Estimating the reliability of encapsulated wire bonds


Accelerated thermal cycling


Cyclic Temperature Range, ΔT Mean Cyclic Life, Nf


, cycles (Fig. 3)


Minimum Needed Acceleration Factor, AF(N)


100˚C 480 11.8


Table 1. Accelerated reliability test and product operational cycles.


Accelerated thermal cycling


Cyclic Temperature Range, ΔT Equivalent Strain Range, Δε


100˚C 1.2%


Table 2. Equivalent strain ranges for the thermal cyclic excursions.


and the experimentally determined fatigue lives.


The reliability goal for the product was


3,650 operating cycles with a cyclic thermal excursion of 20 to 60˚C, and thus a ΔT of 40˚C, with a failure probability of no more than 10% at the end of the design life. This means that the mean cyclic life has to be 5,659 cycles assuming a Weibull slope of β=3.0 [see Eq. 6 in Ref. 6]. The results of accelerated temperature cycling from 0˚C⇔+100˚C, 0˚C⇔+85˚C and +25˚C⇔+95˚C with values of ΔT of 100˚C, 85˚C and 70˚C, respectively, are given in a Weibull plot in Figure 3. The test specimens could not be


monitored continuously and in-situ; failure searches every 50 cycles were required, which accounts for the ‘punched together’ data most evident in the ΔT=100˚C data. The data in Figure 3 reflect the fact that the results from the 0˚C⇔+100˚C test, which was run first because the results would be produced fastest, were well-behaved. Thus, the 0˚C⇔+85˚C and +25˚C⇔+95˚C tests could be terminated with only 20% and 10% failures, respectively, without too


much danger of major inaccuracies. From the data in Figure 3 the


acceleration factors relative to the product reliability requirement at ΔT of 40˚C can be determined and are given in Table 1. The results from Figure 3 are plotted in a Manson-Coffin plot in Figure 4. Without the ability to determine


cyclic strains resulting from the thermal expansion mismatch between the bonding wire and the encapsulant, the correlation between the mechanical fatigue behavior and the thermal cycling results can be established by matching up the Manson- Coffin plots in Figures 2 and 4. That is shown in Figure 5. From Figure 5, the equivalent cyclic


strain ranges given in Table 2 result from the temperature cycling of the various severities. The strain ranges resulting from the


thermal cycles with values of ΔT of 100˚C, 85˚C and 70˚C are significantly lower than those produced by the mechanical cycling using fatigue ductility bend mandrels of diameters [see Refs. 3-5] of 40 to 100 mils.


85˚C 70˚C 0.88% .68% Product


Operation 40˚C


0.32%


85˚C 880 6.43


70˚C


1,500 3.77


Product


Operation 40˚C


5,659 1.00


psi) results in a projected mean fatigue life for a thermal cyclic operation with ΔT=40˚C of Nf


material properties of the bonding wire (Df = 12.7%, Su


Using Eq. 4 from Reference 1 and the ≅ 40,000 psi and E ≅ 8.99x106


(50%) ≈ 60,000 cycles. This


shows that cyclic operation with ΔT=40˚C is not a significant threat to the wire bond reliability.


References: 1.


Engelmaier, W., “Fundamentals of Fatigue,” Global SMT & Packaging, Vol. 10, No. 8, August 2010, pp. 36-38.


2. Engelmaier, W., “Finite Element Analysis is a Wonderful Tool for Complex Situations, but…Caveat!,” Global SMT & Packaging, Vol. 7, No. 5, May 2007, pp. 52-53.


3. Engelmaier, W., “A Method for the Determination of Ductility for Thin Metallic Materials,” Formability of Metallic Materials 2000 A.D., ASTM STP 753, J. R. Newby and B. A. Niemeier, eds., ASTM, Philadelphia, PA, 1982,pp. 279-295.


4. “Standard Test Method for Ductility Testing of Metallic Foil,” ASTM E 796, Annual Book of ASTM Standards, ASTM, Philadelphia, PA, 1991.


5.


“Flexural Fatigue and Ductility, Foil,” Test Method 2.4.2.1, Test Methods Manual IPC-TM-650, The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, August 1980.


6. Engelmaier, W., “How to Estimate Solder Joint Reliability, Part 1,” Global SMT & Packaging, Vol. 7, No. 8, August


2007, pp. 60-64. Werner Engelmaier will be giving some of his


reliability workshops as Webinars in November and December 2010 and others in Tel Aviv in April 2011. He is available for in-house workshops and


consulting in both English and German on a world- wide basis; for details of the workshops go to


www.engelmaier.com; for more information about the workshops and consulting contact engelmaier@aol.com.


Figure 4. Weibull plot of thermal cycling fatigue results and Manson-Coffin plot for 99Al1Si wire bonds of configuration in Figure 1.


www.globalsmt.net


Figure 5. Matched-up Weibull plots of thermal cycling and mechanical cycling fatigue results for 99Al1Si wire bonds of configuration in Figure 1.


Global SMT & Packaging – Celebrating 10 Years – December 2010 – 43


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