Paresh Limaye IPSI/REMO imec/restricted Reliability From the point of view of the end user Product has to perform as is promised to the customer AND has to last for as long as possible (or until the user gets tired of using the product) Probability that the device performs as expected for an expected duration. From the point of view of product seller Product has to perform as is promised to the customer AND has to last for a certain period (Warranty period/time till an newer version of the product is introduced)
Paresh Limaye IPSI/REMO imec/restricted Reliability Issues Broad categories Component reliability PCB reliability Solder joint reliability Electro-migration Sn whiskers Others…….. Many more
Paresh Limaye IPSI/REMO imec/restricted Reliability – Specific to Lead Free soldering Dealing with products, systems, specifications designed for tin lead backed with 50 years of field data The increased soldering temperature has a significant impact on product reliability. Degradation rate doubles with every 10 o C. Lead-free solder has different mechanical properties compared to SnPb.
Paresh Limaye IPSI/REMO imec/restricted Components Components can be degraded/damaged during the reflow soldering process Thermal load: temperature-time –Damage to internal temperature sensitive structure of component: electrolytes, insulating materials,… –Shift in electrical performance, reduced life span,…
Paresh Limaye IPSI/REMO imec/restricted Components - Moisture Absorbed moisture: rapidly expands during reflow soldering may lead to cracking of the component package: pop-corning. Absorbed moisture may lead to excessive component warpage – Opens/Shorts – Poor quality solder joints
Paresh Limaye IPSI/REMO imec/restricted Printed Circuit Board: laminate Lead-free reflow soldering / Hot Air Solder Leveling puts more thermal load on the board. Risks: Via barrel cracking due to CTE mismatch between Cu barrel and epoxy laminate in Z-direction. Delamination Board sagging Discoloration Important parameters: Glass transition temperature Tg Time-to-delamination: T260, T288 Decomposition temperature Z-expansion: o C
Paresh Limaye IPSI/REMO imec/restricted Printed Circuit Board: Board finish Lead-free HASL (Sn100C, SAC) –High thermal load on PCB: not suitable for thick multilayers Immersion Sn –Sn whiskering in unsoldered areas –Solderability: shelf life, multiple reflow Immersion Ag –Solderability: sensitive to sulphur NiAu –Au Embrittlement: Immersion OK, electroless (?), electroplated NOT OK. –Black pad (PCB manufacturer) –Skip plating (PCB manufacturer) –Soldering to Ni instead of Cu: slower All cases: risk of harmful chemistry residues in via holes. OSP (Organic Solderability Preservative) –Solderability –Invisible quality issues
Paresh Limaye IPSI/REMO imec/restricted Key aspects of SAC solders SnAg3-4Cu leads to increased stress levels! Stiffer material than SnPb: significantly higher E-modulus. The same deformation leads to a higher stress level. Stronger than SnPb: can bear higher stress levels Lower plasticity than SnPb. Higher solidification temperature leads to increased stress levels in the component/joint after solidification when thermal mismatch is present. Creep rate (deformation under constant load) is 10 to 100 times slower than for SnPb.
Paresh Limaye IPSI/REMO imec/restricted Solder joint: SAC SA3-4C may lead to failures elsewhere than in the solder! Intermetallic layer PCB pad lifting Component pads and body (ceramic chip) SA3-4C solder joints are more susceptible to shock. SA3-4C solder joints are less resistant to strong vibrations Increasing trend to move towards lower Ag content solders SAC 105 etc. Metallurgical Mess!!!
Paresh Limaye IPSI/REMO imec/restricted Thermo-mechanical Fatigue Package Board Package Board Package Board Package has lower CTE – 7-12 ppm/C Board has higher CTE – ppm/C Thermo-mechanical load is taken up by the solder
Paresh Limaye IPSI/REMO imec/restricted Thermal Cycling Solder joints experience creep-fatigue Fracture is initiated and the crack grows until the joint is mechanically separated
Paresh Limaye IPSI/REMO imec/restricted Recrystallize/Refined Grain Zone Cracking of the joints accompanied /preceded by recrystallization in the region of high strain accumulation Microstructure – Damage Zone
Paresh Limaye IPSI/REMO imec/restricted Solder joint: Thermo-mechanical fatigue How is reliability ensured? Identify the loading mechanism during operation Identify failure mode and failure distribution Accelerate failure mode in testing Compare to qualification standards (themselves based on acceleration models and past experience/field data)
Paresh Limaye IPSI/REMO imec/restricted Solder joint: Thermo-mechanical fatigue Accelerated thermal cycling test: e.g. 0C- 100C, 1 cycle/hour Determination of failure distribution: e.g. Weibull distribution Determination of acceleration factor with respect to field condition based on failure model: e.g. Coffin-Manson, Norris-Landzberg Lifetime estimation under field conditions
Paresh Limaye IPSI/REMO imec/restricted Is SAC Reliable enough? Is it as reliable as SnPb? NO SINGLE ANSWER Stress level dependency (J.-P. Clech)
Paresh Limaye IPSI/REMO imec/restricted Thermal cycle experiments 10 mm x 10 mm x 0.68 mm WLCSP device daisy chained– 64 I/O Assembled on 2.5 mm thick board (High Tg) Two pad sizes: 250 m and 450 m Sn 4%Ag 0.5% Cu – 300 m and 450 m preformed BGA Spheres
Paresh Limaye IPSI/REMO imec/restricted Acceleration Factors:SAC Norris-Landzberg equation: N.Pan et al., HP, 2005; Salmela et. al., Nokia, 2006 have published models for SAC Acceleration factor more sensitive to maximum test temperature as well as to the temperature amplitude. Increased acceleration factor compared to SnPb at constant dwell time.
Paresh Limaye IPSI/REMO imec/restricted Acceleration Factors – Experimental vs Modeled N f =C() n
Paresh Limaye IPSI/REMO imec/restricted Acceleration Factors Salmela’s model accounts for the solder material and the component type used Tends to over predict the AF at higher range and under predicts at lower ranges AF’s based on strain energy density show better correlation with the experimental observations We are far from understanding the real accelerated behaviour of SAC solders Creep mechanisms and their activation Creep behavior of the various new alloys being introduced through the range of temperature of accelerated testing
Paresh Limaye IPSI/REMO imec/restricted Solder joint: Thermo-mechanical fatigue Reliability statements are based on accelerated thermal cycling tests. These tests have been designed for SnPb solders. Accelerated tests give different results depending on test conditions, joint configuration, failure criterion, stress level, times lower creep rate of lead-free solders reduces the acceleration factor of the accelerated test. Risk: accelerated tests may overestimate fatigue resistance of lead-free solders!
Paresh Limaye IPSI/REMO imec/restricted Solder joint: Contamination Solder joint contamination may have a negative impact on the solder joint reliability Pb in lead-free solder joint –Source: SnPb solderable finish, contaminated solder bath –Effect: tendency to form low melting phase SnPbAg (179 o C). Weakened solder joint in last solidified region. Bi in SnPb solder joint –Source: SnBi solderable finish –Effect: tendency to form low melting phase PbBi (96 o C). Severely weakened solder joint. Au in lead-free or SnPb solder joint –Source: NiAu solderable finish –Effect: formation of highly brittle SnAu intermetallics. Au embrittled solder joint.
Paresh Limaye IPSI/REMO imec/restricted Solder joint: Contamination Reports from July 2007 suggest that leaded solder components are becoming scarce Assemblies relying on SnPb solders (high rel. applications) may end up being forced to use lead free components Long term reliability ????? Need to understand –Creep Behaviour of mixed/contaminated alloys –Accelerated and field cycling behaviour
Paresh Limaye IPSI/REMO imec/restricted Intermetallic related issues Other causes of potential solder joint failure Kirkendall voiding related to differences in diffusion between elements at solder/base metal interface. –Effect: weakened interface, cracking along interface –More severe with lead-free soldering because of high Sn content. –AgPd not compatible with SAC(?). –Source of concern, not clear yet. Electroless Ni/Immersion Au Ni 3 P precipitation ENIG requires typically 8% P in Ni. Intermetallics growth leads to Ni 3 P precipitation along interface Brittle interface: cracking Concern for both SnPb as well as lead-free soldering
Paresh Limaye IPSI/REMO imec/restricted Sn whiskering SnPb3-10% has been widely used as a solderable lead finish for components. Ban of Pb leads component manufacturers to go for pure Sn because of its low cost, availability and good solderability properties. Pure Sn whiskers! Tin whisker (inspection definition): A spontaneous columnar or cylindrical filament, which rarely branches, of tin emanating from the surface of a plating finish. (NEMI) Kinked Branched Odd-Shaped Eruptions (OSE)
Paresh Limaye IPSI/REMO imec/restricted What do we know about whiskers? It may create shorts under field operation conditions. It is NOT a production issue! Satellites/Cruise missiles even Nuclear plants affected by this It is not only an issue of pure Sn. Compressive stress in the Sn layer drives whisker growth. No quantitative view yet on impacting parameters. Several mitigation techniques: no clear solution.
Paresh Limaye IPSI/REMO imec/restricted Brittleness Testing of Leadfree solders –Charpy Test Clear ductile to brittle transition for Pb-free solders!
Paresh Limaye IPSI/REMO imec/restricted Brittleness Testing of Leadfree solders – Mini Charpy Test 11 22 E 1 =mgH 1 (initial energy) E 2 =mgH 2 (final energy) L 90°- 1 H 1 =L+Lcos(90- 1 )=L(1-sin 1 ) LL E=mg(H 1 - H 2 )= mgL(sin 2 - sin 1 ) hit point Hammer Cooling block (down to–150°C) Sample location
Paresh Limaye IPSI/REMO imec/restricted Brittle Solder Failures Energy for breaking joints To Be published in the proceedings of EPTC 2007
Paresh Limaye IPSI/REMO imec/restricted Brittleness Testing of Leadfree solders – Mini Charpy Test Test at -88ºC Test at 23ºC K. Lambrinou Potential concern for assemblies that operate in high shock/ extreme temperature environment
Paresh Limaye IPSI/REMO imec/restricted Summary Reliability issues in leadfree electronic assemblies – various sources –Component reliability, PCB reliability, Solder joint reliability, Sn whiskers, Flip Chip related ……….. MANY MORE!!! –Higher melting temperature –Higher solder stiffness –Higher propensity to form intermetallics Various alloys and surface finishes being used: Metallurgical Mess!!! Long term solder joint related effects – Will lead free solders perform as well as tin lead? NO clear answer
Paresh Limaye IPSI/REMO imec/restricted Summary Depends on the loading condition is which the solder joint is expected to fail. Need for identifying creep mechanisms active in field conditions AND accelerating those in testing Need good creep and acceleration models Mixed leadfree-SnPb solders – very little information Sn Whiskers is an issue for high reliability. applications for which we have no solution Low temperature brittle behaviour of LF solder can be an issue.
Paresh Limaye IPSI/REMO imec/restricted Special Thanks to Bart Vandevelde Ingrid De Wolf Geert Willems (www.rohsservice.be)www.rohsservice.b IPSI/REMO Group Dr. Jean Paul Clech, Dr. Robert Darveaux ALSHIRA Partners –Connectronics,TBP (Geel), Alcatel-Lucent (Antwepren) Multiboard, IMEC Gent, Interflux, Electronic Apparatus: (http://www.imec.be/ALSHIRA)