A Model Study of Profiling for Voiding Control at Lead-free Reflow Soldering By Dr. Yan Liu, William Manning, Dr. Benlih Huang, and Dr. Ning-Cheng Lee Indium Corporation of America Clinton, NY, USA Tel: +1 (315) 853-4900, Fax: +1 (315) 853-4320, [email protected]ABSTRACT Voiding is attributed to the flux outgassing within the solder joints when the solder is at molten state. The effect of reflow profile on voiding at microvia for lead-free soldering is strongly dependent on the flux chemistry. In general, wetting is more important than melting outgasing behavior, and can be enhanced by employing a higher melting energy, including both higher peak temperature and longer dwell time. Use of a high soaking energy can help drying out volatiles hence reduce the melting outgasing and result in low voiding, but may also increase oxidation for pastes with poor oxidation resistance and cause a high voiding. Testing oxidation resistance of solder paste beforehand will promise a more accurate selection of soaking energy. Key words: lead-free, solder, SMT, CSP, BGA, soldering, void, voiding, reflow, profile, microvia INTRODUCTION Voiding at reflow soldering has been plaguing industry for decades. The trouble is getting worse with the prevailing of BGA and CSP, particularly in the presence of microvia. Converting into lead-free soldering further aggravates this problem [1]. Occurrence of voids in solder joints typically compromises the reliability [2-5]. For instance, large voids often cause premature failure, or even immediate crack of solder joints upon gentle impact. As a result, curtailing the voiding constantly becomes the major challenge of process engineers. Among all alternatives [6-11], profiling is the most commonly employed approach, mainly due to its flexibility and accessibility. However, up to this point, the profiling process remains primarily as an art. Improvement in voiding control often comes out as results of extensive trial and error. Apparently, a fundamental understanding of the effect of changing profile on voiding becomes critical for a prompt and efficient voiding reduction. In this study, the mechanism behind the relation between profiling and voiding behavior at microvia is investigated, with results discussed below. EXPERIMENTAL In order to increase the sample size for microvia voiding study, a test was designed so that a large number of microvias were arranged as an area array in a test coupon which was subsequently covered with a thick layer of solder paste. Upon reflow, the solder paste melted and formed a thick layer of molten solder simulating the BGA or CSP solder height. This thick molten solder layer would allow the formation of large voids as those observed in BGA or CSP, as reported in a previous study [6-8]. 1. Test Coupon for Voiding (a) (b) Fig. 1 Microvia test boards used for studying effect of solder alloy composition on voiding, (a) overall view of test coupon, (b) closed up look of test coupon. For effect of reflow profile study, a special simulated microvia coupon was used, with a 12 x 12 matrix of tapered wells on copper with 6 mil diameter (mouth), 4 mil diameter (bottom), 3 mil well depth, and 20 mil pitch. This square matrix of microvia or well was confined by tape, as shown in Fig. 1(a). Fig. 1(b) shows a closed up look of the test coupon. Fig. 2 shows a schematic comparison of a typical microvia and the simulated microvia. Fig. 3 shows a schematic view of void formation for each situation. Presence of a thick layer of molten solder on top of simulated microvia allows easy formation of voids due to negligible impact on the curvature of solder dome [8]. This ease of formation of large voids simulates the voiding condition of large solder joints such as that of BGA or CSP.
Transcript
A Model Study of Profiling for Voiding Control at Lead-free Reflow Soldering
By
Dr. Yan Liu, William Manning, Dr. Benlih Huang, and Dr. Ning-Cheng Lee Indium Corporation of America
ABSTRACT Voiding is attributed to the flux outgassing within the solder joints when the solder is at molten state. The effect of reflow profile on voiding at microvia for lead-free soldering is strongly dependent on the flux chemistry. In general, wetting is more important than melting outgasing behavior, and can be enhanced by employing a higher melting energy, including both higher peak temperature and longer dwell time. Use of a high soaking energy can help drying out volatiles hence reduce the melting outgasing and result in low voiding, but may also increase oxidation for pastes with poor oxidation resistance and cause a high voiding. Testing oxidation resistance of solder paste beforehand will promise a more accurate selection of soaking energy. Key words: lead-free, solder, SMT, CSP, BGA, soldering, void, voiding, reflow, profile, microvia
INTRODUCTION Voiding at reflow soldering has been plaguing industry for decades. The trouble is getting worse with the prevailing of BGA and CSP, particularly in the presence of microvia. Converting into lead-free soldering further aggravates this problem [1]. Occurrence of voids in solder joints typically compromises the reliability [2-5]. For instance, large voids often cause premature failure, or even immediate crack of solder joints upon gentle impact. As a result, curtailing the voiding constantly becomes the major challenge of process engineers. Among all alternatives [6-11], profiling is the most commonly employed approach, mainly due to its flexibility and accessibility. However, up to this point, the profiling process remains primarily as an art. Improvement in voiding control often comes out as results of extensive trial and error. Apparently, a fundamental understanding of the effect of changing profile on voiding becomes critical for a prompt and efficient voiding reduction. In this study, the mechanism behind the relation between profiling and voiding behavior at microvia is investigated, with results discussed below. EXPERIMENTAL In order to increase the sample size for microvia voiding study, a test was designed so that a large number of microvias were arranged as an area array in a test coupon which was subsequently covered with a thick layer of solder paste. Upon reflow, the solder paste melted and formed a thick layer of molten solder simulating the BGA or CSP solder height. This thick molten solder layer would allow the formation of large voids as those observed in BGA or CSP, as reported in a previous study [6-8].
1. Test Coupon for Voiding
(a) (b) Fig. 1 Microvia test boards used for studying effect of solder alloy composition on voiding, (a) overall view of test coupon, (b) closed up look of test coupon. For effect of reflow profile study, a special simulated microvia coupon was used, with a 12 x 12 matrix of tapered wells on copper with 6 mil diameter (mouth), 4 mil diameter (bottom), 3 mil well depth, and 20 mil pitch. This square matrix of microvia or well was confined by tape, as shown in Fig. 1(a). Fig. 1(b) shows a closed up look of the test coupon. Fig. 2 shows a schematic comparison of a typical microvia and the simulated microvia. Fig. 3 shows a schematic view of void formation for each situation. Presence of a thick layer of molten solder on top of simulated microvia allows easy formation of voids due to negligible impact on the curvature of solder dome [8]. This ease of formation of large voids simulates the voiding condition of large solder joints such as that of BGA or CSP.
Fig. 2 Schematic of (a) a typical microvia and (b) a simulated microvia.
Fig. 3 Schematic of void formation at (a) a typical microvia, and (b) a simulated microvia.
2. Solder Pastes Three solder pastes were used, as described in Table 1. Table 1 Solder paste samples used in voiding study. Sample Type Alloy Powder Metal
Load A No-
clean 95.5Sn3.8Ag0.7Cu Type 3 88%
B Water wash
95.5Sn3.8Ag0.7Cu Type 3 88%
C No-clean
95.5Sn3.8Ag0.7Cu Type 3 88%
3. Processes PrintingA stencil with a 0.3 inch x 0.3 inch opening and a thickness of 32 mil was used for manually printing paste onto the microvia matrix. Only single print was employed. For each paste, three coupons were prepared.
ReflowThe printed coupons were reflowed under air atmosphere via a BTU convection oven. Three groups of profiles with fixed peak temperature and soaking temperature were used. The soaking temperature was 200°C, while the peak temperature was 230, 235, and 255°C, respectively, as shown in Fig. 4 to Fig. 6. Within each group, the time from ambient to peak temperature was controlled at 2, 4, 6, and 8 minutes, respectively.
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Fig. 4 Reflow profiles with a peak temperature of 230°C and a soak temperature of 200°C. Time from ambient to peak was controlled at 2, 4, 6, and 8 minutes.
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Fig. 5 Reflow profiles with a peak temperature of 235°C and a soak temperature of 200°C. Time from ambient to peak was controlled at 2, 4, 6, and 8 minutes.
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Fig. 6 Reflow profiles with a peak temperature of 255°C and a soak temperature of 200°C. Time from ambient to peak was controlled at 2, 4, 6, and 8 minutes.
4. Outgasing Determination Voiding in solder joints is caused by outgasing from interior of solder joints when the solder is in molten state [10,11]. Accordingly, it appears to be logical to stipulate that voiding extent is proportional to outgasing quantity when the solder is molten. The outgasing quantity Wo, the melting outgasing, is determined by the following equation. Wo (%) = (Ws – Wr)/Wp x 100 Where Wo (melting outgasing) = the weight percentage of solder paste lost as outgasing when the solder is in the molten state. Ws = weight of solder paste after processed with a soaking profile without a spike. Wr = weight of solder paste after reflow process Wp = weight of solder paste before any heating treatment.
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Fig. 7 Soaking profile demonstrated by one designed for 6m reflow profiles.
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Fig. 8 Overlap between 6m soaking profile and 6m reflow profile with 255°C peak temperature.
The soaking temperature for this study was set at 200°C. Fig. 7 exemplifies the soaking profile for 6m reflow profiles, while Fig. 8 illustrates the close simulation of the soaking profile for actual soaking when compared with an actual reflow profile. Fig. 9 shows all four soaking profiles developed for the four sets of reflow profiles, namely 2m, 4m, 6m, and 8m profiles. Each soaking profile is used for reflow profiles with the same heating length but differ in peak temperature.
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Fig. 9 Soaking profiles developed for simulating the soaking of reflow profiles. 5. Void Measurement Void area of solder dome was evaluated using X-ray machine (V.J. Technology) and computer software (PCB Inspector version 4). The average data of three samples for each solder paste was derived. The void content is expressed as area percentage of solder dome. Fig. 10 shows a microvia test coupon with reflowed solder dome. Fig. 11 shows x-ray picture of this test coupon, while Fig. 12 shows a cross-sectional view of voids in a solder dome at simulated microvia.
Fig. 10 Microvia test coupon with reflowed solder dome.
Fig. 11 Example of X-ray pictures of test coupons using simulated microvia and SnAgCu solder pastes.
Fig. 12 Cross-sectional view of voids at simulated microvias in the solder dome formed from SnAgCu solder pastes. RESULTS 1. Effect of Reflow Profiles on Voiding The voiding performance of all three pastes were plotted against reflow profiles utilized, with results shown in Fig. 13.
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Fig. 13 Voiding performance of lead-free solder pastes when reflowed under various reflow profiles. It is interesting to note that the effect of profile on voiding is a strong function of flux chemistry of solder pastes. First of all, paste A exhibits the highest voiding rate. Paste C is considerably lower than paste A, but slightly higher than paste B.
For paste A, 230 and 235°C peak temperature have comparable voiding performance, and both are not sensitive to soaking time. 255°C peak temperature exhibits a lower voiding. This edge over lower peak temperature gradually diminishes with increasing soaking time. Paste B is not sensitive to soaking time. The voiding rate at 230°C is slightly higher than the higher temperature ones. Paste C shows significant sensitivity toward both soaking time and peak temperature. In general, its voiding rate decreases with increase in either peak temperature or soaking time, although at 255°C peak temperature, the voiding seems to increase slightly with increasing soaking time. 2. Outgasing Factor Since voiding is a result of outgasing, the voiding rate for the three pastes is then plotted against melting outgasing, as shown in Fig. 14. Although no clear correlation can be identified, a weak trend can be discerned for all three pastes studied, suggesting a lower voiding with increasing melting outgasing rate. The tentative correlation described above can not be explained by physics law, hence suggesting that there could be some other factors overriding the outgasing factor here. It has been reported that the voiding is greatly affected by solder wetting, with better wetting typically resulting in a lower voiding [6-11], since a system with better wetting will have less chance to have flux entrapped within the solder joint when the solder is in the molten state. If the flux is excluded from interior of solder joint, its outgasing will not contribute to the voiding phenomenon. 3. Heating Energy Factor For a given flux chemistry, the wetting behavior could be affected by the reflow profile in the following manner. First, the fluxing reaction typically increases with increasing temperature and time, or heating energy. On the other hand, the oxidation of metal surface also increases with increasing temperature and time, or heating energy. Furthermore, the oxidation is also in proportion to the surface area of metallization [11]. Consequently, the possible role of wetting factor on voiding in this study may be revealed by examining the heating energy of reflow profiles versus the voiding behavior. The heating energy may be approximated by integrating the area (∆T x ∆t, where T is temperature and t is time) covered under the reflow profile curve. However, since a higher temperature has a greater impact than a lower temperature on flux vaporization or fluxing reaction, the area to be measured had better be focused at or above the temperature where the flux starts to show sign of reaction or vaporization.
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Fig. 14 Relation between melting outgasing rate and voiding for paste A, B, and C. In the case of fluxing reaction, similar to many other fluxes, the fluxes used here become active at around 150°C [10,11]. On the other hand, the thermal gravimetric analysis data for the three fluxes also indicate that the fluxes begin to vaporize significantly at around 150°C, as shown in Fig. 15.
Accordingly, the threshold temperature for determine the heating energy of a reflow profile is set at 150°C. In the case of solder paste reflow process, the melting temperature of solder is another crucial parameter. Below this temperature, the solder paste is not coalesced yet, thus can be oxidized excessively due to the large surface area exhibited by powder. Since solder is still solid therefore can not spread and wet, the oxide removed by flux can be reformed again under hot air. Hence, increase in heating energy below the melting temperature inevitably results in significantly more oxidation than helping wetting. In this study, the area under the soaking curve but above 150°C line is denoted as soaking energy, and is exemplified by the shaded area in Fig. 16. Above the solder melting temperature, the solder paste coalesces and forms a much reduced surface area. Additional heating energy often results in a greater wetting and has less impact on oxidation. In this study, the melting outgasing is caused by the extra heating energy of the reflow profile above that of melting temperature of solder. In this work, the melting energy is approximated by area below the reflow curve but above the soaking profile as well as above the 200°C line, as shown in Fig. 16. This heating energy, as represented by the red area, is denoted as melting energy. The melting energy and soaking energy for the reflow profiles studied in this work are shown in Fig. 17. The soaking energy increases with increasing soaking time, as expected. The melting energy increases not only with increasing peak temperature, but also with increasing soaking time due to extended dwell time.
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Fig. 17 Soaking energy and melting energy for reflow profiles used in this study.
Soaking EnergyFig. 18 shows relation between soaking energy and melting outgasing. Although data scattering is quite significant, the melting outgasing roughly decreases with increasing soaking energy. This is expected, since soaking reduces volatiles before solder melt. The greater the soaking energy, the less amount of volatiles remains in flux prior to solder melting. The effect of soaking energy on voiding is shown in Fig. 19. In more incidences, the voiding decreases initially, then increases with increasing soaking energy. The initial decrease in voiding can be attributed to a decrease in melting outgasing, as reflected in Fig. 18. However, the voiding increases subsequently despite a further reduced melting outgasing, and is attributed to the oxidation caused by extensive soaking.
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Fig. 18 Relation between soaking energy and melting outgasing.
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Fig. 19 Relation between soaking energy and voiding. For paste A, an excessive soaking causes considerably more voiding, reflecting a poor oxidation resistance of this paste. Reflow at 255°C helped reducing voiding initially, presumably due to a better wetting. However, this positive high temperature wetting effect diminishes with further increase in soaking energy. Fig. 20 shows the poor wetting or dewetting of paste A at 6m and 8m 230°C peak temperature systems. Similar trend is also observed at other peak temperature systems. Paste B is fairly insensitive toward soaking energy, indicating a very good oxidation resistance. At peak temperature of 235 and 255°C the voiding is better than 230°C, suggest wetting is enhanced by the higher peak temperature. Fig. 20 shows a very consistent wetting results at various soaking conditions. No dewetting can be discerned under all test conditions, including 235 and 255°C reflow conditions.
Paste C shows voiding decreases rapidly with increase in both soaking energy and peak temperature. The former indicates a good oxidation resistance, while the latter reflects a good compatibility with high temperature reflow process. The high voiding at 230°C and low soaking reflects the adverse impact of melting outgasing, and is a condition should be avoided during profiling for C. It is interesting to note that at 255°C, the voiding of paste C is very low at low soaking energy, as seen in Fig. 19, despite the highest melting outgasing observed in Fig. 18. These data together indicate that a good wetting of paste C at 255°C is the most logical explanation for the low voiding. Fig. 20 shows good wetting of paste C at various soaking conditions for 230°C systems.
Fig. 20 Wetting of pastes A, B, and C on microvia test coupons using 230°C peak temperature systems. Melting EnergyThe effect of melting energy on voiding and melting outgasing is shown in Fig. 21 to 24. In general, voiding decreases with increasing melting energy, despite the simultaneously increasing melting outgasing. This relation is more profound in Fig. 21 and 22 for shorter profiles (2m and 4m), and less obvious in Fig. 23 and 24 for longer profiles (6m and 8m). A higher melting energy promises a better wetting. The stronger influence of this factor relative to that of melting outgasing indicates that wetting is a more dominant factor than melting outgasing. The reduced dominance of melting energy in Fig. 23 and 24 can be attributed to the increasing oxidation caused by a greater soaking energy for 6m and 8m profiles. This is consistent with the observation that wetting is a more dominant factor than melting outgasing.
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Fig. 21 Relation between melting energy, melting outgasing, and voiding for 2m profile systems.
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Fig. 22 Relation between melting energy, melting outgasing, and voiding for 4m profile systems.
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Fig. 23 Relation between melting energy, melting outgasing, and voiding for 6m profile systems.
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Fig. 24 Relation between melting energy, melting outgasing, and voiding for 8m profile systems. DISCUSSION Wetting or Outgasing?Wetting has been reported to be an important factor in affecting the voiding of solder joints for SMT and BGA applications [6-12]. Prevailing of microvia technology has resulted in significantly higher incidence of voiding in solder joints, particularly in the case of lead-free soldering process. It has been speculated that the adoption of microvia may have introduced a new mechanism for voiding due to the presence of "dead corner" in microvia. The concern was: a solder paste with good wetting may not be sufficient to promise a solder joint with low voiding. If any flux gets stuck in the dead corner, merely counting on good wetting may not be enough to exclude it from the interior of solder joints, and reduced outgasing from flux may be more critical in assuring low voiding. Study results here indicate that, similar to SMT experience, although outgasing is directly responsible for voiding, wetting is still the more dominant factor in regulating voids. The simulated microvia used in this study is a relatively shallow well, as can be seen in Fig. 9. Although it did successfully aggravate the voiding problem significantly, as demonstrated by Fig. 8 and 9, it may not precisely duplicate the "dead corner" of some deep pocket-shaped microvia. In other words, the outgasing may still have chance to be a dominant factor in regulating voiding in those situations. Flux Chemistry FactorFig. 19 shows flux chemistry can cause huge difference in voiding behavior. The flux chemistry also dictates the approaches in profiling in order to reduce the voiding. Flux A desires a hot and short profile, flux B is very open to any profile, while Flux C desires a hot and long soaking profile. It appears that, as long as wetting is the dominant factor in voiding, and good wetting is still a challenge for lead-free
soldering, a high reflow temperature will always be desirable. A flux with good oxidation resistance can benefit from a long soaking to further reduce the melting outgasing hence get a further reduced voiding. This oxidation resistance of fluxes possibly can be tested separately and easily, therefore allows the number of profiling trials to be reduced for optimizing voiding performance. Surface Finish FactorThe study here was conducted on copper substrate. This substrate tends to oxidize upon extended soaking, as reflected by the discoloration of test coupons for 8m condition for paste A and B in Fig. 20. If a more wettable and a more oxidation resistant surface finish is employed, the relative dominance of wetting and melting outgasing may alter. How About Tin-Lead?The dominant influence of wetting over outgasing on voiding here could be caused by the intrinsic poor wetting properties of lead-free solders. For SnPb solders, the wetting in general is much better, therefore the voiding versus profile relation may not be the same as lead-free soldering system. It may be similar to the surface finish factor discussed above, and minimizing outgasing may be more important under those situations. CONCLUSION Voiding is attributed to the flux outgassing within the solder joints when the solder is at molten state. The effect of reflow profile on voiding at microvia for lead-free soldering is strongly dependent on the flux chemistry. In general, wetting is more important than melting outgasing behavior, and can be enhanced by employing a higher melting energy, including both higher peak temperature and longer dwell time. Use of a high soaking energy can help drying out volatiles hence reduce the melting outgasing and result in low voiding, but may also increase oxidation for pastes with poor oxidation resistance and cause a high voiding. Testing oxidation resistance of solder paste beforehand will promise a more accurate selection of soaking energy. REFERENCES 1. B. Huang and N.-C. Lee, “Prospect of Lead Free
Alternatives for Reflow Soldering”, in Proceedings of IMAPS, Chicago, IL, Oct. 1999.
2. D. T. Novick, “A Metallurgical Approach to Cracked Joints,” Welding J. Res. Suppl. 52, (4), 154S-158S (1973).
3. A. der Marderosian and V. Gionet, “The Effects of Entrapped Bubbles in Solder for The Attachment of Leadless Ceramic Chip Carriers,” in Proc. 21st IEEE International Reliability Physics Symposium, Phoenix, Arizona, pp. 235-241 (1983).
4. V. Tvergaard, “Material Failure by Void Growth to Coalescence,” in Advances in Applied Mechanics, vol. 27 (1989), Pergamon Press, pp. 83-149.
5. M. Mahalingham, M. Nagarkar, L. Lofgran, J. andrews, D. R. Olsen and H. M. Berg, “Thermal Effects of Die Bond Voids in Metal, Ceramics and Plastic Packages,” in Proc. 34th IEEE Electronic Components Conference, New Orleans, Louisiana, pp. 469-477 (1984).
6. H. Jo, B. Nieman, and N.-C. Lee, “Voiding of Lead-Free Soldering at Microvia”, in Proceedings of IMAPS, Denver, CO, Sept. 2002.
7. A. Dasgupta and N.-C. Lee, "Effect of Lead-Free Alloys on Voiding at Microvia", Apex, Anaheim, CA, Feb. 2004
8. C.S. Chiu, N.-C. Lee, K. Randle, and C. Parrish, “Voiding in BGA at Solder Bumping Stage”, ISHM, 1997.
9. W.B. Hance and N-C. Lee, "Voiding in BGA", in Proc. of 1995 ISHM, Los Angeles, CA, pp 535 (1995)
10. W. Ohara and N.-C. Lee, “Voiding Mechanisms in SMT”, China Lake’s 17th Annual Electronics Manufacturing Seminar, 1993
11. Ning-Cheng Lee, “Reflow soldering processing and troubleshooting SMT, BGA, CSP, and Flip Chip Technologies”, Newnes, pp.288, 2001.
12. J. H. Lau, C.P. Wong, Ning-Cheng Lee, S.W. Ricky Lee, "Electronics Manufacturing with Lead-Free, Halogen-Free, & Conductive-Adhesive Materials", McGraw-Hill, 2003.
