The Impact of Reflowing A Pbfree Solder Alloy Using A Tin/Lead Solder Alloy Reflow Profile On Solder Joint Integrity

The electronics industry is undergoing a materials evolution due to the pending Restriction of Hazardous Substances (RoHS) European Directive. Printed wiring board laminate suppliers, component fabricators, and printed wiring assembly operations are engaged in a multitude of investigations to determine what leadfree (Pbfree) material choices best fit their needs. The size and complexity of Pbfree implementation insures a transition period in which Pbfree and tin/lead solder finishes will be present on printed wiring assemblies. Ball grid array is one component style that has generated concern with respect to mixed finish scenarios. To better understand the reliability effect of mixed surface finish manufacturing, an investigation was conducted to evaluate the solder joint integrity impact of reflowing a Pbfree solder alloy using a tin/lead reflow profile. In this study, ball grid array components with tin/silver/copper (SAC) solder spheres were processed using a tin/lead reflow profile and then subjected to thermal cycle testing from -55oC to +125oC. Solder joint life measurements and failure analysis revealed premature solder joint failures due to non-uniform microstructure and poor wetting characteristics. BACKGROUND Rockwell Collins has followed the European Union (EU) activities/efforts pertaining to the drafted Waste from Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Materials (RoHS) directives very closely since their inception. The RoHS directive requires the elimination of lead, mercury, hexavalent chromium, and two specific fire retardants from electronic assemblies by July of 2006. Avionics equipment and systems were neither specifically included or excluded from the directives. At present, Rockwell Collins is coordinating with EU representatives to define avionics’ status as detailed by the RoHS directives. Rockwell Collins has adopted a proactive approach to the implementation of Pbfree processes by conducting internal investigative efforts, and participating in a number of industry Pbfree solder process focused efforts. The differences in process requirements for Pbfree and tin\lead solders raise both material and logistical concerns for electronic assembly. Pbfree solders require higher temperatures to reflow. Pbfree solders are alloyed with a wider number of metals which creates the potential for a far wider variety of intermetallics to be present in a solder joint. The more complicated compositions can result in solder joint microstructures that are not as thoroughly studied as current tin\lead solder microstructures. These material concerns are intensified by the possibility of Pbfree solders being unintentionally used in either processes designed solely for tin\lead solders or environments where material interactions are poorly understood, e.g. reworking a tin\lead solder joint with Pbfree solder. These mixed finish scenarios could negatively impact solder reliability. Rockwell Collins currently utilizes lead containing solder alloys in our soldering processes. The scope and complexity of Pbfree implementation ensures a transitional period in which both Pbfree and tin\lead solder finishes will be present on the same printed wiring assemblies. Understanding the potential impact of mixed finish scenarios on avionics product reliability is critical. OBJECTIVE The objective of the study was to evaluate the solder joint integrity impact of reflowing a Pbfree solder alloy using a tin/lead reflow profile on BGA components. PROCEDURES Test Vehicle The test vehicle used in the investigation was 0.082 inches thick, contained 18 layers of 0.5 ounce copper, and used an electroless nickel/immersion gold (ENIG) finish. The test vehicle laminate was FR4 per IPC-4101/26 with a minimum Tg of 170oC. The test vehicle contained 11 component test locations. Figure 1 illustrates the test vehicle design. Figure 1: Test Vehicle Design Test Component The test component was an 256 Daisy Chained, I/O 256, 17mm x 17mm square, 1.0 mm pitch Field Programmable Gate Array utilizing a Sn/4.0Ag/0.5Cu (SAC 405) soldersphere alloy. The BGA component was originally to have tin/lead solderballs but was unintentionally supplied with Tin/Silver/Copper (SAC) solderballs. Process time constraints did not allow for the component supplier to replace the BGAs with Sn/Pb solderballs thus allowing the opportunity to investigate the interaction of a SAC solder alloy with a Sn/Pb soldering reflow process. Figure 2 illustrates the test component pwb pad pattern. Figure 2: Test Component Pad Pattern Assembly Test Vehicle Assembly Flow The test vehicles were processed as shown in the flow diagram in Figure 3. Figure 3: Flow chart of assembly process Solder Paste Process The first step of the test vehicle assembly process was the application of solder paste. An MPM Ultraprint 2000 using Indium’s SMQ92J solder paste was utilized. Indium’s recommend solder paste print parameters (e.g. print speed, print pressure) were followed. The solder paste stencil was fabricated from 0.005 inches thick stainless steel material and utilized laser cut apertures. Component Placement The test vehicles were immediately transferred to the Universal GSM 4685 machine shown in Figure 4 upon completion of the application of solder paste. The test vehicles were fully populated. Figure 4: Universal GSM 4685 Machine Reflow The test vehicle was reflowed with a Heller 1809EXL Convection Reflow Oven. This oven had ten temperature zones for solder reflow. The conveyor speed was 34 inches per minute and the nitrogen flow was 1500 cubic feet per hour (approximately 150ppm O2 concentration). The oven used the high convection setting, and the boards were placed on the rails. The reflow profile as measured on the test vehicle is shown in Figure 5. Figure 5: Thermal Profile Cleaning The boards were allowed to cool to room temperature after reflow and then placed an Electrovert Aquastorm in-line cleaning system for removal of solder flux residues. The inline cleaner utilized Kyzen SSA saponifier/de-ionized water to remove the test vehicle flux residues. The in-line cleaner is shown in Figure 6. Apply Solder Paste on Component Pads MPM 2000 Place Components On Board Universal GSM Solder Reflow Heller1900 Wash Boards Electrovert Aquastorm Aqueous Cleaner 4685 8 9 Figure 6: Inline Cleaning System Assembly Inspection The solder joint quality and placement accuracy of all test vehicles were x-ray and visually inspected following the cleaning process. The inspection criteria were in accordance with Rockwell Collins Workmanship and the IPC-JSTD001 specifications. No solder joint anomalies were observed during the x-ray inspection analysis. Thermal Cycle Parameters/Methodology The temperature cycle range used in the investigation was 55°C to +125°C with a minimum 11 minute dwell at each temperature extreme and a maximum temperature ramp of 10°C/min. Components for Rockwell Collins designs are evaluated against specific benchmarks. Components are expected to pass 2000 thermal cycles failure-free to be used in flight-critical, high performance applications. Other benchmarks are 1000 and 500 thermal cycles, which represent ground-based high performance and commercial avionics applications, respectively. The continuity of the BGA component was continuously monitored throughout thermal cycle testing by an event detector in accordance with the IPC-9701 specification. Each component was treated as a single resistance channel. An event was recorded if the resistance of a channel exceeded 300 Ω for longer than 0.2 μsec within a 30-second period. A failure was defined when a component either: • Exceeded the maximum resistance for 15 consecutive events • Had five consecutive detection events within 10% of current life of test or • Became electrically open. Once a solder joint was designated a failure, the event detection system software excluded it from the remainder of the test. TEST RESULTS The test results indicated the first BGA solder joint failures occurred after 137 thermal cycles with a 276 thermal cycle population average being recorded. Typical BGA solder joint failures measured during other area array component investigations [1] has been in the 1000-2000 thermal cycle range. Metallographic analysis was conducted to determine solder joint failure characteristics. Figures 7 and 8 illustrates the BGA solder joint failure observed during the metallographic analysis. The red arrows in Figure 8 point to the solder joint crack. Figure 7: Macro View of BGA Solder Joint Failure Figure 8: Magnified View of BGA Solder Joint Crack The BGA solder joint failure is non-typical in a number of ways: (1) a solder joint failure at 137 thermal cycles would be considered premature and an indication of either a component fabrication or manufacturing error; (2) the solder joint crack was located at the solderball/pwb pad side of the solder joint – typical BGA solder joint cracks are located at the solderball/component pad side; (3) the solder joint crack was located at the solder matrix/solder joint intermetallic interface – typical solder joint crack location is in the solder matrix. Inspection of the solder joint revealed four distinct regions of differing microstructure. Each region has been designated as a Zone (A-D) for ease of discussion. Figure 9 illustrates the zones and Table 1 lists the microstructure observed in each zone.