Lead-free and Tin-lead Rework Development Activities within the Nemi Lead-free Assembly and Rework Project

With the upcoming European ROHS legislation and other global movements to lead-free assembly, the NEMI lead-free rework group investigated and developed lead-free rework processes for medium to high-end computer products. The work concentrated on development of lead-free hot air convection rework for PBGA, CBGA, and uBGA, and lead-free hand soldering rework for TSOP and 2512 chip components on 93mil and 135mil thick test vehicle boards. Leadfree and tin-lead rework profiles along with visual and X-ray inspection will be presented and discussed. The lead-free and tin-lead rework was completed successfully and test boards were submitted for ATC reliability testing for up to 6,000 cycles from 0°C to 100°C which is ongoing at this time. INTRODUCTION Numerous investigations have been performed on the assembly of lead-free electrical components onto printed circuit boards using lead-free soldering materials. Few have explored the reworkability of these lead-free assemblies. Array packages are one of the more challenging types of components for rework. For this work lead-free and tin-lead PBGA, uBGA and CBGA component were evaluated. The rework process included removal, site redressing, and part replacement using hot air convection rework equipment. TSOP and 2512 chip rework components were also evaluated for tin-lead and lead-free rework on the same boards. The rework study was performed in two phases. Phase one emphasized the development of the rework profile. Both tin-lead and lead-free (SnAgCu) assemblies on two relatively thick boards (93mil and 135mil) and two surface finishes (Electrolytic NiAu and Immersion Silver) were evaluated. The rework profile was developed with consideration of the component, board, solder paste and standard specifications. Phase two of the study used the profiles and parameters developed in Phase 1 to rework test boards at uBGA, PBGA, CBGA TSOP and 2512 chip locations. Both reworked and first-pass “as-assembled” assemblies were subjected to accelerated thermal cycling (from 0°C to 100°C for 6,000 cycles). Results were compared. J-STD-020B (Ref.1) required that the lead-free soldering peak temperatures for large components (> 350Cumm package volume) stay below 245°C and small components (<350Cumm package volume) stay below 250°C. This experiment aimed to verify if the developed rework profiles conformed to this standard. EXPERIMENTAL Board and Components Used and Rework Locations Rework development was conducted on the NEMI Payette reliability test vehicle. The board is shown in Figures 1 (topside) and 2 (bottomside). Most of the components reworked were on the topside. The bottomside has one PBGA544 component and some TSOP and 2512 chip components. Board specifications are shown below: High Temp Laminate FR4 (Tg: 170°C) Surface Finish: Imm Ag and Electrolytic NiAu Board Thickness: 0.093” and 0.135” Board Dimension: 16.5” x 7.25” Number of copper layers: 14 Component specifications are shown below: PBGA544 1mm pitch, 35 x 35mm body size Ball Alloy: 63Sn37Pb or SnAgCu Board Location: U29 (Topside), U30 (Bottomside) UBGA256 1mm pitch, 17mm x 17mm body size, 256 I/O uBGA (Plastic Overmold), Ball Alloy: 63Sn37Pb or SnAgCu Board Locations: U40 and U41 (Both Topside) CBGA 937 I/O 10Sn90Pb alloy spheres 1mm pitch, 32.5 x 32.5 mm body size 4.33 mm thick (die & substrate) Overall height (including balls) 5.14 mm SnAgCu alloy spheres 1mm pitch, 32.5 x 32.5 mm package size 1.5 mm thick (substrate only) Overall height (including balls) 2 mm Board Locations: U27 and U28 (Both Topside) The rework process, including component removal, site redressing, paste deposition and new component attachment, was evaluated on lead-free assemblies using new process parameters (i.e. increased processing temperatures). Solder Paste Used for PBGA544 and uBGA256 Tin-Lead: (63Sn37Pb) 90wt% metal content Lead-free: (Sn3.9Ag0.6Cu) 89.3wt% metal content Solder Paste Type: No Clean, Type 3 Mini-Stencil used for uBGA256 Thickness: 6mils Aperture opening: 20mil Mini-stencil Printing on Component Sphere for PBGA544 The mini-stencil was cleaned with alcohol and a lintfree wipe after every print to prevent solder paste clogging. The paste volume used was 2493mils. The paste volume to print on the ball spheres was based on the ball diameter. Solder Paste for Paste Dispense Process (In syringes) for CBGA933: 63Sn37Pb: No-clean, 87% metal content Type 4 Sn3.9Ag0.6Cu: No-clean 84% metal content Type 3 Paste Deposition Equipment for CBGA: Paste Dispensing System Localized Hot Air Rework Convection Equipment For BGA Components Rework Atmosphere: Air (for CBGA and PBGA) Rework Atmosphere: N2 (for uBGA) Rework Equipment for TSOP and 2512 Chip: Hand soldering iron and desolder station No-clean cored wire for lead-free rework: Sn3.9Ag0.6Cu No-clean cored wire for tin-lead: Sn37Pb Liquid no-clean rework flux pen Redressing Equipment: Non-contact scavenger system (for CBGA and uBGA) Manual Solder wick (for PBGA) Thermocouple Attachment: Type K Thermocouples – 36 Gauge Thermally Conductive Adhesive Kapton Tape (heat resistant tape) Post Rework Analysis: Electrical Resistance Measurement X-Ray System Visual Inspection Temperature Profiling Criteria and Thermocouple Locations The tin-lead and lead-free rework thermal profiles had to satisfy the J-STD-020B temperature requirements. The temperature on the solder ball joint had to be high enough to guarantee reflow while not exceeding the maximum temperature allowed on the top of the component The values for these requirements were dependent on the solder paste alloy used and the alloy spheres present on the component. Table 1 summarizes the rework temperature profiling criteria. Existing rework equipment used in tin-lead rework was used for the lead-free rework with higher heater settings developed to rework lead-free SnAgCu soldered components. Thermocouple Calibration and Setup Prior to the development of any rework profile, the thermocouples used had to be verified to ensure that proper temperatures were recorded. A simple test was performed using boiling water to verify that all thermocouples used did not deviate more that +/-1°C