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© ET-Trends LLC 1 Ken Gilleo, PhD Ocean State - Rhode Island L-F SMTA Boston 2000
© ET-Trends LLC 2 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 3 Metallurgical Solder phase Cu 6 Sn 5 component Tin phase Sn -phase Cu 6 Sn 5 -phase Cu 3 Sn Copper - PWB (shown with Sn only for simplicity) Binary and tertiary alloys form complex structures with many organometallic compounds
© ET-Trends LLC 4 L-F Alloy Political Issues Japan –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 5 Alloys Sn Cu Zn? Mg Ba Ce Ag In Bi Mostly tin plus, hold the lead Some 5-metal alloys
© ET-Trends LLC 6 Popular Alloys TAC: Sn-Ag-Cu –Promoted 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 20 o C lower than Sn-Ag-Cu –Licensed from Oatey by Alpha-Fry who will sublicense Compiled by Dr. Alan Rae
© ET-Trends LLC 7 The Real L-F Problem L-F o C Higher - was Sn/Pb already too hot?
© ET-Trends LLC 8 Collateral Damage? Equipment –oven; N 2, 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? Can they take the heat and thermal effects L-F
© ET-Trends LLC 9 Flux Issues Lead-free alloys dont 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 10 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 11 Laminates Laminates can degrade at 260 o C on multiple passes Dimensional stability issues Solder masks; adhesion, color change, degradation Issue - finding an effective compromise on electrical properties, flammability, mechanical properties Before or after? FR4 in jeopardy
© ET-Trends LLC 12 Packaging Materials Epoxy Molding Compounds (EMC) may need to be modified or changed to totally different resins. Formulations are very complex; many formulations have >10 components. Additives O OCH 2 CH CH 2 O OCH 2 CH CH 2 Epoxy Resin OH Hardener Catalyst A Silica Fillers Coupling Agent A A A A A O OCH 2 CH CH 2 Epoxy Molding Compound (EMC); could be in jeopardy
© ET-Trends LLC 13 Long-chained macromolecules of small repeating block monomers. Properties like strength, flexibility, melting point and electricals can be engineered into the polymer.
© ET-Trends LLC 14 The inventor of the IC Jack Kilby, uses Conductive Adhesives
© ET-Trends LLC 15 Resin, hardener. accelerator
© ET-Trends LLC 16 Conductive Adhesives Metal-filled composite Thermoset (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 17
© ET-Trends LLC 18 Classes of Joining Materials Solders - metallurgical POLYMER-BASED –Isotropic Cond. Adh. (ICA) –Anisotropic Cond. Adh. (ACA) –Hybrids –Non-conductive
© ET-Trends LLC 19 Iso- vs. Anisotropic
© ET-Trends LLC 20 Conductive Particles silver powder silver flake + Metal coatings: Ag, solder custom penetrating particles + Anti-corrosion films still the standard + Conductive Polymer Coating No real success yet! copper use blend
© ET-Trends LLC 21 Typical CA Composition Pre-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 22 Silver is Dominant Moderate cost (some impact) Oxide is electrically conductive Malleable; can be shaped Only moderately abundant Chemically reactive Safe for humans (biocide) (but why silver?)
© ET-Trends LLC 23 ICA Adhesive Model Polymer binder oxide & surfactant Electrical pathways The many interfaces add electrical resistance Mechanical & Electrical properties are mostly independent
© ET-Trends LLC 24 Adhesive Assembly Processes Stencil Adhesive Place Component Cure Adhesive Stencil Solder Remove or deactivate flux Place Component Reflow Solder Runs on todays SMT lines Cookson - Foxboro, MA
© ET-Trends LLC 25 Adhesive Solder 50% reduction Adhesive Oxide-Tolerant Solder Dewetting Oxide <1% reduction Half as much volume Much more compatible
© ET-Trends LLC 26 Digitizer made by SOLDERLESS SMT Poly-Flex Circuits - Parlex
© ET-Trends LLC 27 Test the System MECHANICAL –Bond Strength –Die Shear –Shock; drop Bond Strength After: –Heat Age –Chemical Exposure –Temp. & Humidity ELECTRICAL –Volume Resistivity –Electrical Stability –Heat Age –Thermal Cycle –Temp. & Humidity REWORKABILITY SOLDER: 4-6 m Per junction CA: 8-12 m Per junction
© ET-Trends LLC 28 Junction Instability Many PWB finishes oxidize Most metal oxides are insulators Oxidation can occur under CAs Most CAs can form unstable junctions OXIDIZED METAL AFTER AGING METAL INITIAL
© ET-Trends LLC 29 One Proven Solution: Penetrating Particles Penetrating Particles Metal oxide METAL Oxide-Penetrating particles Poly-Solder used by Poly-Flex a Parlex Co.
© ET-Trends LLC 30 Junction Resistance - 85%RH/85 o C Sn/Pb solder, the reference, shows no change 1000 hrs +18% incr. +15% incr. -8% -3%-10% -22% 1000 hrs 500 hrs -8% -3% PLCC resistor PWB FinishBare CuSilver InkSolderGold 85%rh/85 o C Note resistance drop Components are solder-coated Penetrating Particle Type Adhesive:
© ET-Trends LLC Silver Migration? Silver is encapsulated by resin.
© ET-Trends LLC 32 Flip Chips with CAs Work well, in production Underfill provides high strength Underfill eliminates migration concerns Used for memory, RFIDs
© ET-Trends LLC 33 Advantages Low Thermal Processing Stress Eliminate Solder Mask Excellent 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 -Particle Emission Laptop mouse pad, polyester max 150 o C
© ET-Trends LLC 34 Limitations Lower mechanical shock No self-alignment Some 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 35 Comparison CharacteristicSOLDER Volume Resistivity ohm.cm ohm.cm Best Adhesive Typical Junction R m <25 m Thermal Conductivity W/m-deg.K3 - 8 W/m-deg.K Shear Strength (1206)>2000 PSI> 1600 PSI no change Mechanical ShockPass 6 ft. x 6Fail 6 ft., once Thermal FatigueYesLower, gradual Proc. Temp o C o C Fine PitchGoodExcellent T & H (85%/85C)
© ET-Trends LLC 36 Conclusions Adhesives run on std. SMT lines Much lower process temperatures Low mechanical shock issue Need to move away from silver A niche product without a breakthrough
© ET-Trends LLC 37 Prognosis Copper adhesives will succeed –metal-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
KBG Ken Gilleo - ET-Trends David Blumel Alpha Metals with Reworkable Wafer-Level Underfill
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