Statement of Work (SOW) inemi Board Assembly TIG BiSn-Based Low-Temperature Soldering Process and Reliability Project Version 1.4 Date: December 1, 2015 Project Leader: Raiyo Aspandiar, Intel Corporation Co-Project Leader: Scott Mokler, Intel Corporation inemi Staff: Haley Fu Problem Statement: Recently there have been some drivers from most ODMs and some OEMs to adopt low-temperature soldering for the assembly of consumer electronic products, such as cell phones, tablets, and mobile computers. The motivations for these drivers have been three fold: environmental, economic and technical. The environmental driver is related to the electronic products life cycles. The economic driver is related to the reduction in manufacturing assembly costs, particularly the energy costs expended in running the reflow soldering ovens. The technical driver is related to the dynamic warpage of area array components, such as Flip chip BGAs, which are getting thinner, and their termination pitches are getting finer. These factors are resulting in a larger dynamic warpage of these components at the reflow temperatures being used for SAC solder pastes. These in turn cause a significant drop in solder joint yield due to the formation of Head-on-Pillow (HoP) and solder bridging defects. Lowering the peak reflow temperature diminishes the dynamic warpage and enhances solder joint yields. Solders in the BiSn system, particular compositions close to the eutectic (58wt% Bi) are prone to brittle fractures at high strain rates. This leads to a high risk of brittle fracture of BiSn-based solder joints under of mechanical shock and drop conditions for electronic products. To mitigate this risk, solder paste manufacturers have explored two pathways. One is through the development of a more ductile metallurgy within the BiSn solder system, by reducing the bismuth content, and addition of elemental dopants, both of which enhance the solder bulk mechanical properties by modifying its microstructure. The other is through the incorporation of polymer resins in the solder paste, that, during the reflow soldering process, cures after the solder wets to the metallic surfaces of the board and the component, and forms a polymeric reinforcement around the solder joint after the assembly cools down. There is sparse independent Page 1 of 10
assessment of these two pathways, both in terms of the processibility of these new solder pastes and the reliability of the solder joints thus formed by their use. Purpose: The purpose of this project is to assess the surface mount processibility and reliability of the solder joints formed when enhanced BiSn-based solder pastes are used for assembling electronic components on printed circuit boards. Scope: The scope of this project, separated out for each of its multiple ingredients, is listed below. Solder Pastes: The enhanced BiSn solder paste to be evaluated include those that have a ductile metallurgy as well as those that have resin incorporated within to enable polymeric reinforcement of the solder joint after the reflow soldering step. SnAgCu (SAC) 305 and eutectic BiSn (+Ag) composition solder pastes will be used for comparison to the current and BiSn system baselines, respectively. Components: The components evaluated will include high density flip chip BGAs, QFNs, QFPs, chip components, and other non-ic components such as switches, though hole pin connectors, and sockets. Board Surface Finishes: Two surface finishes will be evaluated. OSP, which is widely used for mobile computers today and ENIG, which is used in most consumer hand held products. Surface Mount Process Steps: The surface mount process steps that will be evaluated will include stencil printing, reflow soldering and rework of specific components. Wave soldering using low-temperature BiSn solders is not part of the scope. Instead, a pin in paste process using the low-temperature BiSn solder pastes will be developed for formation of the through hole pin solder joints of some of the components being evaluated. SAC wave soldering will be used for assembling these components on the baseline control boards. Reliability Tests: The main reliability test to be conducted is the Mechanical Shock/Drop Test since the highest risk for BiSn solders is brittle failures under the high strain rates occurring during these tests. The Mechanical Shock/Drop test will therefore be part of the initial phase of the project. Other reliability tests in subsequent phases will include accelerated Thermal Cycling, Transient Bend, Plug and Un-plug test for connectors, and Vibration Test. Product Validation: The product validation phase will comprise of assembling a number of product boards using the best performing solder paste from the ductile metallurgy and the resin reinforced categories and then subjecting these boards to standard Product level functional and reliability tests. Page 2 of 10
What the Project Is / Is Not: This Project IS: Evaluation of BiSn-based low-temperature melting (<200C) solder pastes Evaluation of selected BiSn-based solder pastes with ductile metallurgy and with or without resin for reinforcement Evaluation of solder joints for specific components types (SAC-ball BGAs, BTCs, chip components, through hole connectors, non-ic components) Evaluation of solder paste suitability and capability for specific board assembly process steps (stencil printing, no-clean reflow soldering process, rework) and its Surface Insulation Resistance (SIR) This Project IS NOT: Evaluation of any low-temperature melting solder paste Evaluation of all available BiSn-based solder pastes with and without resin for reinforcement. Evaluated of corner glue and corner fill resin reinforcement configurations. Continuous improvement of the selected solder paste material to satisfy the reliability requirements. Evaluation of solder joints for all available component types Evaluation of solder paste suitability and capability for all board assembly process steps or all fundamental solder paste properties Use equivalent pallet and stencil design at different assembly sites Evaluation of two specific board surface finishes Evaluation of all available board surface finishes Assessment of Mechanical Shock/Drop Reliability, Accelerate Thermal Cycling Reliability, and specific product validation tests Assessment of BiSn-based solder joint microstructural characteristics under selected assembly and reliability conditions Assessment of solder joint reliability under other stress/temperature conditions Assessment of bulk material properties of BiSnbased solders Scope of Work: This project will have two checkpoints which may cause the variation of detailed experiment scope from the initial plan. The experiment scope, resource matrix and schedule will be refined and agreed by the project members at each checkpoint before moving to the next steps. Page 3 of 10
Step 1: Definition Review prior work and make recommendations for testing needed. Investigation should take into account needs of electronic product sectors represented by inemi membership. 1. Identify and select candidate solder paste materials Poll the supplier base, keying in on candidate materials that are commercially viable with consideration for market segment applications. Two key categories are solder pastes with ductile BiSn metallurgy when compared to standard BiSn (+Ag) eutectic composition and solder paste containing resin for reinforcement of the solder joints. 2. Identify and select components to be evaluated Components selected should cover the main termination geometries such as balls, gull wing pins, bottom terminations, through hole pins, chip component terminations, and include non-ic components such as connectors, sockets, switches, etc. Where possible, components should be daisy chains to facilitate electrical testing of solder joints during process development and reliability tests. 3. Identify key performance characteristics to be investigated and define reliability test and requirement criteria Review results of prior industry and member company investigations. Make recommendations for performance tests needed. Prior studies have indicated that a) BiSn solder are prone to brittle fracture under mechanical shock stresses, and hence, mechanical shock/drop tests are a priority; b) resin containing pastes require specific stencil printing parameters and reflow profiles significantly different from standard non-resin solder paste, and hence, paste printing and reflow process development is a key requirement.; c) Rework of certain components, such as QFNs is difficult with resin reinforced solder pastes since the resin bond has to be broken on the thermal pads before the component can be removed. 4. Define test vehicle(s) and test methodologies, leverage standards where possible: Specify test vehicle criteria required for performance testing. Agree on a minimal number of test vehicle designs and test requirements. Two Test vehicles are warranted. One is for process development and the other is for mechanical/shock testing. Daisy chaining of the board traces to match those of the components is necessary. New JEDEC shock board design, which uses 4 components instead of 15, will be employed. Step 2: Assembly Process Development Develop, manage, and execute assembly process development using various solder pastes selected. 1. Design Process Development Test Vehicle and Mechanical Shock Test Vehicle Use suggested land patterns from component suppliers. Mechanical Shock Test vehicle will be used for large BGA components, only. 2. Procure parts and test vehicles Obtain needed evaluation materials. Consider lead times needed to synch with evaluation schedule. Solicit participation from supply partners. Test Vehicle Boards will need to be fabricated as a suitable PCB supplier. 3. Design Stencil Apertures and order Stencil Stencil apertures will need to be designed based on the component type and their particular characteristics. Will use equivalent pallet and stencil design at different assembly sites. Page 4 of 10
4. Develop Stencil Printing Process for selected solder pastes Establish optimum stencil printing parameters for each paste using transfer efficiency as the key output variable. Start with paste supplier specified settings for critical to function input process variables. 5. Develop reflow profile for selected solder pastes Using a reflow profiling unit of the process development test vehicle, develop the reflow profile to meet the desired range for key profile parameters as given by the solder paste supplier. 6. Initial Process Check Builds Small quantities of boards built for each solder paste to determine feasibility of process. 7. Optimize Process Steps Adjust process parameter settings to maximize solder joint yield. Electrical test is to be used on the daisy chained components for solder joint yield determination and IPC-A-610 can be used for visual quality checks on the non-daisy chained components. Voids should be measure for BGAs and QFNs. 8. Assemble Process Verification boards Assemble 6 boards per solder paste. Measure solder joint yield, record defect type and location and voids for BGAs and QFNs. 9. Run SIR test on boards Use SIR pattern on the board located on BGA site. 10. Run Rework Feasibility Evaluation on QFNs and BGAs Use standard rework process and equipment. Rate ease of rework to established process step rating table. 11. Collate and Analyze Process Development Data and Write Report Separate out the data for each solder paste used. Checkpoint #1: Assess process development results and determine candidate solder pastes to evaluate under mechanical shock Step 3: Mechanical Shock Test Develop, manage, and execute mechanical shock testing. 1. Optimize assembly process for Mechanical Shock Test Vehicle Boards and Assemble Boards Multiple legs are expected. Baseline SAC, baseline BiSnAg, ductile BiSn paste (at least one), resin reinforced solder paste (at least one). 2. Set up Mechanical Shock Equipment per JEDEC specification and execute Shock/Drop test The G levels with duration of the applied force will need to be established. Strain Gauge attachment and strain measurement will need to be done. Drop number vs failure data will be collected. 3. Analyze Mechanical Shock Data and write report Data will need to be analyzed and plotted in suitable manner (Weibull or other) to facilitate comparison of shock resistance between the various solder paste legs. Failure Analysis will need to be done to determine failure interfaces. Page 5 of 10
Checkpoint #2: Assess mechanical shock results and determine candidate solder pastes to evaluate under further reliability tests. Mechanical Shock equivalency to SAC leg is a desired requirement. Step 4: Accelerated Temperature Cycling and Other Reliability Tests 1. Assemble Process Development boards for Accelerated Temperature Cycling Test Multiple legs are expected. Baseline SAC, baseline BiSnAg, ductile BiSn paste (at least one), resin reinforced solder paste (at least one). 2. Set up Thermal Cycling Chamber and execute accelerated Thermal Cycling Test Thermal Cycling test protocol will need to be decided on. Connections between the boards and the data loggers will need to be set up. Interim readings will need to be recorded. Failed boards will be pulled out for analysis. 3. Execute Plug/un-Plug test Establish Plug/un-plug test protocol from industry usage. If specific equipment needed, then procure equipment. Conduct test and record data. 4. Execute transient Bend Test Use IPC-9702 standard. A design of the new test board may be required. Document Data. 5. Execute Vibration test Vibration Test parameters need to be established based on typical requirements for consumer electronic products. Document data. 6. Analyze data from all tests above and publish report Data will need to be analyzed and plotted in suitable manner (Weibull or other) to facilitate comparison of reliability under the various stress conditions between the selected solder paste legs. Failure Analysis will need to be done to determine failure location and modes. Step 5: Product Validation Validate the low-temperature solder paste assembly process and specific reliability requirements on a product board, using the best performing ductile and resin reinforced solder pastes from earlier phases. 1. Select Product Design and Validation tests Design should be for a consumer electronics market. Validation tests should follow standard product functionality and reliability protocols. 2. Select at least two manufacturing Sites to assemble and test the Product Board EMS and/or ODMs sites are preferred. 3. Procure BOM, and Assemble Boards Work will solder paste, component and board suppliers. Coordinate product builds with the selected manufacturing sites 4. Subject Assembled Boards to Validation Tests and Compile Results Document manufacturing yield and test yield, defect paretos, etc. Step 6: Completion Write and Publish Final Report to inemi members. Upon project team members agreement, identify public conferences to publish part or whole of the project work over the next year. Page 6 of 10
Anticipated Outcome: Validate feasibility of assembling high density boards with BiSn solder pastes (ductile metallurgy and resin reinforced) o Stencil Printability o Solder Joint Yield expected o Defects observed o Voiding levels in BGAs and QFNs o Reworkability Validate Board Level Reliability of solder joints formed with ductile BiSn and resin reinforced solder pastes o Comparison of these solder joints vs. SAC solder joints and standard BiSnAg eutectic solder joints under following stress conditions Mechanical Shock and drop Thermo-mechanical Fatigue Transient Bend forces Vibration Forces Push forces encountered in plugging cables into connectors Participant Profile: inemi s member companies encourages the participation of individuals from different disciplines and divisions within their organizations to contribute on the range of tasks outlined in the project plan. This project team encourages the participation of low-temperature solder paste manufacturers, component suppliers, PCB fabricators, EMS, ODMs and OEMs The following companies have participated in the meetings for developing this project plan: Alcatel- Lucent, Alent, Altera, Bosch, Bose corporation, Celestica, China Solder Technology Group, Creation technologies, DfR Solutions, Dow Electronic Materials, Euroflux, Flextronics, IBM, Indium Corporation, Intel, ist, Jabil Circuits, Kaifa, Lenovo, Nihon Superior, Senju, Shinko, Tamura, Universal Instruments, University of Maryland, Wistron Corporation, and YINCAE. Resources Required from Participants: The resources required are listed below: solder pastes, component test vehicles, board test vehicle design and fabrication. Potential participants and in-kind contributions are identified during the project formation. The detailed resource allocation will be managed in a separate table based on the project signup. 1) Solder Pastes: Provide at least 2 kg of following solder pastes. a. Standard SAC305 b. BiSnAg c. Ductile metallurgy BiSn based d. Resin reinforced BiSn based Obtaining the ductile metallurgy and resin reinforced solder paste from more than one supplier is preferred to broaden the investigated solder paste types. Page 7 of 10
2) Component Test Vehicles: Provide at least 200 quantities of the following component types: a. Flip Chip BGAs i. >30 mm. body size, >= 0.65mm ball pitch ii. <17mm, <=0.4mm ball pitch b. Through hole connector for Pin-in-Paste evaluation c. USB Connector for plug/un-plug test d. Area Array socket ~50mm inches body size e. QFP i. 14 x14mm body size, 100 leads ii. 28x28mm body size, 208 leads f. QFN - 10x10mm body size, 0.5mmpitch g. Chip Capacitors i. 1005 ii. 201 iii. 402 h. Tactile Switch, surface mount 3) Board Test Vehicles: a. Process Development Board i. Design Resources ii. Fabrication b. New JEDEC Shock Test Board i. Design Resources ii. Fabrication 4) Stencil procurement for Process Development Board and Shock Test Board 5) Board Assembly Resources: a. Development /Optimization of stencil printing process for each paste type b. Programming of the Pick and Place machine for each board test vehicle c. Development of Reflow profile for each paste type for each of the two test vehicles using recommended reflow profile parameters from solder paste suppliers d. Assembly of the two Test vehicles boards (at least 50 of each at each assembly site) through the stencil printing, pick and place, reflow soldering and test process steps e. Board Assembly Test data collection and collation 6) Failure Analysis of assembled boards, preferably at more than one site 7) Data Analysis for printed solder paste volume, solder joint yield, reliability tests. 8) Conduct Mechanical Shock tests, record and collate data 9) Conduct Accelerated Thermal Cycling tests, record and collate data 10) Conduct Vibration Tests, record and collate data 11) Conduct Transient Bend Test, record and collate data 12) Conduct Plug/Un-plug test, record and collate data 13) Identify a Product Board for Validation, build small samples of such Product Boards, conduct Product Reliability Tests, collect and collate resulting data 14) Analyze results, write and publish findings to inemi members. This will be done by a subset of the team members. Page 8 of 10
Project Schedule: 2015 2016 2017 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Segment Steps Task Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Project Initiation Define Scope Define Resources Needed 1 Define Test Vehicles Select Solder paste Materials Draft and Ratify SOW Finalize Member Sign-ups and Check Point #1 Resource Commitments Check Point #2 Design Test Vehicles Order PCBs Draft Experimental Plans for 2 Process Development Receive All Components Execute Process Development Plans and Collect Data Preliminary Report End Point Build Mechanical Shock test 3 Boards and Conduct Mechanical Shock Test Mechanical Shock Results Report 4 Accl. Temperature Cycling Other Rel tests 5 Product Validation Phase Completion 6 Final Report External Publication Definition Execution Project monitoring plans There will be open lines of communication among participants. All project requirements and commitments will be reviewed with participants before the project begins. Project participants will meet bi-weekly to review various aspects of the project and make plans for next phases of the project. There will be one meeting each at an Asia region friendly time and one meeting at a North America region friendly time. All Designs of experiments and the resource allocation will be submitted to the inemi Technical Committee for review to make sure the team has an appropriate scope and sufficient resource commitment to accomplish the planned tests. Minutes of each meeting minutes will be provided through e-mail to the participants. A 1:1 follow-up with individuals will be done on an as-needed basis. Workshops and face-to-face meetings will be scheduled as determined by the project team. Progress reports, comprising of a short series of PowerPoint slides showing the work done to date, will be provided upon request for presentation at regularly scheduled inemi meetings. The approximate man-months per quarter per team member will be tracked based on input from team members. The approximate number of people on the project per quarter will be tracked through inemi's WebEx account. Page 9 of 10
Outcome of the project Successful completion of this project will include the publication and presentation in the public domain of the knowledge acquired from the activities of this project on the low-temperature BiSn solder paste technology investigated. Deliverables of this project include the following: Webinar and associated slides for project members summarizing the data and conclusions from the project activities on the relevant low-temperature materials and processes. Final Report containing all the data, its analysis, and conclusions about the viability of the lowtemperature soldering materials and processes investigated. Project results will be shared with the industry in order to drive alignment amongst the supply chain and the users. Knowledge assessment results will be shared through presentations at leading industry conferences and journals, subject to group participant approval process. IF existing standards warrant updating, this will be done through the relevant standards publication entities. NOTE: All changes to SOW must be approved by the TC (version control) Page 10 of 10