Printed Circuit Design & Fab - November 2008 - (Page 36)

LEAD-FREE rEliaBility The Influence of final finish on Lead-free assembly reliability Understanding the end use application of a PCB is critical in the selection of right solderable final finish for Pb-free applications. by MOHAMMAD HOSSAIN, NIkHIL LAkHkAR, pULIGANDLA VISwANADHAM and DEREJE AGONAFER Tin-Silver-Copper (SAC) solder alloys have been introduced as primary alternatives to the eutectic tin-lead solder. The use of surface finishes is a very integral process in the assembly of printed circuit boards. The purpose of a surface finish in the manufacturing process is to provide and to maintain a solderable surface for component assembly. This is accomplished by protecting copper pads from oxidation throughout the fabrication process and up to the solder reflow process. Final finishes used in the PCB industry include Electroless Nickel/Immersion Gold (ENIG), Hot Air Solder Level (HASL), Organic Solderability Preservative (OSP), Immersion Silver (ImAg) and Immersion Tin (ImSn). It is important to understand the effects of these PCB finishes on solder joint reliability, as well as to provide an optimized process to support a particular finish. In this study, three PCB surface finishes were examined, namely Finish Type ImAg Typical Thickness ~ 3 to 4 m • Low percentage of silver dissolved into solder joint • Solderability • Micro-voids • High friction coefficient and tarnishing is concern in press-fit connections. ENIG 2 to 4 m Ni & 0.05 to 0.25 m Au over Ni • Corrosion resistance • Flatness for fine-pitch technology • Solderability • Shelf life • Surface contrast • Contact resistances • High aspect ratio capability OSP ~ 0.2 to 0.6 m • A water based organic compound that selectively bonds with copper to provide an organometallic layer that protects copper from oxidation • Consistent Coating Process • Narrow process window • Cost • Fatigue failures on large BGA packages • Extraneous Ni plating • Soldermask compatibility with the Ni bath • Probes often scratch finish and leave copper exposed hence cant do testing after plating. • Difficult to soldermask via pad • Dependent on solder paste, problems with large SMT pads. • Gumming up of test probes Advantages Disadvantages ENIG, ImAg and OSP. A comparison of the above-mentioned surface finishes is shown in taBlE 1. Test Vehicle and Assembly All the boards were populated with CTBGA 288 packages. Each package is daisy chained to produce an overall risknet resistance of 2 ohms. The package description is as follows: 4 rows, 0.5 mm pitch, ROHS- Compliant epoxy mold compound, die attach epoxy, die size 8.5 mm² and a 4-layer substrate. The component solder balls were made of Sn4.0/Ag0.5/Cu alloy and the no clean solder paste used for reflow attachment was SAC 95.5Sn/3.8Ag/0.7Cu standard paste. The solder paste constitutes around 25% to 30% of the volume of the joint after screen-printing and the reflow process. All boards were subjected to the same reflow process with a peak reflow profile slope of (Temperature vs Time) 0.5o C/sec. during the ascending time, a soak time of 30 to 38 sec., a peak temperature of 249.3o C, a cool down slope of 1.2o C/sec. and the time above liquidous is 35 sec. After assembly, the boards were x-rayed for initial defects. Some defects such as bigger solder balls and solder bridging were found; however, it was not clear in this study how much they impacted the bend or drop reliability of boards. The configuration for the boards for the bend and drop tests essentially follows JESD22-B1111 with minor differences in the number of components placed, since the same board design was used for both the drop and bend test. Strain gauges were placed on bare and populated boards on both sides of the panels, and replicate readings were taken for confirmation. For cyclic bending, the validity of the constant strain assumption in a four-point-bend test has been confirmed by strain gauge measurements at all component NOVEMBER 2008 taBlE 1. Advantages and disadvantages of PCB surface finishes. 36 printEd CirCuit dESign & fAB

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - November 2008

Printed Circuit Design & Fab - November 2008 Contents Our Line Market Watch Around the World Happenings ROI Positive Plating Ten Tips to Improve Manufacturability 3D Chip-Package-Board Modeling Improving Circuit Simulation With The Addition Of Real Measurements Ad Index PCB West: Interview with NBS Design Inc. The Influence of Final Finish on Lead-Free Assembly Reliability The Lead-free Soldering Challenges for Peelable Resists Off the Shelf Marketplace BGA Bulletin

Printed Circuit Design & Fab - November 2008

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