How to Develop a Qualification Test Plan for RoHS Products Mike Silverman, Ops A La Carte LLC Fred Schenkelberg, Ops A La Carte LLC Craig Hillman, PhD, DfR Solutions Key Words: RoHS, qualification, underplate, delamination SUMMARY & CONCLUSIONS The subject matter we consider in this paper are the significant reliability uncertainties around Lead-Free Solder and how to best consider these risks and mitigate them so as not to take a hit in the area of reliability during the lead-free transition. Like the rest of the electronics industry, your products will transition to Restriction of Hazardous Substances (RoHS) compliance. This includes the transition to Lead-Free Solder, and at this time, there are significant reliability uncertainties around Lead-Free Solder. Even if your product does not need to be compliant, the materials and processes that make up your product are changing. During this time of rapid transition, there is a significant new body of knowledge to understand to determine the areas of greatest risk to the reliability of your product. In this paper, we will highlight a few of these significant risk areas and how to best mitigate these risks during the transition. 1 INTRODUCTION Lead-free solder materials have been used in the electronics industry for over 60 years and therefore the processing conditions and their impact on materials and reliability are well understood. Converting to lead-free products and processes introduces many risks some of which are better understood than others. Even though the deadline of July 1, 2006 [1] came and went, many companies are still struggling with this issue and are either still trying to become compliant, or have developed substandard methodologies to meet the deadline, only to find out that they created a ticking time bomb of reliability that is just waiting to go off. This paper explores the main risks involved and shows how to evaluate the risks and set up qualification programs to mitigate these risks. If these steps are taken, the reliability of lead-free products and processes can yield products just as reliable as their leaded counterparts. 1.1 Nomenclature RoHS: Restriction of Hazardous Substances Sn: Tin Pb: Lead SnPb: Tin-Lead formulation of solder MSL: Moisture Sensitivity Level Pb-Free or Lead-Free: Part of the RoHS directive [1] is to restrict the use of lead on electronic systems. 2 LEAD-FREE RISKS SnPb solder materials have been used in the electronics industry for over 60 years and therefore the processing conditions and their impact on materials and reliability are well understood. Converting to Pb-free materials and processes introduces many risks some of which are better understood than others. The following table describes some of the changes inherent with Pb-free, the failure mechanisms they may induce and the testing/inspection procedures used to screen for them. The test criteria called out in this Pb-free qualification document are intended to screen for many of these potential failure mechanisms. Area of Concern Moisture sensitivity Heat damage Impacted Item Failure Mechanism Plastic IC packages, optocouplers, other polymer based components All passive components, circuit boards Popcorn delamination at higher reflow temperatures. Heat damage of IC packages Cracking, dielectric breakdown (capacitors), PCB delamination, warping, or via cracking Cold joints or weak joints fracture in use environment Cracked solder joints Poor wetting All solder joints Solder fatigue Solder joints, particularly on high CTE components Solder joints particularly on higher mass components Sn and SnCu plated components Insufficient process window creates poor solder joints Poor solder joints, damaged components Incomplete hole fill, fillet lifting, damage to board Board surface with noclean paste residue Mechanical shock Sn whiskers Surface mount process control Rework process Wave solder process Electrochemical migration Solder joint failure during shipping or dropping Shorting Occasional solder joint failures in use environment Joint failures or cracked vias in use environment Failed through hole, cracked vias, weak joints Shorting between biased traces in a moist environment Table 1a – Pb-Free Risk Table, Failure Mechanisms Area of Concern Moisture sensitivity Heat damage Poor wetting Solder fatigue Mechanical shock Sn whiskers Surface mount process control Rework process Wave solder process Electrochemic al migration Testing Method Moisture sensitivity testing. J-STD-020C Heat resistance MIL-STD 202G #210F Decomposition temp. Time to delamination Package planarity JESD22-B108A Solderability J-STD-002B J-STD-003A Thermal cycle JESD22-A104-B, HALT Vibration Shock test NEMI / JEITA recommended procedures Precondition and assembly JEDEC Standard 22A113D Rework components followed by reliability testing Thermal cycle HALT Vibration Bellcore GR-78CORE J-STD-004 SIR Inspection Technique Visual inspection C-SAM Visual inspection, functional verification Wetting balance, visual inspection, x-sectioning, lead pull Electrical continuity Visual inspection Electrical continuity Visual inspection SEM X-ray, X-section, Inspection, reliability test X-ray, X-section, Inspection, reliability test Electrical continuity, visual inspection Visual and resistance after 35C/85%RH exposure at 50V Table 1b – Pb-Free Risk Table, Inspection Methods Note: For the board in this study, solder fatigue and mechanical shock do not seem to have a significant risk factor and are not recommended for evaluation. This is based on the assumption of adequate packaging during transport and the stable operating temperature (very little if any thermal cycling during use). 2.1 The Approach The basic approach is to verify adequate qualification of the individual components has occurred. By requesting and reviewing detailed vendor data most of the risk for your product is either identified or reduced. For example, if a vendor is unable to produce or has inadequate MSL rating test results, then the risk for popcorning defects is present. With engineering judgment, considering the processing parameters, we can decide to conduct proper MSL evaluation to determine true risk of using the component. 2.2 Qualification Requirements – First Level This level is reserved for products with lower perceived risk of failure due to introduction of lead-free materials and processing. Products determined to be in this level are relatively simple and constructed exclusively with components such as passives, through-hole and/or coarse pitch (>0.5 mm) surface mount leaded packages. Some exceptions may apply based on specific product application or use environment. To ensure that all materials can survive the elevated temperatures expected with lead free assembly, all components must be evaluated individually prior to assembly. Types of failure mechanisms being screened for include heat damage, moisture induced cracking/delamination, poor solderability, and weak joints. The following information permits the identification of specific risks and creates a baseline of information on Pb-free assemblies and processes. Materials Used • • • • • • • • • • • • • • • • PCB type PCB Manufacturer PCBA Assembler (list if sub contracted, In House, sub supplier) PCB glass transition temperature PCB decomposition temperature PCB manufacturer certified heat resistance PCB Thickness PCB # Layers 1 or 2 side populated Pad finish type (i.e.ImAg, OSP, etc.) List the surface mount lead-free alloy (i.e. Sn-3.5Ag0.9Cu) Solder paste manufacturer and product # (list all suppliers) SIR test results from solder paste supplier. Flux type (no clean, water soluble, etc.) Wave solder Pb-free alloy Hand Solder / Rework (Wire) Pb free alloy Process Information The information requested below is important to both supplier quality and reliability engineering in relation to lead free process and reliability impact. The questions are meant to establish a baseline for these items relative to initial lead free process management. The vendor must maintain a consistent process going forward or inform customer of changes. 1. Is the product built with a single or dual reflow process? 2. List the peak temperature distribution across the PCBA/Panel. 3. Time within 5C of peak. 4. List the time above liquidus temperature. 5. Provide time / temperature reflow profile. Provide location on PCB / Panel for thermocouple probes (attach picture / diagram) and temperatures for those locations during the reflow process. 6. List the minimum peak solder joint temp measured on board (under highest thermal mass component). 7. Wave solder process flow and maximum solder pot temperature / duration (if applicable). 8. Soldering iron temperature (Temp +/-) for rework and hand solder. Maximum allowable hand-solder duration. 9. 10. 11. 12. 13. Provide general overview of part storage and factory floor management for components according to MSL level (for moisture sensitive components). Detailed part management is subject to on site audit. Is nitrogen used in reflow? Procedures for rework to include inspection criteria and soldering iron temperature. Inspection criteria used for lead-free solder joints. This will include criteria for sub contracted assemblies. Provide general overview regarding isolation and tracking of leaded components / materials from Pb-free components / materials. Detailed part management is subject to on site audit. Component Information – Heat Resistance and not acceptable). Lead plating situations as outlined in table 1 will require testing according to NEMI/JEITA recommended procedures: 1) Storing at 60°C/95%RH for 1000 hours note 3 & 4 followed by SEM analysis; 2) Thermal cycling 1000 times from -55°C/85°C note2 followed by SEM analysis; and 3) Store at room atmosphere conditions for 1000 hours note 3 & 4 followed by SEM analysis. Criteria: Maximum allowable whisker length is 50 microns (separate criteria for FFC/FPC/Connector Mating). NOTES: 1. 2. All components used to build a Pb-free product must be rated for temperatures at least 10°C higher than peak assembly process temperature. Heat resistance testing should be performed following MIL-STD 202G #210F with 90-120sec above Pb-free solder liquidus with ≥ 10 seconds at or above peak (+10C). Deviation for time above liquidus may be allowed based on process TAL (must be a minimum of 20% greater than process TAL). MIL-STD-202G #210F should be followed for wave solder and soldering iron heat resistance. Components that are hand soldered, reworked, or touched up should be rated for a soldering iron temperature at least 10°C higher than process conditions. Recommended min sample size is 10/lot for 3 lots. 5. 6. Component Information – Moisture Sensitivity 7. Determining the moisture level for surface mount components should be done following J-STD-020C for Pbfree or JEITA ED 4701 (Test Method 301A for Pb-Free). Components qualified to J-STD-020B may be acceptable if temperature rating is deemed sufficient. Sample sizes are defined in the specifications as well as inspection and pass/fail criteria. A minimum of 3 reflows is required. A minimum of 60 seconds above liquidus or duration 20% higher than actual reflow process time above liquidus (whichever is longer) is required. SMT type components that are wave soldered will follow procedures detailed in JEITA ED 4701 (Test Method 301A for Pb-Free). A minimum of Level 3 (JEDEC) or Level E (JEITA) is required for all components. Level 4 components (JEDEC) or Level F/G (JEITA) may be approved if factory management is considered acceptable. Components that do meet minimum of Level 4 or above are not acceptable. 3. 4. 8. 9. 10. Lead Plating The Pb-free lead plating material is important in evaluating the risk for tin whiskers. Table 1 outlines requirements for Tin Whisker testing for plating materials determined to be a risk. Finer pitch components plated with Sn based lead finish are most susceptible to shorting due to whisker growth. A 1.3 μm nickel underplate is preferred for Sn finishes as this prevents copper diffusion into the Sn which contributes to compressive stress in the Sn layer (the primary driving force for Sn whiskers for Cu base material). SnCu plating is known to be a high risk for Sn whisker growth and should be avoided when possible (bright Sn is the highest risk 11. 12. 13. Sample size: 10 per condition. Samples should be taken from 3 lots, divided equally between each condition (total samples 30). Recommended thermal cycle condition: 20C/min, 10 min dwell at each temperature. For Cu base material, testing duration for high reliability application will be 4000 hours for ambient and temperature / humidity (non-FFC/FPC/Connector mating). Whisker evaluation will need to take place every 1000 hours. Testing duration for high reliability application will be for 2000 hours for ambient and temperature / humidity (FFC/FPC/Connector Mating). Whisker evaluation will take place after 2000 hours only. Measurement to be by SEM following NEMI guideline. Measurement method per NEMI (full length of whisker must be measured). Full scan of each pin needs to be performed to find longest whiskers, while more detailed scan to be performed on a minimum of 10 pins per device. Detailed scan must be measured using SEM at 400X minimum, with magnification up to 4000x for whisker verification. Sampling of pins needs to be from each side. Parts that are reflowed through manufacturing process need to be reflowed through equivalent profile (nonsoldered) prior to Tin Whisker testing. Profile to be provided. Test details (including any deviations) and reporting requirement must include SEM images, number of pins measured per device, inspection method, whisker length and count reporting. Representative photos of longest whiskers observed to be provided in a clear reporting format. Testing is to be performed on the component part number to be used. Testing is required for each manufacturing location / process used. In addition to whisker length reporting, minimum reporting must also include part number(s) tested, base material, manufacturing location, manufacturing date, lot # (s), test date(s), plating type (% each element), thickness, and underplate thickness (if applicable). Specify whether parts are annealed (including condition). Post production annealing of 150C for 1 hour is preferred. All Components EXCEPT: FFC/FPC/Connector Mating End Finish Lead Pitch Lead Pitch Comments > 0.5mm ≤ 0.5mm Acceptable Acceptable Sn/Ni (>1.3 μm Ni) Acceptable Acceptable SnBi (1-4%Bi) Acceptable Acceptable SnAgCu & SnAg Sn (matte) Acceptable Testing Reflow or annealing Required may help reduce Sn whisker density (conflicting industry data). Acceptable Testing For high reliability SnCu Required applications, testing may be required for >0.5mm pitch Sn (bright) Unacceptable Unacceptable Semi-bright Sn should be treated similar to SnCu FFC/FPC/Connector Mating End ONLY Finish Min Min Comments Conductor Conductor Spacing Spacing >270um ≤270um Acceptable Testing Best tin based solution Sn/Ni (>1.3 μm Ni) Required Acceptable Testing Nickel Underplate SnBi (1-4%Bi) Required preferred SnAgCu & SnAg Acceptable Testing Required Nickel Underplate preferred Sn (matte) Acceptable Unacceptable Testing Required Unacceptable Unacceptable Unacceptable Nickel Underplate preferred Nickel underplate required note g Semi-bright Sn will not be acceptable SnCu Sn (bright) Criteria Note g Whisker length Criteria Min Conductor Spacing ≤140um Min Conductor Spacing >140um Max Whisker Length 20um Max Whisker Length 50um Notes: (a) Whisker testing includes both mating and PCB mounting end for connector (b) Conductor spacing is the minimum tolerance, not nominal value. Supplier specification must be provided that shows key dimensions including conductor spacing (nominal +/- tolerance). (c) Testing will be performed mated to the connector used in application. Both FPC/FFC and connector (if applicable) will be evaluated for tin whisker. A matrix shall be provided showing connector / FFC (FPC) combination tested. (d) Gold / Nickel Underplate is preferred for high reliability applications (e) Where possible, minimum spacing should be increased to mitigate whisker risk. (f) High Reliability applications may require testing for spacing above 270um for any Sn based plating type. (g) Will be allowed on exception basis only. Exception will be based on product application and risk. In addition these factors, minimum conductor spacing must be >270um. Table 2 – Tin Whisker Test Matrix Sample Distribution 1. Full sample size must be provided from each manufacturing location and solder supplier used. This includes sub supplier qualification. 2. 3. 4. 5. Full or partial sample for each bare PCB supplier will be required. Agreement regarding sample size and distribution will be made prior to qualification start. If different manufacturing lines are used (at time of evaluation), then sampling must be from each line. Sampling rate will be determined prior to evaluation. Distribution of key components from each manufacturer used. Key component(s) and build mix will be determined prior to evaluation. A control group will be required for comparison purposes. This control group may consist of the same product assembled with SnPb solder or previous generation of SnPb product of same complexity. 2.3 Qualification Requirements – Second level This level is reserved for products with moderate perceived risk of failure due to introduction of lead-free materials and processing. Products determined to be in this level are more complex and will likely have one or more of the following surface mounted components: plastic leadless packages, fine pitch QFPs (≤0.5 mm lead pitch), plastic ball grid arrays (≤ 27 mm body size) or chip scale packages with ball pitches ≥1mm (ball pitch of less than 1mm is generally require solder fatigue evaluation). In addition to the Level 1 requirements of proving process capability and functionality after assembly, this level of qualification requires testing for mechanical and thermally induced fatigue failure. This Level requires that the product also pass qualification Level 1. 2.3.1 Precondition / Assembly Assembly of product/test boards should be done at optimum process conditions (including wave or hand solder when applicable) followed by reworking of predetermined components on some boards. In some cases exploring additional assembly conditions may be required. For example, preconditioning components and boards prior to assembly is an effective way to prove that worst-case assembly conditions still produce reliable products (i.e. a sufficiently wide process window exists). A test plan specific to each product will be developed and sample sizes for each assembly condition agreed to prior to testing. 1. Minimum sample sizes for each condition are outlined in Table 2 for prime and rework. Final sample size is will depend on testing performed. 2. Assemble components onto boards through the optimum lead free soldering process. 3. Perform rework of selected components. 4. Prove each component (passive and active) meets electrical performance specifications either individually or by functionally testing the entire board. 5. In cases where predetermined IC packages are selected for preconditioning, follow JEDEC Standard 22-A113D and expose to level 3 moisture conditions. Preconditioning boards prior to assembly should take place per J-STD-003A (8 hr steam age for Sn plated finish, 35C/85% for 24 hours for OSP finish, and 85°C/85%RH for 12 hours for immersion Ag). 6. Perform C-SAM and X-ray on key IC package components after assembly and rework if required. Minimum sample sizes are shown in Table 2. 7. Visually inspect (with up to 40x magnification) all components on boards for defects (cracking, delamination, etc.) and record all observations. Provide component inspection standards (i.e. pass/fail criteria) used for this inspection. 8. Visually inspect (with up to 40x magnification) solder joints for defects. Inspection standard IPC-A-610D. 9. To establish a baseline prior to reliability testing, major components on at least PCBA should be cross sectioned after assembly while those on another PCBA should be subjected to Die & Pry. Inner-metallic layer thickness should be in the range of 1-2um. 10. Criteria: Component delamination from C-SAM must meet criteria according to J-STD-020C. Zero electrical failures are allowed. No critical soldering defects (voiding should be less than 25% of joint area as revealed in X-ray inspection). 11. Use these assembled boards for reliability tests according to test plan. 12. Test and failure analysis results will be provided 2.3.2 Vibration and Shock Concurrent testing for both vibration and shock shall be performed when determined to be necessary. Testing details will be prescribed according to the product type and expected use environment. Typical tests may include non-operational vibration followed by shipping shock (pack drop). In some instances mechanical shock and vibration testing may be preceded by thermal shock. Operational testing in a system level may also be performed. Sample sizes and pass/fail criteria will be determined during creation of the detailed test plan (sample size will typically be 5 or greater for each test type). 2.3.3 Thermal Cycling This testing is required primarily for level 2 components. Follow procedures described in JESD22-A104-B, condition J. The specific requirements described below fit many product types, however, alternative thermal cycling test protocols and evaluation plans are potentially acceptable. 1. Sample size ≥ 20. A separate group of an additional 10 reworked components will also undergo thermal cycle (if applicable). 2. The complete functional PCBA must be thermal cycle tested. 3. Cycle 1000 times from 0 to 100°C with a ramp rate of 1020°C/min and a 10 min dwell time (measure temperature on the largest thermal mass component on the PCB). 4. Visually inspect joints and confirm functionality of complete PCBAs after 500 and 1000 cycles. 5. In cases where proving electrical functionality is not feasible, solder joints on BGA components can be evaluated after 1000 cycles using dye and pry. 6. Criteria: Zero solder joint failures accepted unless product 7. life requirements allow for a failure. a. A failure is defined by a functional test error or a joint that is cracked 50% through (as revealed by dye and pry). b. Visual inspection and functional test specifics to be agreed upon prior to commencement of test. Additional evaluation may include cross sectioning, lead pull, and component shear. Issues found during additional evaluation, including visual inspection, will be reviewed prior to determination of product acceptability. Test and failure analysis results will be provided. Solder joint failures will be assessed and corrective actions taken. 2.3.4 Highly Accelerated Life Testing (HALT) Listed below are minimum acceptable test conditions according to current practice. 1. HALT is primarily a board level test that must be performed within a multi-stress (temperature and vibration) chamber. 2. During the HALT process, thermal cycling and vibration are to be simultaneously applied. 3. The temperature responses on critical components must be monitored with thermocouples to insure adequacy of the dwell periods selected. 4. The temperature range (between highest and lowest dwell temperatures) is to be a minimum of 80-degrees C unless otherwise technology limited. 5. The product is to be functionally operational and monitored during “HALT” stressing. 6. Sample size preferred is 2 units. 7. Where a previously established baseline is available, the product must meet or exceed prior limits. Where no prior baseline is established, comparative results to leaded control sample must be met. 8. Supplier conducted HALT must include reporting test results according to agreed upon format, including full failure analysis and corrective action on anomalies observed. 9. Criteria: Component delamination must meet criteria according to J-STD-020C. Zero functional failures, or fully cracked solder joints. 3 CONCLUSION Converting to lead-free products and processes introduces many risks some of which are better understood than others. This paper has explored the main risks involved and showed how to evaluate the risks and set up qualification programs to mitigate these risks. If these steps are taken, the reliability of lead-free products and processes can yield products just as reliable as their leaded counterparts. REFERENCES 1. Statuatory Instrument 2006 No. 1463, Environmental Protection, The Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment Regulations, 2006. BIOGRAPHIES Mike Silverman, CRE Ops A La Carte, LLC 20151 Guava Court Saratoga, CA 95070 USA e-mail: [email protected] Mike is founder and managing partner at Ops A La Carte, a Professional Business Operations Company that offers a broad array of expert services in support of new product development and production initiatives. The primary set of services currently being offered are in the area of reliability. Through Ops A La Carte, Mike has had extensive experience as a consultant to high-tech companies, and has consulted for over 200 companies including Cisco, Ciena, Apple, Siemens, Intuitive Surgical, Abbott Labs, and Applied Materials. He has consulted in a variety of different industries including telecommunications, networking, medical, semiconductor, semiconductor equipment, consumer electronics, and defense electronics. Mike has 25 years of reliability, quality, and compliance experience, the majority in start-up companies. He is also an expert in accelerated reliability techniques, including HALT and HASS. He set up and ran an accelerated reliability test lab for 5 years, testing over 300 products for 100 companies in 40 different industries. Mike has authored and published 8 papers on reliability techniques and has presented these around the world including China, Germany, and Canada. He has also developed and currently teaches 8 courses on reliability techniques. Mike has a BS degree in Electrical and Computer Engineering from the University of Colorado at Boulder, and is both a Certified Reliability Engineer and a course instructor through the American Society for Quality (ASQ), IEEE, and Effective Training Associates. Mike is a member of ASQ, IEEE, SME, ASME, PATCA, and IEEE Consulting Society. Currently he is the IEEE Reliability Society Santa Clara Valley Chapter Chair and the IEEE Consultants’ Society Santa Clara Valley Chapter Vice Chair. Fred Schenkelberg, CRE, CQE Ops A La Carte, LLC 20151 Guava Court Saratoga, CA 95070 USA e-mail: [email protected] Fred Schenkelberg is a Senior Reliability Engineering Consultant at Ops A La Carte. He is currently working with clients using reliability assessments as a starting point to develop and execute detailed reliability plans and programs. Also, he exercises his reliability engineering and statistical knowledge to design and conduct accelerated life tests. Fred has conducted over 75 assessments in 20 different industries around the world. Fred has an excellent knowledge in reliability techniques across the entire product life cycle and has specific expertise in Accelerated Life Testing (ALT), Restriction of Hazardous Substances (RoHS/WEEE/Lead-Free Compliance), and Warranty Analysis/Improvement. Fred has developed and performed Accelerated Life Tests for Mechanical, Electrical, Chemical, and a combination of each of these types of products for over 50 different companies in 20 different industries. Fred sat on several national workshops for 2 years and has developed a methodology for creating RoHS-specific tailored Reliability Test Plans. Fred copresented with Eric Arnum, editor of “Warranty Week Magazine” and has chaired sessions at the Warranty Chain Management Symposium for the past 2 years. Fred is also an accomplished trainer, educator, facilitator, mentor, and coach. He has developed training courses in over a dozen different disciplines within reliability and has trained thousands of engineers and managers in a variety of reliability topics in various industries. Fred joined HP in February 1996 in Vancouver, WA. He joined HP’s ESTC Group in Palo Alto, CA., in January 1998 and co-founded the HP Product Reliability Team. He was responsible for the community building, consulting and training aspects of the Product Reliability Program. He was also responsible for research and development on selected product reliability management topics. Prior to joining HP ESTC, he worked as a design for manufacturing engineer on DeskJet printers. Before HP he worked with Raychem Corporation in various positions, including research and development of accelerated life testing of polymer based heating cables. He has a Bachelors of Science in Physics from the United States Military Academy and a Masters of Science in Statistics from Stanford University. Fred has been an active member of the RAMS Mgmt. Committee for 4 years, on the ASQ National Committee for 2 years, and an officer with IEEE Reliability Society Santa Clara Chapter for 5 years. Craig Hillman, PhD DfR Solutions 5110 Roanoke Pl. Ste. 101 College Park, MD 20740 USA e-mail: [email protected] Dr. Hillman‘s specialties include best practices in Design for Reliability (DfR), strategies for transitioning to Pb-free, supplier qualification (commodity and engineered products), passive component technology (capacitors, resistors, etc.), and printed board failure mechanisms. Dr. Hillman has over 40 publications and has presented on a wide variety of reliability issues to over 200 companies and organizations. Dr. Hillman received his Post-doctoral fellowship from Cambridge University, his PhD in Materials from University of California Santa Barbara, and his B.S. in Metallurgical Engineering and Material Science from Carnegie Mellon.