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Published byLillian Griffith
Modified over 3 years ago
Polymer Solders: the Cool Lead-Free AlternativeKen Gilleo, PhD L-F SMTA Boston 2000 Ocean State - Rhode Island © ET-Trends LLC
Outline Lead-Free Solders: Success or Failure?Polymers Vs. Metallurgical Solders Conductive Adhesive Technology Pros and Cons of CAs for SMT Conclusions and Prognosis © ET-Trends LLC
component Metallurgical Solder Copper - PWB(shown with Sn only for simplicity) h phase Cu 6 Sn 5 component Tin phase Sn -phase Cu6Sn5 e -phase Cu3Sn Copper - PWB Binary and tertiary alloys form complex structures with many organometallic compounds © ET-Trends LLC
L-F Alloy Political IssuesJapan Using their own proprietary alloys Alloys; too many and too diverse? A patent “minefield” from global perspective? USA Industry: WHY are we doing this? Government: electronic Pb, minor envir. issue ITRI and NEMI are endorsing TAC (Tin-Silver-Copper) World in General No global cooperation mechanism? Low consensus, too many “my alloy” agendas? Ambiguous deadlines and not in synch! © ET-Trends LLC
Sn Alloys Cu Zn? Mg Ba Ce Ag In Bi Some 5-metal alloysMostly tin plus, hold the lead Mg Ba Ce Ag Sn In Bi Some 5-metal alloys © ET-Trends LLC
Popular Alloys “TAC”: Sn-Ag-CuPromoted by several organizations, appears to be viable Patent issues, “Doctrine of Equivalence” issues in USA? Sn-Cu, Sn-Ag; appears to be viable, mostly patent-free Sn-Ag-Cu-Sb Much process experience with this alloy Many have licensed from AIM as Castin® Sn-Ag-Cu-Bi Majors in Japan are using this alloy system Bi supply no issue - low % Bi used Process at 20oC lower than Sn-Ag-Cu Licensed from Oatey by Alpha-Fry who will sublicense Compiled by Dr. Alan Rae © ET-Trends LLC
The Real L-F Problem 30 - 60oC Higher - was Sn/Pb already too hot? L-F© ET-Trends LLC
Collateral Damage? EquipmentL-F Can they take the heat and thermal effects Equipment oven; N2, faster ramp, different materials tester & probes; harder flux residue Substrate; warp, delamination, degradation Coatings; solder masks; degradation Board Cleaning; more difficult Package substrate; warp, delamination encapsulant; popcorn devices like photo, MEMS, low k dielectric? © ET-Trends LLC
Flux Issues Lead-free alloys don’t wet as well:Sn without Pb oxidizes more easily Nitrogen probably required High temperature fluxes; more voids in reflow? Fluxes - halogen-free for rel. and environ Newer epoxy fluxes may not be viable Expect new fluxes and processes They may be harder to clean © ET-Trends LLC
Cleaning Baked-on fluxes?Many users still clean, even no-clean flux, to improve conformal coating adhesion, etc. Nitrogen atmosphere can reduce residue toughness “Stronger” cleaning machines from Speedline Electrovert © ET-Trends LLC
Laminates Laminates can degrade at 260oC on multiple passesDimensional stability issues Solder masks; adhesion, color change, degradation Issue - finding an effective compromise on electrical properties, flammability, mechanical properties FR4 in jeopardy Before or after? © ET-Trends LLC
Packaging Materials Additives O OCH2CH CH2 Epoxy Resin OH Hardener Catalyst A Silica Fillers Coupling Agent Epoxy Molding Compounds (EMC) may need to be modified or changed to totally different resins. Formulations are very complex; many formulations have >10 components. Epoxy Molding Compound (EMC); could be in jeopardy © ET-Trends LLC
Long-chained macromolecules of small repeating block monomersLong-chained macromolecules of small repeating block monomers. Properties like strength, flexibility, melting point and electricals can be engineered into the polymer. POLYMERS Put the fire out with COOL CONDUCTIVE ADHESIVES Polymers © ET-Trends LLC
The inventor of the IC Jack Kilby, uses Conductive Adhesives© ET-Trends LLC
Resin, hardener. accelerator© ET-Trends LLC
Conductive Adhesives Metal-filled compositeThermoset (a few thermoplastics) Filler determines type Very different than solder Used commercially for SMT and FC Two types Isotropic (best for SMT) Anisotropic; random or patterned © ET-Trends LLC
© ET-Trends LLC
Classes of Joining MaterialsSolders - metallurgical POLYMER-BASED Isotropic Cond. Adh. (ICA) Anisotropic Cond. Adh. (ACA) Hybrids Non-conductive © ET-Trends LLC
Iso- vs. Anisotropic © ET-Trends LLC
+ Conductive Polymer CoatingConductive Particles still the standard silver powder silver flake use blend custom penetrating copper particles + Metal coatings: Ag, solder + Anti-corrosion films + Conductive Polymer Coating No real success yet! © ET-Trends LLC
Typical CA CompositionPre-polymers or monomers (epoxy is common) Hardener and accelerators Silver filler; major component (>79% wt.) Additives for wetting, flow, adhesion Mix under vacuum © ET-Trends LLC
Silver is Dominant Moderate cost (some impact)(but why silver?) Moderate cost (some impact) Oxide is electrically conductive Malleable; can be shaped Only moderately abundant Chemically reactive Safe for humans (biocide) © ET-Trends LLC
ICA Adhesive Model Electrical pathways Polymer binder oxide &The many interfaces add electrical resistance surfactant Electrical pathways Mechanical & Electrical properties are mostly independent © ET-Trends LLC
Adhesive Assembly ProcessesStencil Adhesive Place Component Cure Adhesive C Adhesives run cool Remove or deactivate flux Stencil Solder Place Component Reflow Solder C Runs on today’s SMT lines © ET-Trends LLC Cookson - Foxboro, MA
Solder Adhesive Differences <1% reduction 50% reduction OxideHalf as much volume <1% reduction 50% reduction Oxide Adhesive Oxide-Tolerant Much more compatible Solder Dewetting © ET-Trends LLC
SMT Line for Adhesives Digitizer made by SOLDERLESS SMT© ET-Trends LLC Poly-Flex Circuits - Parlex
Test the System MECHANICAL Bond Strength After: ELECTRICALDie Shear Shock; drop Bond Strength After: Heat Age Chemical Exposure Temp. & Humidity ELECTRICAL Volume Resistivity Electrical Stability Thermal Cycle REWORKABILITY SOLDER: 4-6 mW Per junction CA: 8-12 mW Per junction © ET-Trends LLC
Junction Instability Many PWB finishes oxidizeMost metal oxides are insulators Oxidation can occur under CAs Most CAs can form unstable junctions METAL INITIAL OXIDIZED METAL AFTER AGING © ET-Trends LLC
One Proven Solution: Junction-Stable Penetrating Particles Metal oxideOxide-Penetrating particles Junction-Stable © ET-Trends LLC Poly-Solder used by Poly-Flex a Parlex Co.
Penetrating Particle Type Adhesive:1000 hrs +18% incr. +15% incr. -8% -3% -10% -22% 500 hrs PLCC-44 1206 resistor PWB Finish Bare Cu Silver Ink Solder Gold 85%rh/85oC Note resistance drop Junction Resistance - 85%RH/85oC Sn/Pb solder, the reference, shows no change Components are solder-coated © ET-Trends LLC
- Silver Migration? + Ag Silver is encapsulated by resin.© ET-Trends LLC
Flip Chips with CAs Work well, in productionUnderfill provides high strength Underfill eliminates migration concerns Used for memory, RFIDs © ET-Trends LLC
Advantages Low Thermal Processing Stress Eliminate Solder MaskExcellent Fine Pitch Wide Process Window Less Used: (1/4 to 1/6 as much by wt.) No Flux, No Residues, No Cleaning No Lead No a-Particle Emission Laptop mouse pad, polyester’ max 150oC © ET-Trends LLC
Limitations Lower mechanical shock No self-alignmentSome require non-oxidizing surfaces Higher electrical resistance Higher thermal resistance Much more difficult to rework Higher cost (than Sn/Pb; L-F?) Silver: limited supply, aquatic toxicity © ET-Trends LLC
Comparison .000015 ohm.cm .00006 ohm.cm Typical Junction R 8 - 15 mWCharacteristic SOLDER Best Adhesive Volume Resistivity ohm.cm ohm.cm Typical Junction R mW <25 mW Thermal Conductivity W/m-deg.K 3 - 8 W/m-deg.K Shear Strength (1206) >2000 PSI > 1600 PSI T & H (85%/85C) no change no change Mechanical Shock Pass 6 ft. x 6 Fail 6 ft., once Thermal Fatigue Yes Lower, gradual Proc. Temp. oC oC Fine Pitch Good Excellent © ET-Trends LLC
Conclusions Adhesives run on std. SMT linesMuch lower process temperatures Low mechanical shock issue Need to move away from silver A niche product without a breakthrough © ET-Trends LLC
Prognosis Copper adhesives will succeedmetal-coated conductive polymer coated - boost strength? Mixtures with solder; now used in inks Mechanical properties will be the key Cost probably lower than L-Fs Metallurgical joints likely to dominate © ET-Trends LLC
with Reworkable Wafer-Level Underfill
Jeopardy Q 1 Q 6 Q 11 Q 16 Q 21 Q 2 Q 7 Q 12 Q 17 Q 22 Q 3 Q 8 Q 13
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