Leveraging Existing Market Knowledge to Ensure a Successful Transition to Pb-Free Medical Products Randy Schueller, Ph.D. & Cheryl Tulkoff DfR Solutions Outline Intro Medical Electronics Testing Reliability Testing to Identify Common Pbfree issues.
Medical Electronics Very diverse! What are medical electronics? Is it a realistic category? Some implanted in the body; some outside Some portable; some fixed Some complex; some simple Some control; some monitor; some medicate All connected by the perception that one s life may be dependent upon this product Creates a powerful emotional attachment/effect Assuring reliability becomes critical
Pb-Free Medical Electronics A main question is What can we learn from other Pb-free electronics that will help us most effectively test medical electronic devices as we transition to Pb-free? Types of Testing First we should understand that there are different types of testing Feasibility (or Functional) Testing V&V: Validation & Verification Production Testing Reliability Testing Safety / Regulatory Testing
Feasibility or Functional Testing Feasibility Testing Functional testing confirm that design meets basic performance requirements Is it possible? Proof of concept Does it work Failures undesirable V & V Testing V&V: Validation & Verification Conformance to specifications & standards Industry standards like IPC, JEDEC, ISO, FDA, IEC Environmental Testing Failures Undesirable
Production Testing Production Testing Statistical Optimize design & manufacturing Failures undesirable Reliability Testing (our focus) Two primary objectives: 1. Prove the product can withstand the environment it is going into. ATC, S&V, THB, Heat age, etc. 2. Reveal any weaknesses (in design or process) so risks can be accessed and improvements made. HALT, ESS, Analytical techniques Some failures are expected.
Safety Testing Safety / Regulatory May overlap with some others Some fails may be desirable Varies based on industry Key Elements of a Product Reliability Plan Reliability Requirement & Targets Reliability Organization Structure Reliability Activities (Reports, Tests, Analyses) Schedule Supply chain management /oversight Listing of relevant standards, specifications, procedures
General Reliability Management Needs Create & work to reliability plan Define and Identify external services Test Failure Analysis Reliability Training How to ID the Best Reliability Tests Key Points: Must test at increased stresses, not actual expected stresses, to create failures then use this information to improve reliability Only true upper stress limits for reliability testing are test equipment capability & technology limits (solder melt points, Tg of polymers, etc.) Should not drive failure mechanisms not possible in the field. Should target certain failure mechanisms (but unexpected failure mechanisms should be investigated). Have ability to generate a failure distribution function.
How to Create Reliability Tests General Reliability Testing Approach Perform FMECA (Failure Modes, Effects & Criticality Analysis) / QFD (quality functional deployment) to determine likely service fails Identify stressors Plan to simulate stressors in test Determine methods to identify failures (x-ray, x- section, dye-n-pry, etc.) What are the Pb-Free Failure Mechanisms that should be Stressed? Pb-free issues can be sorted by the product design, material selection and expected user environment. The following information may be included in a product FMEA.
Medical Product with BGAs A product with BGAs (incl CSPs & FC) should be concerned with: Head-in-pillow defects Pad cratering Achieving optimum reflow temperature Testing & Analysis Ball pull or board bend testing Evaluate assembly process using x- sectioning, optical microscopy along edges, etc. Head-on-Pillow Defects HnP joints have become significantly more prevalent since Pb-free (many field failures are being reported). A HnP joint will pass X-ray inspection, in-circuit testing, and functional testing. Failures occur in the field after large volumes of products have been shipped (worst case situation).
What is the Root Cause mechanism? HnP occurs when the flux on the exterior of the paste dries out prior to reaching liquidus temperature. In the case of warping, the paste and ball are not in contact at liquidus and come back together upon cooling. Result is oxidized surfaces that prevent intermixing. The higher temperatures required for lead-free assembly exacerbates both causes. What variables impact HnP? Supplier Issues: Flux activity, slump resistance, tackiness. Sphere oxidation IC package warpage Process Issues Poor paste printing (insufficient volume). Component placement (insufficient pressure, off pad). Reflow Too much time in preheat. Insufficient time above liquidus (TAL). Oxygen content in reflow too high
Detection X-ray can detect gross examples Cross sectioning is most effective. Pad Cratering SAC solder is less compliant than SnPb so tensile stress is transferred to the laminate.
Laminate Cracking Leads to Trace Fracture Trace routed externally Functional failure will occur Bending Force IPC 9708 Ball Pull Test Quick test after BGA ball attach No expensive pins required Almost as sensitive as pin pull o BGAs only o Highly dependent on solder ball so process control is critical
IPC 9702- Bend Test Used to characterize fracture strength of board level interconnects Failure modes from this test are not easily differentiated High speed test Short duration Failures in quick succession HALT Testing is also effective in reproducing pad crater defects.
BGA Visual Inspection BGA (Ball Grid Array) Perimeter Inspection Use of optical fiber to inspect solder balls on the perimeter of the package Most common failure site under BGAs Magnification: 200x 3D X-Ray with m-ct Inspection Option CT models for 3D sample analysis, virtual micro-sectioning and internal dimensional measurements for crack, void and reverse engineering Potentially reduce the number of timeconsuming microsection analyses that are needed Non-destructive
Hand Held Medical Products Expected failure mechanisms include: Shock damage to BGAs or other high stress solder joints (pad cratering or SJ failure - especially with ENIG surface finish). Thermal cycle failure also a possibility. Tests and Analysis Stressors are shock testing or board bend testing. Analyze failures with x-section and dye-n-pry. Brittle failure with SAC on ENIG SAC solder with ENIG surface finish can result in brittle failure at the intermetallic layer. JEDEC (JESD22- B111) standard testing 1500 G s, 0.5 ms pulse width Board Level Drop Test Reliability of IC Packages, Chai TC, et.al., Institute of Microelectronics
Thermal Cycling Hand held products will experience indoor/outdoor temperature swings. The number of cycles should be projected (near worst case) and an ATC plan developed. Analyze for solder joint cracking with x- section and dye-n-pry. Does the Medical Product contain Fine pitch or Flex Circuit Components? Tin whiskers is the primary concern. Ensure appropriate mitigating measures are being taken
Avoid Bright Tin (shells & shields) Whiskers also found to grow in screw holes. Ref: L. Flasche & T. Munsun, Foresite, Inc. 9/09. Ref: Emerson D-Sub Connectors with bright tin shells have been known to grow whiskers that can short our pins (if connector is unmated). Contact Pressure on Flex Cables Flex Circuits with Connector Mating Pressure from contacts with the soft polymer substrate produces whiskers. Don t use Sn plating in mated flex with a spacing less than 200 micrometers. Use gold plating with such conditions.
High humidity environments Risk is metal migration due to flux residue or cleanliness issues (flux can bake onto the board and is more difficult to clean off). Test with THB (example 130C/85%RH/Bias) Measure ionic cleanliness Example of Dendritic Growth Elapsed time 12 sec. *From Contamination Studies Laboratory, Inc., http://www.residues.com
Flux Residues Residues of no-clean soldering? Water-soluble dicarboxylic acids Hygroscopic polyethylene glycol ethers List of potential weak organic acids (WOAs) Benzoic, Butyric, Formic, Lactic, Malonic, Oxalic, Propionic, Succinic, Citric, Glutaric, Adipic, Malic Optimum flux Acids are neutralized after soldering process Residual wetting agents are minimized Cleanliness Controls: Ion Chromatography Contamination tends to be controlled through industrial specifications (IPC-6012, J-STD-001) Primarily based on original military specification 10 g/in 2 of NaCl equivalent Calculated to result in 2 megaohm surface insulation resistance (SIR) Not necessarily best practice Best practice is contamination controlled through ion chromatography (IC) testing IPC-TM-650, Method 2.3.28A Pauls General Electric NDCEE DoD* IPC* ACI Chloride ( g/in 2 ) 2 3.5 4.5 6.1 6.1 10 Bromide ( g/in 2 ) 20 10 15 7.8 7.8 15 *Based on R/O/I testing
Sources of Contaminants (cont.) Ion Cl Br Fl PO 4 SO 4 NO 4 Weak Organic Acids Possible Sources Board Fab, Solder Flux, Rinse Water, Handling Printed Board (flame retardants), HASL Flux Teflon, Kapton Cleaners, Red Phosphorus Rinse Water, Air Pollution, Papers/ Plastics Rinse Water Solder Flux High Temperature Products Medical equipment that runs hot will require forced air cooling. If Immersion silver surface finish is used then creep corrosion should be evaluated with corrosion testing. Plated through-hole vias can fracture so should be tested (IST, Thermal cycle, etc.)
Creep Corrosion of ImAg Copper sulfide grows in the presence of sulfur compounds. Greatly enhanced with airflow bringing in S containing gasses and particulates Summary Reliability testing of Pb-free products should take into consideration the various defects and failure mechanisms that have been associated with this change. Stress tests and failure analysis techniques should be designed to explore these known mechanisms to ensure sufficient robustness is designed into the products.
THANK YOU! Any Questions? rschueller@dfrsolutions.com