameters play an important role in maximum placement accuracy.
. From the results of these tests, a permissible deviation of 20% from the solder mask opening is a safe value for solder mask openings greater than or equal to 85% of the solder ball diameter. In these cases, the solder mask height on the copper pad is less than or equal to 10 micrometers.
. If the solder mask-defined fiducial shifts, the solder mask-defined landing pad also shifts by the same distance. The permissible deviation is the same for both shifted and unshifted solder masks.
. For the test combination with a solder mask opening = 70% of the solder ball diameter, some solder balls sat on the solder mask without contacting the copper. During reflow, these solder balls did not solder.

　　This study reveals that placement accuracy depends on component pitch and board layout. For copper-defined landing pads, the solder balls must contact the pad. Therefore, a placement offset of 40% deviation from the pad is permissible. For solder mask-defined landing pads, there is a strong relationship between solder ball diameter, solder mask opening, and solder mask height. The permissible placement offset is 20% deviation from the solder mask opening.

　　The extrapolation of these study results down to 100 micrometer pitch dimensions is shown in Table 2, assuming that for CSPs, the copper pad size should be 70% of the solder ball diameter, and for flip chips, the solder mask opening size should be at least 85% of the solder ball diameter.