Managing Temperature Sensitive. Components in Pb-free Assemblies. Server and Technology Group

Transcription

1 Managing Temperature Sensitive Components in Pb-free Assemblies J.R. Wilcox, C.T. Grosskopf, M.S. Kelly, M.S. Cole IBM Integrated Supply Chain

2 Pb-free Solder Assembly at IBM IBM Server team... designs and assembles complex PCBAs for server class products; systematically transitioning to Pb-free assembly.... has developed rigorous practices for ensuring extended field performance and reliability of Pb-free assemblies.... expects its suppliers to take equal care in designing and manufacturing the components and sub-assemblies provided for use in IBM systems.... shares best practices learned from our Pb-free assembly experience IBM Power Symposium, RTP, NC

3 Topics Component Process Sensitivity Industry Specifications J-STD-075 Evaluation and Classification Using Sensitivity Data in Assembly Process Definition Post Solder Attach Temperature Sensitivity Take home messages IBM Power Symposium, RTP, NC

4 IBM ES 44V5066 for Power Assemblies DOES: Define allowable RoHS material selections (through 97P3864). Define minimum qualification testing for IBM Power Assemblies. Establish minimum functional reliability through accelerated stress testing: ATC (-40C / 90C / 1cph) 750 cycles [not powered] TH&B (85C / 80RH / 15V) 500 hours [not powered] Random Vibration (2G (rms) /10 min) and up [powered] Require product functional survival of 5X reflow cycles.! DOES NOT: Identify procedures for proper definition of a Pb-free solder assembly process. Classify process sensitivity of the assembled power product IBM Power Symposium, RTP, NC

5 Elevated Solder Processing Temperatures Peak Temperature o C SnPb SnAgCu C 217C SnAgCu melting pt. Full Pb-free assembly drives a minimum 230C solder joint. PCB and attached components reach higher temperatures: 245C 260C. 230C SnAgCu solder joint temperature Process Windows SnPb SnAgCu C SnPb melting pt. PTH & REWORK SMT REWORK SMT REFLOW IBM Power Symposium, RTP, NC

6 Temperature Sensitive Devices Temperature sensitive device families currently acknowledged by the electronics industry: aluminum and polymer aluminum capacitors film capacitors molded tantalum capacitors (polymer & fused) (exception: standard MnO 2 type non-fused are not sensitive) fuses inductors and transformers with wire coils non-solid state relays LEDs (Light Emitting Diodes) IBM Power Symposium, RTP, NC

7 Component Process Sensitivities The more common process sensitivities encountered today are limitations with respect to: pre-heat time and T time above liquidus (217C) peak reflow temperature time within 5C of peak T number of reflow passes flux exposure cleaning processes SMT Temperature Sensitivities (solder reflow) Other Process Sensitivities wave solder pot temperature preheat temperature solder contact time Temperature Sensitivities (post solder process) encapsulant cure moisture bake IBM Power Symposium, RTP, NC

8 Process Sensitive Devices Overheating process sensitive devices during SMT reflow or PTH wave soldering can result in serious reliability failures: Immediate electrical or catastrophic fails Early fails in the life of the electronic product. Examples. If thermally overstressed, Aluminum and polymer aluminum capacitors may exhibit bulging or venting of the metal can; Fuses can experience reduction in the maximum limiting current of the fuse element (nuisance blows). Inductors and transformers with coils can experience degradation of the lacquer coating of the coil wire, leading to intermittent shorts IBM Power Symposium, RTP, NC

9 Temperature Sensitive Components The negative consequences of over-temperature exposure varies. Component Type SMT Aluminum Capacitors Polymer Aluminum Capacitors Film Capacitors Polymer Tantalum Capacitors Double Carbon Layer Capacitors Inductors / Transformers Fuses Non Solid State Relays LEDs SMT Connectors Time Dependant Failure Mode Seal damage; early electrolyte dry out Increased loss tangent Fairly robust Delamination Breakdown of conductor coating; intermittent shorts Reduced limiting current - - Warpage; Loss of dimensional tolerance 9 Building 2011 IBM the Power on demand Symposium, supply RTP, chain NC IBM Corporation

10 Temperature Sensitive Components 100 F electrolytic capacitor after Pb-free reflow After M. Wickham, et al., 2003 Cracks in the end of the polyester capacitor After M. Wickham, et al., 2003 SMT electrolytic capacitor body burn as a result of Pb-free hand soldering operations IBM IBM Power Symposium, RTP, NC IBM Corporation

11 Temperature Sensitive Component Standards J-STD-020: Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices applies to surface mount soldered components only presumes different expected reflow profiles based on package volume and height assigns different reflow profiles for SnPb and Pb-free solder assembly Tpeak = 245C, 250C, or 260C [Pb-free] assigns Moisture Sensitivity Level (MSL) Rating not formally applicable to passive components and connectors IBM Power Symposium, RTP, NC

12 Process Sensitivity Standards: J-STD-020 J-STD-020 is widely accepted and adhered to by suppliers of integrated circuit components. As increasingly complex server board assemblies transition to Pb-free assembly, some concerns with the defined J-STD-020 limits have been identified. Extended time above liquidus (TAL) conditions are of primary concern high thermal mass card assemblies built using complex fixtures and shielding can extend TAL times well beyond the rated 150 second limit second required TAL time exposes components beyond their rated capability IBM Power Symposium, RTP, NC

13 Temperature Sensitive Component Standards J-STD-075: Classification of Non-IC Electronic Components for Assembly Process targets non-ic components but can be applied to any commodity soldered to a PCB (connectors, etc) requires all components be tested to same worst case Pb-free reflow profile metrics, e.g., highest peak temperature addresses Pb-free wave soldering as well as Pb-free surface mount soldering SMT: Tpeak = 245C, 250C, or 260C (package body temp); PTH: Tpeak = 275C (wave solder pot temp) assigns Process Sensitivity Level (PSL) Rating IBM Power Symposium, RTP, NC

14 Process Sensitivity Standards: J-STD J-STD-075, recently published, has not yet been widely adopted by the passive component industry. Devices that cannot meet baseline Pb-free soldering profiles without degradation are classified as being Process Sensitive Defines base Pin-Through-Hole (PTH) wave solder profile Defines base Surface Mount (SMT) reflow solder profile SMT profile is linked to J-STD-020 reference profiles Components deemed process sensitive per J-STD-075 may or may not be an issue on any given card assembly e.g., a large mass passive may only be rated to 250C, and therefore sensitive, but never reaches 250C in actual PCBA designs. Must explicitly profile the sensitive component on the card in question e.g., ceramic caps have 3C/s cooldown limit but never encountered in practice IBM Power Symposium, RTP, NC

15 J-STD-075: Evaluation and Classification J-STD-020 defines temperature and duration ranges for SMT reflow profile parameters e.g., Table 5-2 in J-STD-020 states that the time above liquidus (TAL) requirement is seconds J-STD-075 instead requires that the device under evaluation be tested at the maximum value of the range. e.g., the device must be capable of withstanding 150 seconds above 217C to avoid a TAL sensitivity rating IBM Power Symposium, RTP, NC

16 J-STD-075: Evaluation and Classification Classification Reflow Profile; J-STD-020, Figure 5-1 worst case reflow profile metrics IBM Power Symposium, RTP, NC

17 J-STD-075 Evaluation and Classification Base SMT reflow profile, per Table 5-2, J-STD-020D reflow profile metrics Requirement of 3 reflows stated in clause 5.6 of J-STD IBM Power Symposium, RTP, NC

18 J-STD-075: Evaluation and Classification Tables 5-1, 5-2, 5-3 in J-STD-075 provide the PSL (Process Sensitivity Level) classification scheme PSL classification will always contain 2 characters and may require an optional 3rd character 1st character identifies process: W = wave, R = reflow 2nd character identifies maximum peak temperature limitation 3rd character identifies other process limitations as defined in Table 5-3 solder process (W or R) 3rd character is omitted when device is not sensitive (R0, W0), or when the only sensitivity is peak temperature (R5, W4) XYZ peak temperature limit (number) other process sensitivities (letter) IBM Power Symposium, RTP, NC

19 J-STD-075: Evaluation and Classification IBM Power Symposium, RTP, NC

20 J-STD-075: Evaluation and Classification IBM Power Symposium, RTP, NC

21 PSL Classification Example SMT aluminum capacitor 100 F, 25V Size = 6.8mm x 6.8mm x 8mm Supplier A, Qualification assembly profile metrics: Preheat C for 120sec Peak temp 260C for 5 sec max Time above 217C of 70sec Number of reflows = 2X IBM Power Symposium, RTP, NC

22 PSL Classification Example Peak reflow temperature requirement per table 4-2 in J-STD-020D for a device of this size is 245C Package Volume: 6.8 x 6.8 x 8.0mm = 370mm 3, Package Height: 8.0mm is 2.5mm Peak temperature of 260C, exceeds requirement of 245C But what about other profile metrics? IBM Power Symposium, RTP, NC

23 PSL Classification Example Preheat C for 120sec: Not worst case. requirement is C for 120 seconds Time above 217C of 70sec: Not worst case. requirement is 217C for 150 seconds 5 sec within 5C of peak temp: Not worst case. requirement is 30 seconds Number of reflows = 2 times: Not worst case. requirement is 3 reflow passes This device from Supplier A is not qualified to reflow profile extremes and is therefore deemed temperature sensitive. A PSL rating must be generated IBM Power Symposium, RTP, NC

24 PSL Classification Example PSL = R4N Table 5-3 PSL 3 rd Character R = Reflow, W = Wave Table 5-2 Reflow Solder PSL Classification IBM Power Symposium, RTP, NC

25 PSL Data Collection and Management PSL information for each supplier part number (PN) must be gathered and stored PSL rating Identify limiting values for each sensitivity for example, Peak Temperature Time Above Liquidus (TAL) Comments fields for information on cleaning or other limitations Information not easily obtained, nor in readily usable format IBM Power Symposium, RTP, NC

26 PSL Data Collection and Management Need software tool to identify all sensitive devices on assembly Bill of Materials (BoM) For devices with multiple sources, must pull data for all possible suppliers and identify worst case Need to monitor temperature of all identified sensitive devices during solder reflow profile development Many thermocouples; possibly multiple profile boards Where possible, modify profile settings to ensure temperature limitations for all devices are not exceeded Special processes required for very sensitive devices IBM Power Symposium, RTP, NC

27 IBM Example PCBA BoM Review Complete Bill of Materials IBM PN, Quantity, Ref Des Location, Front/Back Convert into spreadsheet format. Pull BoM details from part number data base Bounce PN data off of Temperature Sensitive BoM Scrub Tool Supplier/ Supplier PN, Package, I/O count, Lead spacing, Lead size, (BGA) Ball metallurgy Data not available from PN database must be found by searching individual supplier s web sites or contacting supplier directly IBM Power Symposium, RTP, NC

28 BoM Scrub Tool Output Temperature Sensitive Components IBM Power Symposium, RTP, NC

29 Using PSL Data in PCBA Development Define Thermocouple locations for each initial attach process (SMT, wave) and each special rework process: Along with the T/C s at traditionally critical sites, e.g., QFNs PCB laminate BGAs must include indentified temperature sensitive SMT components electrolytic capacitors, tantalum capacitors older fuses and LEDs very large inductors connectors/ battery holders/ switches and temperature sensitive PTH Parts wave solder and rework is the most difficult lead-free solder process IBM Power Symposium, RTP, NC

30 TSC Monitoring Example 4 Crystals 4 Oscillators 6 SMT Electrolytic Capacitors 1 Tantalum Capacitor 15 total temperature sensitive device placements Temperature Sensitive Components: (15 total) Aluminum Capacitors Polymer Tantalum capacitors/fuses Crystals Oscillators COMPONENT BODY MEASUREMENT (PLASTIC / METAL BODY) SMT Electrolytic Capacitor Example LEAD SOLDER JOINT MEASUREMENT (ATTACH TO EXPOSED FILLET) IBM Power Symposium, RTP, NC IBM Corporation

31 Using PSL Data in PCBA Development Finding out after that a device requires hand soldering after profile development is not optimal. Far better to identify very sensitive devices early in design phase, and not use if at all possible (select alt. devices or alt. suppliers) Designers need to be educated in TSC management practices For new devices, PSL data may not yet be available. Cannot use electrical test or visual inspection to validate profile. Latent damage most typically not immediately evident At times, possible to confirm acceptability of marginal situations directly with component supplier IBM Power Symposium, RTP, NC

32 Post Solder Attach Temperature Exposures Solder reflow is not the only elevated temperature required in circuit card assembly operations. Post solder thermal exposures are typically at lower temperatures (90 125C) but for longer times. Thermal Cures Heatsink Thermal Interface Materials Encapsulants, Adhesives Extended Moisture Bakes Rework preparation Some components known to be sensitive to extended temperature exposures IBM Power Symposium, RTP, NC

33 Post Solder Attach Sensitivity Post solder attach temperature sensitive components not covered by any industry specification. Be wary. Press-fit and socketed devices are not qualified to any solder attach thermal profiles PTH devices are only qualified to solder wave/fountain thermal conditions Only leads are subjected to molten solder Supplier may not have simulated preheat of device body IBM Power Symposium, RTP, NC

34 Post Solder Attach Sensitivity Range of Maximum Temperatures Allowed by Various Suppliers Button Cell Batteries PTH LEDs PTH aluminum electrolytic capacitors Fuses and polymer PTCs (Positive Temperature Coefficient) PTH film capacitors PTH tantalum capacitors 70 to 85C 85C up to Not Sensitive 85C up to Not Sensitive 85C up to Not Sensitive 105 to 125C 125C up to Not Sensitive As was the case for reflow sensitivities, post solder attach temperature sensitivities must be identified uniquely for each supplier part number IBM Power Symposium, RTP, NC

35 Post Solder Attach Sensitivity PTH (organic) LED Failure modes similar to initial attach Immediate electrical or catastrophic fails PTH LEDS with organic encapsulants may expand to point that internal wire bonds break Early fails in the life of the electronic product Aluminum and polymer aluminum capacitors may exhibit bulging or venting of the metal can Button cell batteries may have seals broken and lose electrolyte Broken stitch bond in PTH LED Rated to 105C; baked at 125C. polymer Al capacitor Excessive bake temperature Capacitor shorted & burned IBM Power Symposium, RTP, NC

36 Post-Solder Processes of Concern Short duration bakes Adhesive and encapsulant cures e.g., heatsink attach, underfill encapsulation, device encapsulation 130C for 90 minutes 150C for 40 minutes Long duration bakes Moisture bake required prior to SMT rework to ensure no damage to nearest neighbors of device being removed or to underlying PCB laminate 125C for 24 and 48 hours IBM Power Symposium, RTP, NC

37 Post-Solder Processes of Concern Card level temperature cycling Screen for soldering defects 0 to 100C for 1 cycle/hour, 15 minute dwells Some device families are not qualified to these conditions Some suppliers state that their indicated maximum temperatures only apply if no bias is applied during the stress IBM Power Symposium, RTP, NC

38 Using Post Solder Attach Sensitivity Data Can use similar method of storing post solder attach sensitivity data as with solder process PSL information Data must be gathered and stored for each supplier part number Short duration bake maximum temperature Long duration bake maximum temperature Thermal cycling peak temperature Peak temperature generally matched short duration bake Some suppliers had to perform additional qualification testing to determine if they could support use of their devices to above processes IBM Power Symposium, RTP, NC

39 Using Post Solder Attach Sensitivity Data Software for PSL management can also be used to identify devices on card BoM that are sensitive to post attach processes For devices with multiple sources, must pull data for all Define processes to accommodate worst case supplier Only need to identify oven/chamber peak temperature Profiling with thermocouples not required If process condition exceeds device limits: Change conditions of the process Longer duration bake at lower temperature Perform process prior to attaching the sensitive device(s) Best solution for press-fit and socketed devices May require hand soldering for soldered devices IBM Power Symposium, RTP, NC

40 Key Messages. J-STD-075 documents a process for managing Process Sensitive Components Get it. Read it. Use it. Demand Process Sensitivity ratings from your component suppliers (PSL). Establish internal procedures to gather and use PSL data (board design, supplier selection, process definition). Enforce component temperature exposure limits in all assembly manufacturing operations. measure temperature exposure use alternate processes if necessary IBM Power Symposium, RTP, NC

41 Key Messages. Do NOT depend on post-assembly functional yield to validate assembly of process sensitive components. Be conscious of post-solder temperature sensitivities not adequately addressed by industry practice today. gather supplier data; use it to ensure process compliance Evaluate your finished products for temperature sensitivity in anticipated customer assembly operations quantify limitations and provide to customer. Expect future improvements to the industry specification framework for component process sensitivity IBM Power Symposium, RTP, NC