HOW THE MOLD COMPOUND THERMAL EXPANSION OVERRULES THE SOLDER COMPOSITION CHOICE IN BOARD LEVEL RELIABILITY PERFORMANCE

Transcription

1 HOW THE MOLD COMPOUND THERMAL EXPANSION OVERRULES THE SOLDER COMPOSITION CHOICE IN BOARD LEVEL RELIABILITY PERFORMANCE AUTHORS: B. VANDEVELDE, L. DEGRENDELE, M. CAUWE, B. ALLAERT, R. LAUWAERT, G. WILLEMS SPEAKER: M. TSURIYA (inemi)

2 OUTLINE Introduction Experimental Setup Results of the Thermal Cycling Experiment Cross-sectional Analysis Conclusions 2

3 OUTLINE Introduction Experimental Setup Results of the Thermal Cycling Experiment Cross-sectional Analysis Conclusions 3

4 USE OF LOW-CTE MOLD COMPOUND DRIVING FACTORS AND CONSEQUENCES Driving factors: to reduce the stress on the silicon chip (CPI: chip-package-interaction) to lower the moisture uptake: SiO 2 based filler can reduce the amount of hygroscopic resin to decrease the MSL and it is less expensive than resin filler To use halogen-free green compounds which is achieved by high amount of SiO particles As a consequence the amount of SiO 2 particles in the epoxy matrix increased up to a filler content of 85%. This leaded to a reduction of the CTE of the mold compound below 10 ppm/ C (CTE of SiO 2 is 0.5 ppm/⁰c). 4

5 USE OF LOW-CTE MOLD COMPOUND LOW-CTE MOLD COMPOUNDS For most packages, the mold compound easily takes more than 50% of the total volume of the package So, the mold CTE has a huge impact on the average CTE of the component 5

6 USE OF LOW-CTE GREEN MOLD COMPOUNDS THE CHANGE-OVER TOOK PLACE BETWEEN The introduction of the green mold compounds is an indication for the introduction of the use low-cte versions in that period. In recent years, also green mold compound with higher CTE are used through changing the resin materials. Use of Green Mould Compounds for plastic packages 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Q Q Q Q Q Q Q Q Q Q (data from a leading semiconductor supplier) 6

7 USE OF LOW-CTE MOLD COMPOUND INCREASED CTE MISMATCH WITH THE PCB FEM Simulation, by assuming the same properties for PCB. Test sample: Use PBGA 27x27mm, 1.27mmpitch 6 versions of PBGA with different Mold CTE Test Boards: Use 4 different thickness of PCB board (incl. one flex board) Simulation Result: At 0.8mm PCB board assembly, 6 versions PBGA failure rate are no different. Mold CTE of the PBGA becomes very important when the PCB is less flexible As local bending of the PCB below the component can less compensate for the increased CTE mismatch between PBGA and PCB. MTTF (cycles) FEM Simulation Result Example: PBGA 27x27 area array 1.27mm pitch 0.8 mm 1.6 mm 2.4 mm No PCB flexing Mold CTE (ppm/ C) 7

8 LOW-CTE MOLD COMPOUNDS OTHER NEGATIVE IMPACTS Higher mold stiffness can lead to brittle fracture (in particular in shock loadings) Low-CTE mold compounds also have a higher stiffness due to the higher SiO filler content Each datapoint on this graph represents a commercially available mold compound 8

9 LOW-CTE MOLD COMPOUNDS OTHER NEGATIVE IMPACTS (2) Higher mold stiffness can lead to brittle fracture (in particular in shock loadings) More issues with head-in-pillow due to package warpage The different CTE of the mold changed the package warpage behaviour during solder reflow 9

10 LOW-CTE MOLD COMPOUNDS OTHER NEGATIVE IMPACTS (3) Higher mold stiffness can lead to brittle fracture (in particular in shock loadings) Higher CTE mismatch between copper wire and mold CTE lead to wire fatigue fractures 10

11 OUTLINE Introduction Experimental Setup Results of the Thermal Cycling Experiment Cross-sectional Analysis Conclusions 11

12 EXPERIMENTAL SETUP PACKAGE DESCRIPTION Test component: 9x9 mm QFN with 64 pins The solderable exposed pad is about half of the package size. Die size is roughly 4x4 mm 2 It makes that this package has a rather low die to package ratio. Three versions are made: Mold CTE Lead Tip Plating Wettable Flank QFN #1 7 ppm/ C Cu Exposed Yes QFN #2 7 ppm/ C E less Tin plating Yes QFN #3 12 ppm/ C Cu Exposed No Wettable Flank 12

13 EXPERIMENTAL SETUP PCB AND BOARD ASSEMBLY PCB: 8-layer boards with the 6 inner layers completely filled with copper to maximize the stiffness of the boards To obtain a CTE as close to that of Cu as possible The total thickness is 2.4 mm. Assembly: Two different solder materials: SnPb and Sn3%Ag0.5%Cu. 2.4 mm PCB 13

14 EXPERIMENT MATRIX QFN type QFN1 QFN2 QFN3 Mold CTE Lead Tip Wettable Flank 7ppm/ ⁰C Cu Exposed Wettable Flank 7ppm/ ⁰C Tin plating Wettable Flank 12ppm/ ⁰C Cu Exposed Non - Wettable Solder Paste SnPb SAC305 0 to 100 C TC 0 to 100 C TC 0 to 100 C TC 0 to 100 C TC 0 to 100 C TC 0 to 100 C TC 14

15 EXPERIMENTAL SETUP THERMAL CYCLING TEST CONDITIONS The IPC-9701 TC1 test condition for solder joint evaluation was selected as the most appropriate test. 0 to 100 ⁰C thermal cycling (air-to-air) Total cycling time = 1 hour ramp up time = 10 minutes dwell-time = 20 minutes In-situ measurement for opens Temperature ( C) Time (min.)

16 OUTLINE Introduction Experimental Setup Results of the Thermal Cycling Experiment Cross-sectional Analysis Conclusions 16

17 THERMAL CYCLING TEST RESULTS WEIBULL DISTRIBUTIONS QFN type Solder N63% β QFN #1 SnPb QFN #2 SnPb QFN #3 SnPb QFN #1 SAC QFN #2 SAC QFN #3 SAC The results clearly reveal that the most dominant parameter is the CTE of the mold. For SAC305 assemblies, the difference between 7 ppm and 12 ppm/c QFN s is a factor three. Solder material and flank wettability have much less on the BLR. 17

18 OUTLINE Introduction Experimental Setup Results of the Thermal Cycling Experiment Cross-sectional Analysis Conclusions 18

19 CROSS-SECTIONAL ANALYSIS GENERAL FINDINGS For all analyzed samples, one or even all four corner joints showed fractures leading to the daisy chain resistance increase. This confirms that the fracture is induced by the CTE mismatch between QFN and PCB, which is highest in the 4 corners. Creep strain (-) The fracture was always inside the solder joint, and not at the intermetallic compounds nor the copper pad itself. The solder fractures are induced through lowcycle fatigue during the temperature cycling. 19

20 CROSS-SECTIONAL ANALYSIS QFN #1: Mold CTE: 7 ppm/ C; Cu exposed tip, Wettable flank SnPb solder paste Failing after 971 cycles This assembly showed the earliest failures. It is related to the non-optimal wetting resulting in almost all solder at the side, however, not really functioning as a strong fillet. 20

21 CROSS-SECTIONAL ANALYSIS QFN #2: Mold CTE: 7 ppm/ C; E less Tin plating on tip, Wettable Flank SnPb solder paste Failing after 1720 cycles Thanks to the fillet, this solder joint can resist more cycles Cross-section shows strong damage which is due to the repeated large expansion mismatch between QFN and board in each cycle, after it has already failed much earlier but while the test was still on going. This can only be explained by the low-cte mold compound used in this QFN package. 21

22 CROSS-SECTIONAL ANALYSIS QFN #1: Mold CTE: 7 ppm/ C; Cu exposed tip, Wettable flank SnAgCu solder paste Failing after 1682 cycles While the SnPb soldered QFN showed very low stand-off heights at one side, the SAC305 soldered QFN s has a quite uniform stand-off height resulting in about double life time The fractures are found near the interface with the lead of the component, however, still in the solder joint itself (no IMC failure). Also here, quite some damage were seen which is related to the large expansion mismatch between QFN and PCB. 22

23 CROSS-SECTIONAL ANALYSIS QFN #2: Mold CTE: 7 ppm/ C; E less Tin plating on tip, Wettable Flank SnAgCu Solder paste Failing after 1901 cycles Similar conclusion as with non-wettable flank QFN And as the solder joint shape is also similar to the non-wettable version, we indeed can expect about the same life time. 23

24 CROSS-SECTIONAL ANALYSIS QFN #3: Mold CTE: 12 ppm/ C; Cu exposed on tip, non-wettable flank SnPb Solder paste Failing after 5323 cycles The QFN with the 12 ppm/ C mold material survives about three time more temperature cycles. The fracture is visible, but there is not such a large damage as seen with the previous cases. 24

25 CROSS-SECTIONAL ANALYSIS QFN #3: Mold CTE: 12 ppm/ C; Cu exposed on tip, non-wettable flank SnAgCu Solder Paste Failing after 5604 cycles A tiny crack is seen over the whole length. Both SnPb and SAC305 solder paste can survives longer than 5,000 cycles when 12ppm CTE mold compound is used in encapsulation. 25

26 OUTLINE Introduction Experimental Setup Results of the Thermal Cycling Experiment Cross-sectional Analysis Conclusions 26

27 CONCLUSIONS The conclusion of the elaborated test was that the impact of the solder material and flank wettability (electroless vs Cu exposed) was minimum (less than 20% of difference) while the mold compound had a huge impact. The characteristic life time of the QFN s with 12 ppm/⁰c was 3 times higher than the QFN s with 7 ppm/⁰c. A difference in solder quality was seen with non-uniform stand-off heights for SnPb soldered versus uniform stand-off heights with SAC305 solder. This resulted in a two times higher life time for the SAC305 soldered QFN. This was only experienced with the Cu exposed QFN s. For high reliability applications and electronics operating under severe conditions, this mold compound change creates a major reliability concern and requires thorough evaluation. The impact on electronics reliability is considerably greater than that of a change in solder alloy but as yet did not get a similar level of attention. 27

28 NEXT STEPS INEMI NEW COLLABORATIVE EFFORTS Perform a broader experimental program to demonstrate the impact of the mold CTE, and benchmark its effect to other parameters such as D2P ratio, board stiffness and solder composition Characterisation techniques to measure the mold CTE of commercial components Explore mitigation techniques In order to get a broader and generic view on the impact of the mold CTE on BLR, and to create awareness within the world-wide industry, INEMI initiated a project on this topic with major players in the electronics assembly industry. 28

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