Fatigue issues

13 KA INTERCONNECTION ONE LOAD, RESISTANCE SC TO SC

EDMS No

SC joint: combined electrical and mechanical tests Aim – Evaluate possible deterioration of the electrical quality of the joint because of the mechanical efforts How – Measure the resistance of standard joint – Submit the same joints to tensile load (500 N, 1000 N, 5000 N) – Re-measure the resistance EDMS No

Results 1.9 K Joint typeR before mechanical testR after mechanical test 500 N Quadrupole0.1 nΩ Dipole0.09 nΩ Joint typeR before mechanical testR after mechanical test 1000 N Quadrupole0.13 nΩ Dipole0.12 nΩ0.10 nΩ Joint typeR before mechanical testR after mechanical test 5000 N Quadrupole0.1 5nΩ0.14 nΩ Dipole0.11 nΩ EDMS No

13 KA INTERCONNECTION FATIGUE

A fatigue test on a good Cu-Cu connection for tensile efforts stepR8 A [µΩ]R8 B [µΩ] Initial values After LN shock9.611 After compression 500 N 40 cycles After tension 500 N 40 cycles N N N N N N Nbreakage EDMS No

13 KA INTERCONNECTION FATIGUE ON SC TO SC SPLICE

Mechanical test procedure and acceptance limits At room temperature – Fatigue life cycle: 5000 cycles from 20 N to 240 N – Tensile test: minimum force for joint breakage 900 N At 4.2 K – Fatigue life cycle: 5000 cycles from 40 N to 510 N – Tensile test: minimum force for joint breakage 1000 N Total number of tests performed: 34 R.T., K EDMS No

Production samples The check was performed by the execution, by the same operators using that specific machine, of two production samples. After a visual inspection, the samples were tested electrically at the CERN cryolab and thereafter mechanically at the EIG (Ecole d’Ingénieurs de Genève). Important: no one of the joints tested was broken during the tests. A test was stopped because 1) An upper test limit due to the test set up was reached ( typically 2 KN for the 4.2 K and 5 KN for the R.T. tests) 2) A discontinuity on the loading curve was observed (see case I and case II) and that value was declared the limit of the joint EDMS No

Test equipment EIG EDMS No

Tensile test R.T. EDMS No

Tensile test 4.2 K EDMS No

A fatigue test on a good Cu-Cu connection for tensile efforts stepR8 A [µΩ]R8 B [µΩ] Initial values After LN shock9.611 After compression 500 N 40 cycles After tension 500 N 40 cycles N N N N N N Nbreakage EDMS No

A fatigue test on a good Cu-Cu connection for tensile efforts stepR8 A [µΩ]R8 B [µΩ] Initial values After LN shock9.611 After compression 500 N 40 cycles After tension 500 N 40 cycles N N N N N N Nbreakage EDMS No

SHUNT FATIGUE: MECHANICAL FATIGUE, LFM DOMINATED

Effect of fatigue on electrical resistance Cycles at 77K R measurement at RT F2 broke after 37 cycles at 6.8KN (tensile strenght7-7.9KN-> kept 85% of strength after 300 cycles )

Sample 2 of fast brazing technique has been cycled in LN2 at 6800 N, rupture occurred after 37 cycles Fractography Fast brazing, 15 mm brazed length: Cracks Fatigue striations

Conclusion on low cycle effect The performed fatigue tests indicate that – The soldering should withstand the loads originated from the thermal contraction mismatch without significant increase in the connection resistance – The possible crack propagation coming from local defect seems to be active only when we go over the yield strength We do not have direct estimation of the effect of – Microstructure coarsening – Steady state creep at low temperature BUT – Coarsening effect: it should be linked to the diffusion coefficient D=D0 exp-(Q/kT) and therefore in our case effect limited – Steady state creep. Creep should strongly decrease at T T<228K) Further test with pure thermal cycling will be performed on the final solution

SHUNT FATIGUE: ISOTHERMAL FATIGUE, “CREEP” DOMINATED

Tensile-Shear Strength After TC Tensile-shear strength was tested after thermal cycling for different shunt samples soldered with Sn60-Pb40 KesterMob 39 Impact of thermal cycling rather limited Courtesy A. Gerardin 22

INTERMETALLIC VS. SOLDERING TEMPERATURE

Effect soldering temperature on intermetallic thickness(mech. Properties) [19] EDMS No

Effect of intermetallic on mech. Properties III [19] EDMS No

Aging [15,17] Effect of aging on solder (in function of the Sb content ) Effect of intermetallic thickness on mech. properties EDMS No

Joint strength vs. intermetallic and thermal aging [2] Furthermore, the activation energy for growth of Cu3Sn on polycrystalline copper was found to be 1.27 eV and for Cu6Sn5 it was 0.47 eV, thus explaining the slower growth rate of Cu3Sn. The Cu–Sn intermetallic layer growth behavior can be described by one-dimensional growth parameter, Y, which is related to the square root of the aging time. Y=[Dt]^0.5 where D is the diffusion coefficient and t, the aging time. The diffusion coefficient is given by an Arrhenius expression, D=D0 exp-(Q/kT) where D0 is the diffusion constant; Q, the activation energy; k, the Boltzmann constant and T, the absolute temperature. EDMS No

Joint strength vs. intermetallic and thermal aging [15] The formation of inter-metallic is an exothermic reaction therefore it progresses with time. Aging causes coarsening of the structure that brings to lower capacity to absorb deformation fragilization-> crack propagation increase speed But no decrease of the max load As the coarsening of the structure is dominated by the diffusion coefficient and by the activation energy the coarsening effect for our use should be limited EDMS No

Properties in function of solder composition vs aging [3] EDMS No

FATIGUE EFFECTS ON JOINT EDMS No

Isothermal fatigue life [15] EDMS No

Isothermal fatigue life II [15] EDMS No

Fatigue and crack growth [2] For low frequency and high R (σmax/σmin) crack growth is time dominated-> creep For high frequency and low R> LFM explain the behavior EDMS No

Fatigue Vs. soldering process [17,18,20] It can be obtained by rapid cooling going away from the lamellar eutectic structure towards the fine grain structure. This allows intergranular creep. This structure is not stable evolving towards a coarser structure/ larger grains have lowed free energy but the rate of change is proportional to temperature In the superplastic region larger strain are accepted without fracture [20] In samples quenched after 1 min only Cu6Sn5 was found. In samples quenched after 5 minutes also Cu3Sn was there EDMS No

Fatigue Vs. soldering process [17,18] EDMS No

Fatigue vs. environment [18] The positive effect of vacuum (no oxygen) on the crack propagation is known and seems to be confirmed for soldering alloys EDMS No

Fracture mode of Sn-Pb/copper joints [17] In tensile the fracture happens through the solder bulk But in the case of eutectic cooled at slow rate the fracture takes place in the fragile intermetallic Tests at 77K show displacement of the fracture line between intermetallic and solder and this seems linked to the effect of the anisotrpic crystal thermal contraction of the metallic Sn (tetragonal) EDMS No

Intermetallic properties [18,21] Cu6Sn5 melts at 415 ⁰ C CuSn3 melts at 670 ⁰ C EDMS No

References 1)Measurements of mechanical properties of electronic materials at temperature down to 4.2 K Cryogenics 48 pag )[2] fatigue crack growth behavior of Sn-Pb engineering fracture mechanics 70 pag )[3] tensile fracture of tin-lead solder joints in copper material science and engineering A 379 pag )[4] deformation analysis of lap-shear testing of solder joints Acta Materialia )Deformation of solder joints under current stressing Solids and structure )[6] strengthening of Sn60-Pb40 solder alloy by adding traces of La Materials letters )[7]Improvement of micro structure stability, mechanical properties and wetting prop of Sn-AG-Cu lead free solder adding rare earth elements journal of alloy and compounds )[8] comparative study of wetting behavior of sn-Zn and Sn-Pb microelectronics journal )[9] tests for tin lead solders and solder joints bureau of mine report of investigation number )[10] tensile and shear properties of several solders at cryogenic temperature national aerospace engineering and manufacturing meeting Los Angeles oct )[11]design criteria for solders in cryogenic environments 12)[12] A brief study of soft soldering. The institution of production engineers 13)[13] enhancement of wettability and solder joint reliability by AG coating Alloys and compounds )[14]microstructure and metallic growth effect on shear strength …. Material science and engineering A )[15]SMT soldering handbook Newness 16)[16] ASM welding brazing and soldering handbook 17)[17] The mechanics of solder alloy: wetting and spreading Yost, Hosking, Frear 18)[18] The mechanics of solder alloy: interconnects Frear, Morgan, Burchett, Lau 19)[19] AWS manuel de brasage tendre 20)[20] Solder joint creep and stress relaxation dependence on construction and environmental stress parameter 21)[21] solders and soldering Materials, Design, Production and Analysis for reliable bonding. H. Manko McGraw-Hill 22)[22] Tin solders: A modern study of the properties of tin solders and soldered joints S.J. Nightingale 23)[23]Development of stress strain curves for In80Pb15ag5 journal of electronic materials vol 28 page )[24] Deformation properties of Indium based solders at 294 and 77 K Cryogenics 1991 vol 31 page )[25] Mechanical properties of In based Eutectic alloy solders used in Josephson packaging Cryogenics May 1984 pag )[26] Measured Performance of Different Solders in Bi2223/Ag Current Leads IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 18, NO. 2, JUNE )[27] Magnetoresistance of selected Sn- and Pb-based solders at 4.2 K Material Letters )[28] Mechanical behavior of microelectronics and power electronics under high current density Hua Ye PhD November 2004 University of Buffalo EDMS No