IMC Layers Formed with Various Combinations of Solders and Surface Finishes and Their Effect on Solder Joint Reliability

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1 IMC Layers Formed with Various Combinations of Solders and Surface Finishes and Their Effect on Solder Joint Reliability Per-Erik Tegehall, Swerea IVF 4 th Electronic Materials and Assembly Processes for Space (EMPS) Workshop Aalborg, Denmark, 30 May 2013

2 Failure modes for BGA solder joints exposed to stress Fractures in the solder are ductile fractures whereas fractures in the IMC layers are brittle fractures. 28/05/2013 2

3 Appearances of ductile and brittle fractures Ductile fracture Brittle fracture 28/05/2013 3

4 Main factors affecting the type of failure mode Elasticity modulus of solder, IMC layers, PCB laminate and component substrate Strength of solder, IMC layers, PCB laminate and component substrate Strain rate Time passed since soldering Temperature 28/05/2013 4

5 Factors affecting the type of failure mode, cont d Generally, the strength of solder increases with increasing strain rate whereas the strength of the IMC layer decreases with increasing strain rate. With increasing strain rate, the type of fracture changes from ductile to brittle at a certain strain rate (Ductile-tobrittle transition strain rate DTBTSR) 28/05/2013 5

6 High strain stresses that may cause brittle fractures In-circuit testing Depaneling or breaking of end tabs Insertion or removal of boards in chassis Attachment or removal of fasteners, press-fit connectors, and spring loaded heat sinks Key pad actuation Shipping or handling with insufficient mechanical support Dropping a product to the ground Thermal cycling and vibration Bending of the PCB is the main cause of brittle fractures 28/05/2013 6

7 Impact of lead-free soldering Lead-free solders have higher strength. Intermetallic layers have often lower strength. Laminates in PCBs and components for lead-free soldering have generally higher E-modulus. New moulding compounds for components have also higher E-modulus. 28/05/2013 7

8 Elasticity of SAC compared to SnPb SAC solders with 3-4% Ag and 0,5% Cu has about 30% higher E- modulus than SnPb solder due to precipitation and solid solution hardening SAC105 has about 10% lower E- modulus than SAC405 The E-modulus of SAC decreases with time due to: - structure changes (fewer and larger IMC crystals, i.e. reduced precipitation hardening) - reduced solid solution hardening SnPb solder consisting of a mixture of lead and tin rich phases (Sn-rik fas och Pb-rik fas) SAC solder containing Ag 3 Sn and Cu 6 Sn 5 IMC crystals 28/05/2013 8

9 Compositions and thicknesses of IMC layers The composition and thicknesses of IMC layers are determined by: Finish on surfaces to be soldered (on PCB and components) Composition of solder Soldering profile Use conditions Finishes that solder joints are formed to: Electrolytic Cu (HASL, OSP, immersion silver, immersion tin) Electroless Ni(P) (ENIG, ENEPIG - NiPdAu) Electrolytic Ni (almost exclusively on components) Most common composition of lead-free solders used: Sn3.0Ag0.5Cu (SAC305) Sn1.0Ag0.5Cu (SAC105) Sn3.5Ag Sn0.7Cu0.05NiGe (SN100C) 28/05/2013 9

10 SnPb and SAC solders towards copper The IMC layer formed consists of Cu 6 Sn 5 both for SnPb and SA(C) solders The IMC layer is thicker for solders containing copper The thickness increases with time, the higher temperature the faster growth At temperatures above 60 C, a second IMC layer with Cu 3 Sn is formed The Cu 3 Sn layer often has micro voids Solder joint after soldering Solder joint after aging for 500 h at 150 C SnPb Cu 6 Sn 5 Cu Cu 6 Sn 5 Cu 3 Sn 28/05/

11 Formation of micro voids The micro voids are probably caused by contaminants in the copper plating The amount of voids may vary considerably between various batches and between various PCBs from the same batch They are formed both with SnPb and SAC solders The voids are mainly formed at temperatures above 100 C but may form even at room temperature The voids decrease the strength of the IMC layer Products with voids has caused companies costs of tens to hundreds of million dollars* Formation of micro voids can cause problem on 5-10% of PCBs under the right conditions* Number of drops to failure as a function of time for BGAs aged at 125 C** *Source: P. Borgesen et al., ECTC 2007, pp **Source: Chiu et al., ECTC /05/

12 Examples of solder joints with micro voids* Variation between two PCBs aged under the same conditions SnPb solder joint stored at room temperature for one year Variation on one pad aged for 670 h at 150 C *Source: L. Yin, IMAPS Garden State Chapter Spring 2006 Symposium, May 2, /05/

13 Effect of Ni in SAC solder towards copper The figures show the interface between SAC305 and a pad with Sn0.7Cu0.05NiGe after soldering and after aging. Closest to the copper there is a thin homogenous IMC layer with Cu 6 Sn 5 and on top of that a thicker layer with Cu 6 Sn 5 rods with solder between the rods. The top layer is densified when aged at 150 C to a homogenous layer with Cu 6 Sn 5. When aged, a secondary IMC layer with Cu 3 Sn is formed which is thinner compared to Ni-free solders and is claimed to be less prone to form micro voids. Addition of Co to SAC have a similar effect Other metals are also claimed to reduce the growth of the layer with Cu 3 Sn and reduce the formation of voids, for example Bi, Cs and Mn. Solder joint after soldering Solder joint after aging for 500h at 150 C Cu 6 Sn 5 Cu 6 Sn 5 Cu 3 Sn 28/05/

14 SnPb solder towards electroless Ni Brittle fractures were first observed for BGAs solder using SnPb solder toward electroless Ni (ENIG) The IMC layer formed consist of Ni 3 Sn 4 Electroless Ni contains 5-10% P. The Ni beneath the IMC layer is enriched with P as Ni 3 P. Solder joint after soldering Ni 3 Sn 4 Ni 3 P Ni(P) Between the layers with Ni 3 Sn 4 and Ni 3 P, there is a very thin nanocrystalline layer containing Ni, Sn och P It is believed that it is this nanocrystalline layer that is causing the brittleness Solder joint after aging for 500h at 150 C Ni 3 Sn 4 Ni-Sn-P Ni 3 P 28/05/

15 SAC solder towards electrolytic nickel BGAs with SAC solder balls are sometimes missing solder balls due to the formation of a brittle IMC layer. Solder joints with SAC305 towards electrolytic Ni are usually not prone to brittle fractures. However, sometimes can these solder joints be very prone to brittle fractures. There may be large differences between various suppliers of substrates but also between different batches from the same supplier. DTBTSR for SAC305 towards various finishes* DTBTSR for SAC305 towards different batches of two substrates with Ni/Au* *Source: R. Darveaux and C. Reichman, Proc. Electronics Pack. Techno. Conf., 2006, /05/

16 SAC solder towards electrolytic nickel, cont d For coatings with electrolytic nickel prone to form brittle solder joints, the content of Cu in the solder is important for the inclination for brittle fractures. Low content of Cu decreases strongly the risk for brittle fractures but also high content of Cu decrease the risk. The content of Ag has little influence. The same trend was observed for solder joints to electrolytic Ni that were not prone to brittle fractures but less pronounced. Conclusion: The content of Cu should be as low as possible.* DTBTSR for various solders to electrolytic Ni prone to brittle fractures* *Source: R. Darveaux and C. Reichman, Proc. Electronics Pack. Techn. Conf., 2006, /05/

17 SAC solder towards nickel (both types) The composition of the IMC layer depends on the content of copper. No Cu: Ni 3 Sn 4 <0,4% Cu: (Ni,Cu) 3 Sn 4 >0,6% Cu: (Cu,Ni) 6 Sn 5 0,4-0,6% Cu: (Ni,Cu) 3 Sn 4 + (Cu,Ni) 6 Sn 5 The double IMC layer may also form when aging solder joints with more than 0,6% Cu. The double layer formed during soldering increase drastically the risk for brittle fractures. Double layer formed during aging probably entails less risk for brittle fractures. Number of drops-to-failure as a function of the number of soldering processes for SAC solder with varying content of Cu towards electrolytic NI* *Source: J.Y. Kim, Y.C. Sohn and J. Yu, J. Mater. Res., Vol. 22, No. 3, 2007, /05/

18 Effect of contaminants in the Ni finish The solder mask on the BGA substrate may leach substances to the Ni plating bath. Solder joints to Ni layers plated in contaminated baths are more prone to brittle fractures.* The contaminants in the Ni finish can be found between grains boundaries in the IMC layer. Effect of contaminants from the solder mask on the risk for brittle fractures* *Source: K. Yamamoto, et al.,, IEEE Trans. on Comp. Pack. Techn., Vol. 31, No. 4, 2008, /05/

19 Soldering towards ENEPIG (NiPdAu) Since some years, ENEPIG has been marketed as the universal surface finish since it is lead-free and suitable for soldering as well as wire bonding with both gold and aluminium wires. No Black Pad defects have been reported for ENEPIG It is extensively used as surface finish on substrates for BGAs but is increasingly used also on PCBs. The Pd layer may contain P or consist of pure Pd. The thickness of the Pd layer affects the solder joint reliability. It should be maximum 200 nm thick. With solders containing copper, a thin IMC layer with (Cu,Ni) 6 Sn 5 is formed which is very resistant to brittle fractures. With solders that do not contain copper, for example SnPb and Sn3.5Ag, a thick irregular IMC layer with (Pd,Ni)Sn 4 is formed which may be prone to brittle fractures, especially after high temperature aging. To minimise the risk for brittle fractures with SnPb solders, the thickness of the Pd layers should be in the range nm. 28/05/

20 Thank you for your attention 28/05/