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

Authors: Kuldip Johal and Hugh Roberts et al.
Company: Atotech USA, Inc.
Date Published: 4/1/2004   Volume: 17-2

Abstract: The influence of co-deposited phosphorus within an electroless nickel layer (from low- to high-P) on the reliability of the solder joint integrity and aluminum-wire bond performance was investigated. The solder joint evaluation was conducted using solder maskdefined and non-solder mask-defined BGAs as test vehicles, and using the ball shear technique to establish the level of any "brittle fracture". The type and structure of the intermetallic compound (IMC) created before and after thermal cycling was examined relative to the codeposited phosphorus (from mid- to high-P) within an electroless nickel layer to determine if there is any influence on solder joint integrity. It was found that as the nickel thickness decreased and the gold thickness increased, the highest impact on solder joint integrity was observed.

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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