SMT Magazine

SMT-Oct2017

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68 SMT Magazine • October 2017 lead-free solders and testing of individual solder ball attachments rather than the advanced ball grid arrays (BGAs) electronic packaging assem- blies and for tin-lead solder. It is extremely dif- ficult to correlate test results from the shear of individual balls to the assemblies of BGAs. Re- liability testing using BGA packages, especially land grid arrays, with tin-lead solder is lacking. In addition, controversy exists regarding the re- liability/compatibility of ENEPIG with tin-lead solder 4 , as there are more consistent positive test results for the lead-free solder joints 5 . A hot air solder level (HASL) finish, which is commonly used for the tin-lead solder, lacks the flatness requirement for the fine pitch ball grid arrays (FBGAs). The significance of the PCB surface finish on assembly reliability was recognized by the IPC 9701 standard team in its early development, when the team was narrowing the requirements for the tin-lead solder joint testing for the BGA packaging technologies. The team limited the use of the PCB finishes to solder preservative (OSP) and HASL to minimize the potential for premature failures by using other finishes. Such restriction was implemented to avoid significant cost and schedule burden on smaller facilities, which generally lack knowledge and experience on the nuances of unique surface finishes such as electroless nickel/immersion gold (ENIG). This specification allowed unique surface fin- ishes only for comparison to the baseline finish. The A revision, which also includes recommen- dations for Pb-free solders, allowed only the use of an OSP finish since the tin-lead HASL was not compatible to Pb-free solders. Other surface fin- ishes including silver (Ag) and tin (Sn) were ac- ceptable only for a manufacturer's internal data comparison. Also, an ENIG finish could be used for an internal data comparison; however, it was warned that the risk of introducing unintended brittle failure (black pad) could occur. The EN- EPIG was not introduced at this time; therefore, this specification does not discuss this specific surface finish. With the industry implementation of Re- striction of Hazardous Substances (RoHS), the use of a tin-lead HASL finish for PCB with excel- lent solderability and solder joint reliability has diminished even though this still is the domi- nant finish for high-reliability applications with tin-lead solder. The process consists of immers- ing PCBs in a tin-lead alloy followed by solder removal using air knives, which blow hot air across the surface of the PCB [to remove excess solder]. The lead-free HASL finish has gained some interest for RoHS use, but it suffers from increased copper dissolution and lacks the flat- ness requirement needed for finer pitch array packages. An ENIG finish provides the flatness requirements with excellent solderability, but it suffers from the "black pad" potential failure and lacks gold wire bondability that is required for hybrid (wire and solder) technologies 6 . ENEPIG has provided the best solution for the black pad defect by depositing an additional layer of electroless palladium over nickel. The palladium is not etched away during the gold plating process so the potential for the oxida - tion of nickel (black pad) is eliminated. However, ENEPIG is costlier than ENIG. ENEPIG also pro- vides excellent solder joint reliability for lead- free solder joints, but industry debates continue on its reliability with tin-lead solder assembly. The IPC-4556 specification for ENEPIG is very comprehensive and includes a wealth of information. The thickness specification for ENEPIG specified as: 1) Nickel: 3 to 6 μm (118.1 to 236.2 μin), 2) Palladium: 0.05 to 0.15 μm (2 to 12 μin), and 3) Gold: a minimum thickness of 0.030 μm (1.2 μin). All measurements are to be taken on a nominal pad size of 1.5 mm x 1.5 mm (0.060 in x 0.060 in) or equivalent area. The new amendment sets an upper limit for the gold thickness at 2.8 μin (0.7 μm) to discour- age requests for a much higher immersion gold RELIABILITY OF ENEPIG BY SEQUENTIAL THERMAL CYCLING AND AGING " A hot air solder level (HASL) finish, which is commonly used for the tin-lead solder, lacks the flatness requirement for the fine pitch ball grid arrays (FBGAs). "

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