Manufacturability and Reliability Impacts of Alternate Pb-Free BGA Ball Alloys. June 2007

Transcription

1 Manufacturability and Reliability Impacts of Alternate Pb-Free BGA Ball Alloys Greg Henshall Michael Roesch Kris Troxel Helen Holder Jian Miremadi HP Global Engineering Services The information contained herein is subject to change without notice.

2 Situation BGA suppliers are changing Pb-free solder ball alloys Many BGA suppliers are in the process of changing Pb-free solder ball alloys. The variety of Pb-free alloys for solder ball terminations is increasing. The change is being driven by drop reliability concerns with conventional Pb-free alloys within the handheld industry (especially cell phone manufacturers). There are data showing that some of the solutions look promising. For some packages, the change involves moving from a silver (Ag) content of 3-4% down to 1-3%. Addition of other alloying elements which affect undercooling, formation of various IMCs, matrix properties & microstructure. Such changes can increase the melting point of the solder ball by as much as 10 C. SAC 305/405 Alternative SAC alloys SAC + dopants A few examples of alternative solders. More choices becoming available. page 2

3 Impact summary PCA manufacturing & reliability can be affected by the change in ball alloy PCA Manufacturing Impact Pb-free BGA ball alloy change may have an impact on printed circuit assembly (PCA) manufacturing due to higher melting point. Some EMS providers have had unexpected yield losses due to low Ag alloys when they were not aware of their presence. The change to low Ag ball alloys may require a change to PCA manufacturing processes. Reliability Impact Improperly assembled low Ag parts are a significant reliability risk, since they may pass electrical test but still have unacceptable solder joints (see photos). The drop reliability of well manufactured low Ag parts seems to be better than current SAC alloys. Thermal fatigue reliability of new alloys is not currently understood. Additional factors (BGA and PCB pad finish) are critical for reliability. Unmelted solder ball Unacceptable solder joints CSP Package PCB CSP Package PCB Incomplete solder joint formation for a 1% Ag ball alloy assembled at the low end of typical Pb-free reflow process window. page 3

4 Outline HP study of alloy composition impact on reflow 240 CSP (SAC 101) BGA (multiple alloys) Literature study of alloy composition impact on reliability Mechanical shock Thermal fatigue Part number change page 4

5 HP has performed limited experiments to gain understanding of BGA solder ball composition impact on reflow process window. Peak temperature (solder joint) HP experiments Typical production profile lower limit 255 C 230 C 225 C Margin Process window 60 s 120 s Duration above 217 C 225 C : Lower limit for near-eutectic SAC under laboratory conditions based on experimental work (HDPUG), but confidence level is not high enough for production. Need to have margin. page 5

6 240 CSP experimental details Solder ball alloy: SAC 101 Doped with 0.02% Ni Also doped with In amount not known, but presumed to be very small. Reflow behavior of other low Ag alloys (SAC 105, SAC 125+Ni, etc.) may be similar. Experiments Round 1 3 reflow profiles: 225 C/60s; 240 C/90s; 255 C/120s SAC 305 solder paste used for assembly Metallography to assess completeness of reflow Round 2 2 reflow profiles: 230 C/60s; 230 C/90s SAC 305 solder paste used for assembly Same metallography procedure as Round CSP packages and test boards gratefully provided by Philips Semiconductor (now NXP). page 6

7 240 CSP test vehicle 18 x 18 array 0.30mm diameter ball 0.50mm pitch 4 sites populated All 15 sites printed with solder paste. Measured peak temperatures (Round 1) Profile 1: Peak Temperature= 226 C, TAL = 58.9 s Profile 2: Peak Temperature= C, TAL = 92.9 s Profile 3: Peak Temperature= C, TAL = s page 7

8 Results of Round 1 experiments suggest impact of low Ag content on lower limit of process window. 226 C 60 sec 240 C 90 sec 256 C 120 sec Joints poorly formed Appearance typical of well formed joint page 8

9 240 CSPs Unmelted solder and poor joints using 225 C/60s profile Package body Unmelted SAC 101 solder balls PCB Partially melted paste and unmelted solder balls. 225 C /60 s above 217 C page 9

10 240 CSPs micrographs showing variability in joint formation for 230 C/60s profile. 2 parts on the same test board show different degrees of joint formation. Fully melted paste and ball. Well formed joints. 230 C /60s. Part Fully melted paste but unmelted solder balls. Poorly formed joints. 230 C /60s. Part Hewlett-Packard Development Company, L.P. page 10

11 240 CSPs micrographs showing good joints for 230 C/90s profile. Fully melted paste and ball. Well formed joints. 230 C/90s. Combined with results from previous slide, 230 C/60 s appears to be right on the cliff of forming sound joints for this combination of ball alloy, ball size, & solder materials. page 11

12 BGA experimental details Solder ball alloys. SANC = Sn 2.3Ag + ~0.2% (Ni+Co) Binary eutectic SAC 350 (Sn 3.5Ag) This alloy theoretically melts at a single temperature of 221 C. SAC 387 (Sn 3.8Ag 0.7Cu) for baseline Assembled to test board using 4 reflow profiles (peak temperature / time above 217 C) 225 C/60s; 230 C/60s; 230 C/90s; 240 C/90s SAC 305 solder paste used for assembly Optical metallography SEM page 12

13 Optical micrographs showing good joints for 225 C/60s profile. SANC (2.3% Ag) SAC 350 Fully melted paste and ball. Well formed joints. 225 C/60s Hewlett-Packard Development Company, L.P. page 13

14 Optical micrographs showing good joints for 230 C/60s profile. SANC (2.3% Ag) SAC 350 Fully melted paste and ball. Well formed joints. 230 C/60s. Similar results for the 230 C/90s and 240 C/90s profiles Hewlett-Packard Development Company, L.P. page 14

15 SEM micrographs showing good joints for SANC with 225 C/60s profile. Package Package PCB PCB Outer ball (#1) Inner ball (#10) Sn 2.3Ag + ~0.2% (Ni+Co) Fully melted paste and ball. Well formed joints. 225 C/60s. page 15

16 SEM micrographs showing good joints for SAC 350 with 225 C/60s profile. Package Package PCB PCB Outer ball (#29) Inner ball (#10) Sn 3.5Ag Fully melted paste and ball. Well formed joints. 225 C/60s. page 16

17 Summary of HP process window study Very low Ag alloy (SAC Ni + In) shows poor solder joint quality for 225 C/60 s profile and marginal solder joint quality for 230 C/60 s profile. Moderate Ag alloy SANC = Sn-2.3Ag + ~0.2% (Ni+Co) shows fully melted solder balls and normal shaped solder joints for 225 C/60 s profile. Binary eutectic SAC 350 (Sn-3.5Ag) shows fully melted balls and normal shaped solder joints for 225 C/60 s profile. This alloy theoretically melts at a single temperature of 221 C, so perhaps this is no surprise. page 17

18 Conclusions based on HP process window study. For alloys with Ag content around 1%: Solder joints must reach a minimum of 235 C with a TAL of at least 60 s. Reflow profiles for some PCAs may need to be modified when BGAs with low Ag alloys are used. At a minimum, reflow profiles may need to be validated, requiring additional engineering effort. Along with careful oven profiling, EMS providers should consider performing metallography (with statistically significant sample sizes) to validate that components with low Ag solder ball alloys are forming proper joints. The number of thermocouples used to validate a profile may need to be significantly increased so that the low-ag component can be shown to meet the conditions given above while all other components meet the existing specifications (no overheating). 1% Ag alloys appear to be incompatible with any current industry Pb-free assembly specifications that require a minimum reflow peak temperature/tal of 230 C/60 s. Such specifications are likely adequate for the SANC (2.3% Ag) and SAC 350 (3.5% Ag) alloys studied. The actual limit in Ag content at which significant change to the lower limit of the reflow profile is needed has not been precisely identified. It appears to be between 1% and 2.3% Ag based on our limited study. Other variables may impact this value, so caution is warranted for any alloy with less than 3% Ag. HP encourages more evaluation to validate these findings. page 18

19 Outline HP study of alloy composition impact on reflow 240 CSP (SAC 101) BGA (multiple alloys) Literature study of alloy composition impact on reliability Mechanical shock Thermal fatigue Part number change page 19

20 Introduction to preliminary literature study. Initial review of the literature was performed to determine the impact of Ag content and micro alloy additions on solder joint reliability. Establish the level of confidence in the assertion that low Ag alloys provide improved mechanical performance under drop/shock & other rapid mechanical loading conditions (e.g. in-circuit test). Gain understanding of the impact of alloy composition, especially Ag content, on the thermal fatigue resistance of BGA/CSP solder joints. Must we give up thermal fatigue performance in order to gain drop/shock resistance with low Ag alloys? This preliminary study does not constitute an exhaustive review. Continued review is ongoing. HP encourages others to assess and report work performed in the industry. page 20

21 Reduced Ag content improves drop reliability. Studies to date indicate superior drop performance for low Ag alloys. Exception: one set of data shows that SAC 105+Ni performed about the same as SAC 305 in drop testing. Both performed much better than SAC 405. See slide #25. SAC 125+Ni has performed very well for one supplier (proprietary data). Other results shown on the next 3 slides. Whether these improvements are due to a reduction in under cooling required to solidify the BGA ball, changes to the intermetallic layer composition and/or structure, or to reductions in the stiffness and yield strength of the bulk solder ball is still open to debate. Regardless of the cause, there is now a fair amount of empirical data indicating that the low Ag alloys perform better than near-eutectic SAC in mechanical shock tests. Drop test results are also affected by the package and PCB surface finishes. page 21

22 SAC 101 performs better in drop testing than SAC 405 Applied Technologies data SAC 405 SAC

23 Drop performance for SAC 125+Ni better than for SAC 305 Data of Tanaka et al. Drops to Failure SAC Ni SAC 305 SAC Ni SAC 305 Data of Tanaka et al., proceedings ECTC, p. 78 (2006). 23

24 Low Ag ball alloy drop performance better than high Ag alloy Data of Lai et al. JEDEC drop test results; maximum 30 drops per board. 15 packages per board; minimum 2 boards per test cell. Test boards have Cu-OSP finish. Large effect for Ni/Au pkg. finish; much smaller for Cu-OSP finish. Ball Alloy Pkg. Finish Pkgs. Failed SAC 405 Ni/Au 33.0% SAC 405 Cu-OSP 3.3% SAC 305 Ni/Au 23.3% SAC 105 Ni/Au 6.6% SAC 105 Cu-OSP 0.0% Data of Y-S Lai et al., Microelectronics Reliability, 46, pp (2006). 24

25 Variations in drop performance for different solder ball alloys Unovis Data Eutectic Sn-Pb performs best SAC 405 worst SAC 105+Ni similar to SAC 305 Lack of systematic behavior as a function of Ag content due to change in failure mode. Data provided courtesy of Unovis Solutions. 25

26 Reduced Ag content may reduce thermal fatigue resistance but more work needed. (1 of 2) HP has performed a preliminary review of the literature on the impact of Pb-free solder ball alloy on thermal fatigue resistance. Only a small number of studies were found on the impact of alloy composition on life during accelerated thermal cycle testing (ATC), and data sometimes are conflicting. One set of proprietary supplier data show ATC data for SAC 105 to be much worse than Sn-3.5Ag-0.75Cu but Weibull curves for SAC Ni are essentially equivalent to those of the near-eutectic alloy. This finding suggests that ATC data may be highly dependent on micro alloying elements. Another set of proprietary data shows ATC reliability of SAC Ni approximately equals that of SAC 305 & SAC 205. Another set of proprietary data shows that binary Sn-3.5Ag (SAC 350) has similar ATC performance compared to SAC 387. This finding suggests that ATC performance may be insensitive to Cu content. This finding may depend on the package and PCB surface finish type (Cu or Ni). Data from the open literature show conflicting impacts of Ag content (see slides 28 & 29). page 26

27 Reduced Ag content may reduce thermal fatigue resistance but more work needed. (2 of 2) The performance of low Ag alloys relative to eutectic Sn-Pb is not clear, especially under field use conditions. The impact of significant alloy changes on the acceleration factor that relates field life to accelerated test life is unknown. Overall, the impact of ball alloy composition on thermal fatigue life in the field is difficult to judge at this time. page 27

28 Accelerated thermal cycle performance not sensitive to Ag content in the range 2.1% to 3.8% - data of Kang et al. Average failure life (N50) estimated from ATC failure data of SAC BGA solder joints. Data of Kang et al., ECTC, p. 661 (2004). Only a17% increase in fatigue life (probably within experimental error) for decrease in Ag content from 3.8% to 2.1%. (0-100 C, 120 min. cycle.) 28

29 Large changes in Ag content can have significant impact on ATC reliability Data of Terashima et al. Terashima et al. found that a decrease of Ag content from 4% to 1% decreases the thermal fatigue life (first failure) by a factor of about /125 C, 10 min dwell. All alloys had 0.5% Cu No micro alloying additions Performance relative to eutectic Sn-Pb not reported. S. Terashima, et al., J. Elec. Mater., Vol. 32, No. 12, p.1527 (2003). page 29

30 Conclusions of preliminary literature review on the reliability of alternate Pb-free alloys. Industry studies consistently indicate better drop performance for low Ag alloys than for near-eutectic SAC. Some suppliers have been motivated or pushed by customers to move to low Ag alloys just for this reason. Results depend on the solder pad finishes in addition to alloy composition. Findings assume that reflow conditions produce well formed joints. The impact of ball alloy on thermal fatigue resistance is unclear at this time. Conflicting data for the trend of life vs. Ag content. No acceleration model. Number of studies is small. Not clear how low Ag alloys perform relative to eutectic Sn-Pb. page 30

31 Outline HP study of alloy composition impact on reflow 240 CSP (SAC 101) BGA (multiple alloys) Literature study of alloy composition impact on reliability Mechanical shock Thermal fatigue Part number change page 31

32 HP endorses a change in manufacturer s part number when a change to a low Ag ball alloy is made. Due to potential impacts on reflow process window and reliability, HP considers changing from a near-eutectic SAC ball alloy to one with less than 3% Ag to be a change in form, fit, or function. A change in part number is appropriate in this case. The EMS Forum has published a request that BGA suppliers change manufacturer s part number when changing to a ball alloy with less than 3% Ag. Link to EMS Forum document: HP supports this document. The inemi consortium has published a press release recommending a change in part number for low Ag alloys. Link to HP-endorsed inemi press release: This position is endorsed by other large OEMs and EMS suppliers. page 32

33 Acknowledgments & contact information. The laboratory efforts of Noel Hancock, Al Saxberg, and Cuc Hong are gratefully acknowledged. For questions or comments about this presentation, please contact: Gregory Henshall, PhD Global Engineering Services Hewlett-Packard Co. page 33

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