SMT007 Magazine

SMT007-July2019

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46 SMT007 MAGAZINE I JULY 2019 Type 3 and 4 solder powders give a similar drop in transfer efficiency over time for each solder paste. Type 5 and 6 solder powder sizes show a larger drop in transfer efficiency over time for each solder paste. The no-clean solder pastes are less stable with respect to decreasing solder powder size than the water-soluble sol - der pastes. The no-clean solder pastes tend to dry out more while sitting on the stencil, espe- cially with the smaller solder powder sizes. Reflow Performance of the Solder Pastes Wetting or spread percentage of each sol- der paste was measured using the PR test cir- cuit board with ENIG surface finish. Figure 22 shows the results for wetting. The no-clean solder pastes showed fairly sta- ble wetting of about 70–75% regardless of sol- der powder size. The only anomaly in this trend is the wetting percentage for no-clean Type 5 solder paste, which was near 90%. The water- soluble solder paste shows a trend in decreas- ing wetting as the solder powder size decreases. The wetting for the water-soluble Type 3 sol- der paste was 97% and this decreased to under 70% for Type 6 solder paste (Figure 23). Solder balling was measured using the pull- back patterns on the PR test board. The larg- est overprint percentages that gave zero solder balls, less than five solder balls, and less than 10 solder balls were recorded for each solder paste. The maximum overprint percentage in each category is 1,250%. Table 10 shows the results for the solder pastes that were measur- able with this criteria. The solder balling was greater than 10 on all patterns for the solder pastes that are not Figure 21: Print-and-pause results for each solder paste. No-clean (L) and water-soluble (R). Table 9: Transfer efficiency % drop from time = eight hours to 24 hours. Figure 22: Wetting or spread of the solder pastes. Figure 23: Wetting for WS SAC Type 3 and 6 solder pastes.

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