10/31/2014 PROBLEM: During an unrelated failure investigation, the ISPLSI5256VE-125LT128I TQFP128 microcircuit has transitioned from a 85% Sn 15% Pb finish to a matte tin finish. Production has already assembled 100 circuit – cards, each with 4 devices, and has delivered 25 of them to the field. There are an additional 500 pieces in stock. The next production run for these component is in 2 weeks. The assembly is conformal coated with a rigid polyurethane conformal coating with an estimated average coverage of 40%.The assembly solder used was 63 wt%Sn-37wt%Pb (eutectic tin-lead solder) that has negligible whisker risk. The program manager is looking for a recommendation. Please use the Whisker_Risk_Model_3_2.xls to determine the disposition of these parts. References: Lattice TQFP 128 mechanical datasheet The Lattice ISPLSI5256VE-125LT128I is a TQFP128 (see Fig. 1) with the minimum part spacing computed as shown in Table 1. Table Calculation of minimum spacing between leads – Feature Dimension (mm) Dimension (inch) Pitch (e) 0.4 0.0157 Lead width, (b maximum) 0.23 0.0091 Minimum gap spacing = e b(max) 0.17 0.0067 – Fig. 128 Pin TQFP Lattice package data sheet information – Whisker short circuit risk calculator TQFP128 tin-lead soldered with 40% conformal coating coverage Applied voltage of five volts Whisker length distribution (1,000 hour 85C/85%RH exposure of SAC305 assemblies with clean parts and boards): lognormal µ = -4.978 ln(mm) and = 0.710 σ Ref: S. Meschter, P. Snugovsky, J. Kennedy, Z. Bagheri, E. Kosiba, and A. Delhaise, SERDP Tin Whisker Testing and Modeling: High Temperature/High Humidity Conditions, International Conference on Solder Reliability (ICSR2013), Toronto, Ontario, Canada. May 13-15, 2014.
10/31/2014 PROBLEM: During an unrelated failure investigation, the ISPLSI5256VE-125LT128I TQFP128 microcircuit has transitioned from a 85% Sn – 15% Pb finish to a matte tin finish. Production has already assembled 100 circuit cards, each with 4 devices, and has delivered 25 of them to the field. There are an additional 500 pieces in stock. The next production run for these component is in 2 weeks. The assembly is conformal coated with a rigid polyurethane conformal coating with an estimated average coverage of 40%.The assembly solder used was 63 wt%Sn-37wt%Pb (eutectic tin-lead solder) that has negligible whisker risk. The program manager is looking for a recommendation. Please use the Whisker_Risk_Model_3_2.xls to determine the disposition of these parts. References: Lattice TQFP 128 mechanical datasheet The Lattice ISPLSI5256VE-125LT128I is a TQFP128 (see Fig. 1) with the minimum part spacing computed as shown in Table 1. Table – Calculation of minimum spacing between leads Feature Dimension (mm) Dimension (inch) Pitch (e) 0.4 0.0157 Lead width, (b maximum) 0.23 0.0091 Minimum gap spacing = e – b(max) 0.17 0.0067 Fig. – 128 Pin TQFP Lattice package data sheet information Whisker short circuit risk calculator TQFP128 tin-lead soldered with 40% conformal coating coverage Applied voltage of five volts Whisker length distribution (1,000 hour 85C/85%RH exposure of SAC305 assemblies with clean parts and boards): lognormal µ = -4.978 ln(mm) and σ = 0.710 Ref: S. Meschter, P. Snugovsky, J. Kennedy, Z. Bagheri, E. Kosiba, and A. Delhaise, SERDP Tin Whisker Testing and Modeling: High Temperature/High Humidity Conditions, International Conference on Solder Reliability (ICSR2013), Toronto, Ontario, Canada. May 13-15, 2014.
TQFP128%20example-whisker%20calculator%202014-11-13.docx