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Reliability of Pb-Free Solder Assemblies for Non ... · Medtronic. Due to the highly demanding...
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10μm t 0 OSP t 0 ENIG t 0 ENEPIG Reliability of Pb-Free Solder Assemblies for Non-Implantable, Medical Device Applications Liz Andreasen, David Chriss, Kurt Dornauer, Mike Jankowicz Faculty Advisor: Dr. Carol Handwerker Industrial Sponsor: Dr. Peter Tortorici Project Background Medtronic is pursuing the use of Pb-free solders in their non-implantable/disposable medical devices in response to legislative action in the European Union which bans the sale of Pb containing electronics. All medical devices have previously been exempt from the EU RoHS directive, but the exemption for non-implantable devices is being phased out. The high Sn alloy SAC305 (Sn-3wt%Ag-0.5wt%Cu) is the most widely used lead free solder, and is therefore of primary interest to Medtronic. Due to the highly demanding application and need for high reliability in the medical device industry, extensive understanding of the system is needed to show that SAC305 will perform closely to Sn-Pb solders to be considered a viable alternative for use. Medtronic additionally desires to know how the choice of board surface finish affects the performance of SAC305 solder joints in typical Medtronic assemblies. When solder is flowed over a circuit board, intermetallic compounds (IMCs) form from the elements in the system. The senior design team conducted isothermal aging studies at 75°C, 100°C, 125°C, and 150°C at 500hrs, 1000hrs, 1500hrs, and 2000hrs for 3 surface finishes (OSP, ENIG, ENEPIG) to complement Medtronic’s studies that include thermal cycling and mechanical reliability. The IMC growth at each solder interface was characterized for all conditions to detect trends and determine reliability. The team ultimately recommends OSP or ENEPIG over ENIG due to more consistent growth patterns between samples, lower overall IMC growth, and reduced porosity and cracking. Results and Discussion MSE 430-440: Materials Processing and Design This work is sponsored by the Medtronic Tempe Campus - Medtronic is evaluating the reliability of SAC305 solder (Sn-3wt%Ag- 0.5wt%Cu), a common alternative to Pb-Sn solder. The solder joints interact with multiple elements present in the component metallization, solder and surface finish. The microstructure developed as a result of the presence of these elements will determine reliability. - The objective is to simulate the 10+ year service life at room temperature typical of a non-implantable, medical, electronic device. The goal of the study is to characterize the solder interface and determine the surface finish impact on reliability. This study complements Medtronic’s thermal cycling and mechanical reliability studies for the same systems. - E-Case XT058 10μF capacitors were isothermally aged at four temperatures (75°C, 100°C, 125°C, 150°C) and four times (500hrs, 1000hrs, 1500hrs, 2000hrs) to track the IMC growth for the 3 surface finishes. - One of three surface finishes are applied to the circuit board to improve electrical contact and protect from oxidation. These surface finishes are OSP (Organic Solderability Preservative), ENIG (Electroless Ni Immersion Au), and ENEPIG (Electroless Ni Electroless Pd Immersion Au). - During reflow, elements present at the solder joint interact to form IMCs such as Cu 3 Sn, Cu 6 Sn 5 , (Cu,Ni) 3 Sn, (Cu,Ni) 6 Sn 5 , and (Cu,Ni,Au) 6 Sn 5 [1][2] . Their growth is thermally activated. - IMCs are more brittle than the solder and can cause mechanical failure. Failed joints limit device reliability because electrical continuity is lost. [3] Experimental Procedure - Test vehicles were aged at 75°C, 100°C, 125°C, and 150°C to 500hrs, 1000hrs, 1500hrs and 2000hrs then sectioned along the solid red lines as shown at right. For each time, temperature, and surface finish combination a 10μF capacitor was cut from the board. The capacitors were sectioned, mounted in cold set epoxy, and polished. Sectioning plan for aged test vehicles. The circled components are the capacitors of interest to this study. (Above) Schematic of the 3 surface finishes studied. Solder paste is deposited then components are placed on top of the surface finish and reflowed to secure the components to the board. (A) Schematic of a placed and reflowed E- Case XT058 10μF capacitor (B) Optical micrograph of a typical SAC305- capacitor interface with an OSP surface finish (C) SEM micrograph showing intermetallic growth in the open solder (D) SEM micrograph showing intermetallic growth under the capacitor - EDS results indicate Cu 6 Sn 5 is present in OSP, and (Cu,Ni) 6 Sn 5 is present in ENIG and ENEPIG. There is also a small solubility of Au indicated in the initial (Cu,Ni) 6 Sn 5 layer for ENIG and ENEPIG. As expected from review of the literature, ENIG and ENEPIG exhibit a Ni(P) layer below the solder interface due to the electroless deposition process of the Ni layer. - The microstructures for as reflowed samples show great variability in the shape of the IMC interface. Edges vary from needle-like to scalloped to nearly planar. - In addition to irregular morphology, ENIG shows significant porosity in the (Cu,Ni) 6 Sn 5 layer. Porosity in the other surface finishes is less pronounced. Conclusions Cu 6 Sn 5 Cu 3 Sn - Minimal total IMC growth is seen in samples aged at 75°C and 100°C but observation of the interface morphology shows a smoothing of the scalloped IMC edge with increasing time and temperature. What little porosity that is present initially is densified fully over the 2000hrs studied. - Samples aged at 125°C exhibited minimal growth until 1500hrs where growth was initiated suddenly. Total growth by 2000hrs, however, was still minimal. - Samples aged at 150°C show a growth pattern that is roughly parabolic with time. This is one typical growth rate seen in the literature for this system. [1] A counterintuitive dip in overall IMC thickness is seen at 2000hrs. - IMC growth for 75°C and 100°C samples is highly irregular; we would expect uniform, consistent growth over time instead of what was observed. - 125°C samples exhibit almost linear growth. 150°C samples exhibit irregular growth as observed in the 75°C and 100°C samples. - Morphologies vary greatly across the board with jagged peaks at low temps and time (t 500 75°C) transforming to highly planar, smooth surfaces at higher temps and time (t 2000 150°C). Porosity is minimal in all samples. (Cu,Ni,Au) 6 Sn 5 (Cu,Ni) 3 Sn Ni(P) - EDS line scan results at left for the OSP sample aged at 150°C for 1500 hrs illustrate the general process used to determine phase compositions in all samples. Two distinct plateaus in Cu and Sn content are observed in the IMC containing region of the solder joint. - The first plateau corresponds to the stoichiometric ratio Cu 3 Sn and the second exhibits a ratio between Cu 3 Sn and Cu 6 Sn 5 . - In the first plateau, concentration of Cu falls to approx. 80 at% and Sn rises to approx. 30 at% making a Cu:Sn ratio of 2.7:1 which is comparable to the expected 3:1 ratio seen in literature. [1] - In the second step, the concentration of Cu is approx. 55 at% while Sn is approx. 45 at%. This experimental Cu:Sn ratio is 5.5:4.5, corresponding well with the accepted value of 6:5 [1] 10μm - This counterintuitive result may be explained by the growth of competing IMC layers and/or a changing supply of Sn. Initially, Cu is supplied from the pad to the growing Cu 6 Sn 5 layer and Sn supplied from the solder. [2] - As time progresses, a Cu 3 Sn layer grows between the Cu 6 Sn 5 layer and Cu. - If the growth of the Cu 3 Sn layer outpaces the growth of the Cu 6 Sn 5 layer, it may cause the overall dip in IMC thickness. EDS Line scan results for OSP, 150°C at 1500hrs. The plot shows at% composition versus distance along the yellow line drawn on the corresponding micrograph above. Plot of individual IMC layer thicknesses versus time for OSP at 150°C - OSP and ENEPIG IMC growth patterns were far more predictable and correlated better with literature than ENIG. - Initial IMC formation is significant to overall IMC layer thickness for samples that were aged at low temperatures (75°C, 100°C) but not as significant for high temperature aged samples (125°C, 150°C) for all surface finishes. - For the OSP, ENIG and ENEPIG surface finishes, an aging temperature of at least 125°C was necessary to detect significant IMC growth in 2000 hours. - Observations of cracking and voiding proved inconclusive (but more frequent in ENIG) due to the small sample population size of the study. - For all surface finishes, porosity diminishes within the IMC layer and IMC morphology become less irregular with increasing time and temperature. - Selection of OSP or ENEPIG is advised as ENIG shows the greatest variation and maximum in IMC thickness as well as increased porosity and more frequent voiding and cracking relative to the other surface finishes. - Further study to substantiate these results by expanding the sample population size is advised. - Consideration of these results in context of the larger Medtronic study is advised. References [1] Kumar, S., Handwerker, C., and Dayananda, M. Intrinsic and Interdiffusion in Cu-Sn System. Phase Equilibria and Diffusion, 32, 309-319 (2011). [2] Yoon, Jeong-Won et al. Comparative Study of ENIG and ENEPIG as Surface Finishes for a Sn-Ag-Cu Solder Joint. Electronic Materials, 40, 9, 1950-1955 (2011). [3] Zeng, G. et. al. A Review on the Interfacial Intermetallic Compounds between Sn-Ag-Cu Based Solders and Substrates. Material Science: Material Electronics. 21, 421-440. (2010). Cu 6 Sn 5 Porous (Cu, Ni) 6 Sn 5 Ni(P) Dense (Cu, Ni) 6 Sn 5 - Phenom SEM used in backscattered electron mode to obtain images and to confirm multiphase IMC presence and identify phases formed, the team used Energy Dispersive X-ray Spectroscopy (EDS). Charging effects mitigated by painting with graphite suspension, grounding with Cu tape and use of a charge dissipative sample holder specific to the Phenom SEM. - ImageJ software characterized three parameters‒ average, maximum, and minimum IMC thickness. The total area occupied by each IMC was traced and measured. The area was divided by the length of interface observed in order to produce the average thickness for each IMC layer. - The total IMC thickness is the sum of individual average thicknesses for all IMC species present in a given sample. - Maximum and minimum phase thicknesses for each IMC species were measured directly from the scaled image in ImageJ. The range of these readings approximates irregularity of the interface shape. - Each micrograph was investigated for cracks and microvoids. While a crack in a 2-D image could be representative of a 3-D separation, it does not fully capture the interface. The presence of cracks and microvoids indicate further investigation required and a larger sample size to characterize fully. As reflowed samples for all three surface finishes show the initial IMC growth that occurs during reflow. Ni(P) Sn Cu 92at% 46at% 0at% Cu 3 Sn Cu 6 Sn 5 Solder Matrix SEM micrographs of the OSP system for the 150°C series and the 500hr series. (Inset) Plot of average total IMC thickness over time for the four temperatures. 300μm 10μm SEM micrograph of failed SAC305 interface with an ENIG surface finish aged at 150°C for 2000 hours. 10μm Crack 3cm Cu 3 Sn Cu 6 Sn 5 (Cu,Ni,Au) 6 Sn 5 (Cu,Ni) 3 Sn SEM micrographs of the ENEPIG system for the 150°C series and 500hr series. (Inset) Plot of average total IMC thickness vs time for the four temperatures. - Like OSP, ENIG samples aged at 75°C and 100°C do not exhibit much growth and in some cases even show a decrease in IMC thickness compared to the t 0 case. This is caused by the densification of the porosity originally present in the Cu 6 Sn 5 layer over time at these temperatures. - The 125°C trend is unusual compared to the expected parabolic growth rate reported by Yoon et al. [2] The 150°C trend shows a monotonically increasing growth pattern that is roughly linear. - Like OSP, the morphology of the Cu 6 Sn 5 interface becomes more planar with increasing time and temperature but to a lesser degree than OSP. ENIG samples, even after aging, still exhibited large ranges in layer thicknesses. - Incidence of cracking was observed more frequently in ENIG than other surface finishes but few total samples exhibited any cracking at all. SEM micrographs of ENIG system for the 150°C series and 500hr series. (Inset) Plot of total IMC thickness over time for the four temperatures. Elements present: OSP: Cu, Ag, Sn ENIG: Cu, Ni, Au, Ag, Sn ENEPIG: Cu, Ni, Pd, Au, Ag, Sn Capacitor metallization: Ni-Sn B Recommendations The Team would like to thank: Dr. Mysore Dayananda, Dr. Kathleen Kingma, Jameson Root, John Holaday, and Kate Reeve for their generous support
Transcript of Reliability of Pb-Free Solder Assemblies for Non ... · Medtronic. Due to the highly demanding...
10μm
t0 OSP t0 ENIG t0 ENEPIG
Reliability of Pb-Free Solder Assemblies for Non-Implantable, Medical Device ApplicationsLiz Andreasen, David Chriss, Kurt Dornauer, Mike JankowiczFaculty Advisor: Dr. Carol HandwerkerIndustrial Sponsor: Dr. Peter Tortorici
Project Background
Medtronic is pursuing the use of Pb-free solders in their non-implantable/disposable medical devices in response to legislative action in the European Union whichbans the sale of Pb containing electronics. All medical devices have previously been exempt from the EU RoHS directive, but the exemption for non-implantabledevices is being phased out. The high Sn alloy SAC305 (Sn-3wt%Ag-0.5wt%Cu) is the most widely used lead free solder, and is therefore of primary interest toMedtronic. Due to the highly demanding application and need for high reliability in the medical device industry, extensive understanding of the system is needed toshow that SAC305 will perform closely to Sn-Pb solders to be considered a viable alternative for use. Medtronic additionally desires to know how the choice ofboard surface finish affects the performance of SAC305 solder joints in typical Medtronic assemblies. When solder is flowed over a circuit board, intermetalliccompounds (IMCs) form from the elements in the system. The senior design team conducted isothermal aging studies at 75°C, 100°C, 125°C, and 150°C at500hrs, 1000hrs, 1500hrs, and 2000hrs for 3 surface finishes (OSP, ENIG, ENEPIG) to complement Medtronic’s studies that include thermal cycling andmechanical reliability. The IMC growth at each solder interface was characterized for all conditions to detect trends and determine reliability. The team ultimatelyrecommends OSP or ENEPIG over ENIG due to more consistent growth patterns between samples, lower overall IMC growth, and reduced porosity and cracking.
Results and Discussion
MSE 430-440: Materials Processing and Design
This work is sponsored by the Medtronic Tempe Campus
- Medtronic is evaluating the reliability of SAC305 solder (Sn-3wt%Ag-0.5wt%Cu), a common alternative to Pb-Sn solder. The solder jointsinteract with multiple elements present in the component metallization,solder and surface finish. The microstructure developed as a result of thepresence of these elements will determine reliability.
- The objective is to simulate the 10+ year service life at room temperaturetypical of a non-implantable, medical, electronic device. The goal of thestudy is to characterize the solder interface and determine the surfacefinish impact on reliability. This study complements Medtronic’s thermalcycling and mechanical reliability studies for the same systems.
- E-Case XT058 10μF capacitors were isothermally aged at fourtemperatures (75°C, 100°C, 125°C, 150°C) and four times (500hrs,1000hrs, 1500hrs, 2000hrs) to track the IMC growth for the 3 surfacefinishes.
- One of three surface finishes are applied to the circuit board to improveelectrical contact and protect from oxidation. These surface finishes areOSP (Organic Solderability Preservative), ENIG (Electroless NiImmersion Au), and ENEPIG (Electroless Ni Electroless Pd ImmersionAu).
- During reflow, elements present atthe solder joint interact to formIMCs such as Cu3Sn, Cu6Sn5,(Cu,Ni)3Sn, (Cu,Ni)6Sn5, and(Cu,Ni,Au)6Sn5
[1][2]. Their growth isthermally activated.
- IMCs are more brittle than thesolder and can cause mechanicalfailure. Failed joints limit devicereliability because electricalcontinuity is lost.[3]
Experimental Procedure- Test vehicles were aged at 75°C,
100°C, 125°C, and 150°C to500hrs, 1000hrs, 1500hrs and2000hrs then sectioned along thesolid red lines as shown at right.For each time, temperature, andsurface finish combination a 10μFcapacitor was cut from the board.The capacitors were sectioned,mounted in cold set epoxy, andpolished.
Sectioning plan for aged test vehicles. The circled components are the capacitors of interest to this study.
(Above) Schematic of the 3 surface finishes studied. Solder paste is deposited then components are placed on top of the surface finish and reflowed to secure the components to the board.
(A) Schematic of a placed and reflowed E-Case XT058 10μF capacitor(B) Optical micrograph of a typical SAC305-capacitor interface with an OSP surface finish(C) SEM micrograph showing intermetallic growth in the open solder(D) SEM micrograph showing intermetallic growth under the capacitor
- EDS results indicate Cu6Sn5 is present in OSP, and (Cu,Ni)6Sn5 is presentin ENIG and ENEPIG. There is also a small solubility of Au indicated in theinitial (Cu,Ni)6Sn5 layer for ENIG and ENEPIG. As expected from review ofthe literature, ENIG and ENEPIG exhibit a Ni(P) layer below the solderinterface due to the electroless deposition process of the Ni layer.
- The microstructures for as reflowed samples show great variability in theshape of the IMC interface. Edges vary from needle-like to scalloped tonearly planar.
- In addition to irregular morphology, ENIG shows significant porosity in the(Cu,Ni)6Sn5 layer. Porosity in the other surface finishes is less pronounced.
Conclusions
Cu6Sn5
Cu3Sn
- Minimal total IMC growth is seen in samples aged at 75°C and 100°C butobservation of the interface morphology shows a smoothing of the scallopedIMC edge with increasing time and temperature. What little porosity that ispresent initially is densified fully over the 2000hrs studied.
- Samples aged at 125°C exhibited minimal growth until 1500hrs where growthwas initiated suddenly. Total growth by 2000hrs, however, was still minimal.
- Samples aged at 150°C show a growth pattern that is roughly parabolic withtime. This is one typical growth rate seen in the literature for this system.[1] Acounterintuitive dip in overall IMC thickness is seen at 2000hrs.
- IMC growth for 75°C and 100°C samples is highly irregular; we would expectuniform, consistent growth over time instead of what was observed.
- 125°C samples exhibit almost linear growth. 150°C samples exhibit irregulargrowth as observed in the 75°C and 100°C samples.
- Morphologies vary greatly across the board with jagged peaks at low tempsand time (t500 75°C) transforming to highly planar, smooth surfaces at highertemps and time (t2000 150°C). Porosity is minimal in all samples.
(Cu,Ni,Au)6Sn5
(Cu,Ni)3Sn
Ni(P)
- EDS line scan results at left for theOSP sample aged at 150°C for 1500hrs illustrate the general process usedto determine phase compositions inall samples. Two distinct plateaus inCu and Sn content are observed inthe IMC containing region of thesolder joint.
- The first plateau corresponds to thestoichiometric ratio Cu3Sn and thesecond exhibits a ratio betweenCu3Sn and Cu6Sn5.
- In the first plateau, concentration ofCu falls to approx. 80 at% and Snrises to approx. 30 at% making aCu:Sn ratio of 2.7:1 which iscomparable to the expected 3:1 ratioseen in literature.[1]
- In the second step, the concentrationof Cu is approx. 55 at% while Sn isapprox. 45 at%. This experimentalCu:Sn ratio is 5.5:4.5, correspondingwell with the accepted value of 6:5[1]
10μm
- This counterintuitive result may beexplained by the growth of competingIMC layers and/or a changing supplyof Sn. Initially, Cu is supplied from thepad to the growing Cu6Sn5 layer andSn supplied from the solder.[2]
- As time progresses, a Cu3Sn layergrows between the Cu6Sn5 layer andCu.
- If the growth of the Cu3Sn layeroutpaces the growth of the Cu6Sn5layer, it may cause the overall dip inIMC thickness.
EDS Line scan results for OSP, 150°C at 1500hrs. The plot shows at% composition versus distance along the yellow line drawn on the corresponding micrograph above.
Plot of individual IMC layer thicknesses versus time for OSP at 150°C
- OSP and ENEPIG IMC growth patterns were far more predictable andcorrelated better with literature than ENIG.
- Initial IMC formation is significant to overall IMC layer thickness forsamples that were aged at low temperatures (75°C, 100°C) but not assignificant for high temperature aged samples (125°C, 150°C) for allsurface finishes.
- For the OSP, ENIG and ENEPIG surface finishes, an aging temperatureof at least 125°C was necessary to detect significant IMC growth in 2000hours.
- Observations of cracking and voiding proved inconclusive (but morefrequent in ENIG) due to the small sample population size of the study.
- For all surface finishes, porosity diminishes within the IMC layer and IMCmorphology become less irregular with increasing time and temperature.
- Selection of OSP or ENEPIG is advised as ENIG shows the greatestvariation and maximum in IMC thickness as well as increased porosity andmore frequent voiding and cracking relative to the other surface finishes.
- Further study to substantiate these results by expanding the samplepopulation size is advised.
- Consideration of these results in context of the larger Medtronic study isadvised.References[1] Kumar, S., Handwerker, C., and Dayananda, M. Intrinsic and Interdiffusion in Cu-Sn System. Phase Equilibria and Diffusion, 32, 309-319 (2011).[2] Yoon, Jeong-Won et al. Comparative Study of ENIG and ENEPIG as Surface Finishes for a Sn-Ag-Cu Solder Joint. Electronic Materials, 40, 9, 1950-1955 (2011).[3] Zeng, G. et. al. A Review on the Interfacial Intermetallic Compounds between Sn-Ag-Cu Based Solders and Substrates. Material Science: Material Electronics. 21, 421-440. (2010).
Cu6Sn5
Porous(Cu, Ni)6Sn5
Ni(P)
Dense(Cu, Ni)6Sn5
- Phenom SEM used in backscattered electron mode to obtain images and toconfirm multiphase IMC presence and identify phases formed, the teamused Energy Dispersive X-ray Spectroscopy (EDS). Charging effectsmitigated by painting with graphite suspension, grounding with Cu tape anduse of a charge dissipative sample holder specific to the Phenom SEM.
- ImageJ software characterized three parameters‒ average, maximum, andminimum IMC thickness. The total area occupied by each IMC was tracedand measured. The area was divided by the length of interface observed inorder to produce the average thickness for each IMC layer.
- The total IMC thickness is the sum of individual average thicknesses for allIMC species present in a given sample.
- Maximum and minimum phase thicknesses for each IMC species weremeasured directly from the scaled image in ImageJ. The range of thesereadings approximates irregularity of the interface shape.
- Each micrograph was investigated for cracks and microvoids. While a crackin a 2-D image could be representative of a 3-D separation, it does not fullycapture the interface. The presence of cracks and microvoids indicatefurther investigation required and a larger sample size to characterize fully.
As reflowed samples for all three surface finishes show the initial IMC growth that occurs during reflow.
Ni(P)
Sn
Cu
92at%
46at%
0at%
Cu3Sn Cu6Sn5Solder Matrix
SEM micrographs of the OSP system for the 150°C series and the 500hr series. (Inset) Plot of average total IMC thickness over time for the four temperatures.
300μm
10μm
SEM micrograph of failed SAC305 interface with an ENIG surface finish aged at 150°C for 2000 hours.
10μm
Crack
3cm
Cu3Sn
Cu6Sn5
(Cu,Ni,Au)6Sn5
(Cu,Ni)3Sn
SEM micrographs of the ENEPIG system for the 150°C series and 500hr series. (Inset) Plot of average total IMC thickness vs time for the four temperatures.
- Like OSP, ENIG samples aged at 75°C and 100°C do not exhibit muchgrowth and in some cases even show a decrease in IMC thickness comparedto the t0 case. This is caused by the densification of the porosity originallypresent in the Cu6Sn5 layer over time at these temperatures.
- The 125°C trend is unusual compared to the expected parabolic growth ratereported by Yoon et al.[2] The 150°C trend shows a monotonically increasinggrowth pattern that is roughly linear.
- Like OSP, the morphology of the Cu6Sn5 interface becomes more planar withincreasing time and temperature but to a lesser degree than OSP. ENIGsamples, even after aging, still exhibited large ranges in layer thicknesses.
- Incidence of cracking was observed more frequently in ENIG than othersurface finishes but few total samples exhibited any cracking at all.
SEM micrographs of ENIG system for the 150°C series and 500hr series. (Inset) Plot of total IMC thickness over time for the four temperatures.
Elements present:OSP: Cu, Ag, SnENIG: Cu, Ni, Au, Ag, SnENEPIG: Cu, Ni, Pd, Au, Ag, SnCapacitor metallization:Ni-Sn
B
Recommendations
The Team would like to thank:Dr. Mysore Dayananda, Dr. Kathleen Kingma, Jameson Root, John
Holaday, and Kate Reeve for their generous support
