Journal of SMT Article
IMPACTS OF BULK PHOSPHOROUS CONTENT OF ELECTROLESS NICKEL LAYERS TO SOLDER JOINT INTEGRITY AND THEIR USE AS GOLD- AND ALUMINUM-WIRE BOND SURFACES
Company: Atotech USA, Inc.
Date Published: 4/1/2004 Volume: 17-2
However in the case of higher P content, even at the lower nickel thickness, no signs of any brittle fracture were found. Two types of IMC were created on the thermo-cycled product using lead-free (Sn-Ag-Cu) solder balls, which were of different structural and chemical composition. The needle-like IMC structure is (Ni,Cu)3Sn4 with Ag3Sn co-dispersed into the bulk solder, while the chunky IMC structure is (Cu,Ni)6Sn5. Using a high-phosphorus ENIG system, the phosphorus enrichment after the immersion gold reaction and soldering showed a 30% lower enrichment at the interface of the electroless Nickel/C-Ni-Sn IMC. The mid-range phosphorus electroless nickel layer showed a wide spread of all force/length curves as compared to high-phosphorus layers, which were not negatively influenced by 1000 thermal cycles.
For aluminum-wire bonding, no effect is observed by varying the bulk phosphorus content, neither for "as received" nor after 4 hours at 150°C annealing. Gold-wire bonding showed very good results with high-P electroless Nickel with only one-tenth of the gold thickness of that used for electrolytic nickel/gold. Based on this investigation, the recommendation would be to use a high-P ENIG system for superior joint reliability.
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