Case Study Power Electronics Cleaning - Solvent to ph Neutral: Enhancing Safety, Process Efficiency and Productivity Ravi Parthasarathy, M.S.Ch.E. Senior Application Engineer ZESTRON Americas
Outline Introduction The Issue Test Plan and Result Initial Conclusion Final Recommendation
Introduction Global innovator of custom microwave and RF technology Four (4) manufacturing sites Supports prime space and defense contractors Standard products: Operational amplifiers and video amplifiers, motor drives and linear regulators as well as switching voltage regulators Leading producer of custom hybrids, power hybrids and multichip modules
Introduction Primary products manufactured at case study site: Power modules and discrete power devices with and without wire bonds Product types cleaned: Small power components with die pads and gold wires Power encapsulated modules having AISiC base plates
Introduction Solder paste and flux type: One (1) RMA solder paste (ROL0) Two (2) rosin liquid fluxes (ROM1 & ROL0) and 1 core wire (RA) Vacuum soldering technology Original cleaning process Six (6) step dip tank solvent process Wash cycle Three (3) wash tanks o o o Cleaning agent: PC Board (custom blend) cleaner containing D-Limonene, DPM, IPA and Non-hazardous ingredients (HMIS rating of 2-2-0) 100% concentration 90 F wash temperature 10 min cycle time
Introduction Original cleaning process Six (6) step dip tank solvent process Rinse cycle Two (2) DI-water rinse tanks: 120 F IPA final rinse Dry cycle Nitrogen blow-off to remove remaining IPA
Outline Introduction The Issue Test Plan and Result Initial Conclusion Final Recommendation
Issue Concerns with dip tank cleaning process: Worker safety PC Board cleaner: 95 F flash point Inefficient cleaning process Low production volume due to batch process (increased cleaning times) Multiple recipes required Inconsistent cleaning results Not environmentally friendly Solvent based cleaning system Disposal concerns
Outline Introduction The Issue Test Plan and Result Initial Conclusion Final Recommendation
Test Plan Developed Design of Experiment (DOE) for cleaning process improvement Main criteria: Safe and environmentally sound cleaning system (closed-loop): Aqueous based with no flash point Complete flux removal Excellent compatibility with various substrate material mix One process recipe for all products Efficient operating system: Improve productivity and reduce labor costs Automate wash system offering reliable process control Minimized waste disposal costs
Test Plan and Solution DOE classified into three stages Stage 1: Preliminary Cleaning Trials Evaluate alternate chemistries to assess performance and cleaning effectiveness Confirm optimum process conditions Stage 2: Material Compatibility Assessment Utilizing cleaning process parameters from Stage 1: Assess material compatibility with selected cleaning agent Multiple wash passes Stage 3: Cleaning Trials at Customer Site Repeat inline cleaner trials to confirm results with larger lot size over several weeks (at customer site)
Test Plan and Solution Cleaning process recommendation for initial trials: Cleaning agent: aqueous-based ph neutral Cleaning equipment: spray-in-air inline cleaner Cleanliness assessment methods (as per IPC standards): Visual inspection (IPC-A-610E) Ionic contamination testing (IPC-TM-650 Method 2.3.25) Ion chromatography testing (IPC-TM-650 Method 2.3.28)
Stage 1: Preliminary Cleaning Trials Substrate types used: Several discrete power components with die pads and gold wire bonds Power encapsulated modules (PEMs) with complete casing
Stage 1: Methodology Cleaning equipment: Utilize spray-in-air inline cleaner Initial cleaning trial conducted at the equipment supplier Cleaning agent: Micro phase cleaning agent ph neutral Cleanliness assessment: Substrates returned to ZESTRON Analytical Center for visual inspection Note: time interval between soldering and cleaning: 3-4 days
Stage 1: Methodology Inline Cleaner Operating Parameters Wash Stage Cleaning Agent Concentration 10% Conveyor Belt Speed 0.5 Ft/Min Wash Exposure Time 2 Minutes Pre-wash Pressure (Top/Bottom) 40 Psi / 35 Psi Wash Pressure (Top/Bottom) 50 Psi / 50 Psi Wash Temperature 135-140 F Rinse Stage Rinse Agent Di-water Rinse Pressure (Top/Bottom) 30 Psi / 30 Psi Rinse Temperature 115-120 F Drying Stage Drying Method Hot Circulated Air Drying Temperature 140 F 160 F
Stage 1 Results Visual Inspection: Power Components Before Cleaning After Cleaning
Stage 1 Results Visual Inspection: Power Components Before Cleaning After Cleaning
Stage 1 Results Visual Inspection: PEMs Before Cleaning After Cleaning
Stage 1 Results Visual Inspection Flux residues completely removed No oxidation or dulling observed on aluminum and gold pads No breakage or bending (movement) of gold wires
Stage 2: Material Compatibility Assessment Substrate type used: IGBTs having DuPont QQ550 low temperature glass encapsulate material over gold traces Custom built planar transformers Cleaning agent: Micro phase ph neutral Test methods: Beaker (static) test Inline cleaner (dynamic) test
Stage 2: Methodology Beaker test parameters: Concentration: 15% by volume Temperature: 140-145 F Exposure time: 15 min & 60 min Inline cleaner test parameters: Cleaning agent concentration: 15% by volume Wash temperature: 140-145 F No. of wash passes: 2 & 5 Chemical wash exposure time: 4 min & 10 min (@ 0.5 ft/min)
Stage 2 Results Material Compatibility Assessment Beaker tests Before Compatibility 15 Minute Chemical Exposure 60 Minute Chemical Exposure
Stage 2 Results Material Compatibility Assessment Beaker test results Before Compatibility 15 Minute Chemical Exposure 60 Minute Chemical Exposure
Stage 2 Results Material Compatibility Assessment Inline cleaner tests: Before Compatibility 4 Minute Chemical Exposure 10 Minute Chemical Exposure
Stage 2 Results Material Compatibility Assessment Inline cleaner tests: Before Compatibility 4 Minute Chemical Exposure 10 Minute Chemical Exposure
Stage 2 Results Material Compatibility Assessment Excellent compatibility results achieved through beaker and inline tests ph neutral cleaning agent is fully compatible with: Dupont QQ550 low temperature glass encapsulate material Custom built planar transformers
Stage 3: Cleaning Trials at Customer site Substrates: Various integrated circuits Sample 1: High Power Driver Sample 2: Microcircuit Sample 3: Motor Controller Sample 4: Microcircuit Sample 5: Converter
Stage 3: Methodology Cleaning equipment: Spray-in-air inline cleaner Operating parameters same as used in Stage 1 Cleaning agent: ph neutral formulation Cleanliness assessment (as per IPC Test Methods): Visual analysis (performed at customer site) Ionic contamination (performed at ZESTRON Analytical Center) Ion chromatography (performed at ZESTRON Analytical Center)
Stage 3: Methodology Cleaning process Inline cleaner test parameters: Cleaning agent concentration: 15% by volume Wash temperature: 140 F Conveyor belt speed: 0.5 ft/min Wash exposure time: 2 minutes
Stage 3: Results Cleanliness assessment Visual analysis: Flux residues were fully removed from all sample parts Ionic Contamination: All clean parts passed the tests Ion Chromatography: All clean parts passed the tests
Stage 3 Results Ionic Contamination Uncleaned Cleaned Sample Number Ionic Contamination Values (µg/inch 2 ) Surface Area (inch 2 ) Ionic Contamination Values (µg/inch 2 ) 1 0.97 3.836 0.25 2 11.59 4.219 0.45 3 29.10 2.421 0.52 4 31.61 6.200 5.94 5 1.55 5.498 0.22
Stage 3 Results Ion Chromatography Ionic Species Maximum Contamination Limits Anion Species Always Tested For (µg/in 2 ) Sample 1 Not Clean Sample 1 Clean Sample 2 Not Clean Sample 2 Clean Fluoride (F - ) 3 0.0328 0.1208 0.0339 0.2419 Acetate (C 2 H 3 O 2- ) 3 ND ND ND 0 Formate (CHO 2- ) 3 1.2724 ND 0.4413 ND Chloride (Cl - ) 4 0.3386 0.4243 0.0760 0.2608 Nitrite (NO 2- ) 3 0.1842 0.3985 1.1753 0.2187 Bromide (Br - ) 10 2.4533 1.1391 2.4953 0.2966 Nitrate (NO 3- ) 3 ND ND ND ND Phosphate (PO 4 2- ) 3 ND ND 1.9360 ND Sulfate (SO 4 2- ) 3 ND ND ND ND WOA (Weak Organic Acid) 25 ND 1.3135 ND 0.6750 Cation Species Always Tested For (µg/in 2 ) Lithium (Li + ) 3 ND ND ND ND Sodium (Na + ) 3 0.7629 0.3265 2.4295 0.0958 Ammonium (NH 4+ ) 3 0.1052 0.0590 ND 0.0847 Potassium (K + ) 3 0.4391 0.2065 2.4731 0.1679 Magnesium (Mg 2+ ) 1 ND ND 1.8948 0.7665 Calcium (Ca 2+ ) 1 0.2710 ND 0.1790 ND
Stage 3 Results Ion Chromatography Anion Species Always Tested For (µg/in 2 ) Ionic Species Maximum Contamination Limits Sample 3 Unclean Sample 3 Clean Sample 4 Unclean Sample 4 Clean Sample 5 Unclean Sample 5 Clean Fluoride (F - ) 3 0.2565 0.2794 0.0917 0.3591 0.2565 0.2794 Acetate (C 2 H 3 O 2- ) 3 0 0 0 0 0 0 Formate (CHO 2- ) 3 ND ND ND ND ND ND Chloride (Cl - ) 4 0.2581 0.9778 1.3347 1.3048 0.2581 0.9778 Nitrite (NO 2- ) 3 0.0900 1.1573 0.7793 0.7708 0.0900 1.1573 Bromide (Br - ) 10 0.6384 ND 2.0820 2.1154 0.6384 ND Nitrate (NO 3- ) 3 ND ND ND ND ND ND Phosphate (PO 2-4 ) 3 ND ND ND ND ND ND Sulfate (SO 2-4 ) 3 ND ND ND ND ND ND WOA (Weak Organic Acid) 25 ND ND ND ND ND ND Cation Species Always Tested For (µg/in 2 ) Lithium (Li + ) 3 ND ND ND ND ND ND Sodium (Na + ) 3 1.3532 0 0.6158 0.1351 1.3532 0 Ammonium (NH 4+ ) 3 0.4065 0.1020 0.0057 ND 0.4065 0.1020 Potassium (K + ) 3 1.8198 0.0840 0.6414 0.0889 1.8198 0.0840 Magnesium (Mg 2+ ) 1 ND ND 0.1102 ND ND ND Calcium (Ca 2+ ) 1 ND ND ND ND ND ND
Outline Introduction The Issue Test Plan & Result Initial Conclusion Final Recommendation
Initial Conclusion The micro phase ph neutral cleaning process met the DOE objectives: Safe and environmentally friendly cleaning system: Aqueous-based with no flash point Complete flux removal Excellent compatibility with various substrate materials One process recipe for all products Efficient operating system: Improve productivity and reduce labor costs Automated wash system with reliable process control Minimized waste disposal costs Process was successfully implemented
Material Compatibility Issue However following process implementation Not all new products were evaluated through the DOE: A wide variety of custom dies used in many different devices Intermittent compatibility issues arose on random devices Sporadically, corrosion observed predominantly at wire bond site: Gold wires ultrasonically bonded to bond pad Bond pad typically has aluminum top layer without any passivation Third-party procured packages having DuPont QQ550 dielectric material: Signs of iridescence and dullness occasionally
Material Compatibility Issue Corrosion appears on random Al bond pads it did not appear on every location on the same substrates Thorough investigation lead to galvanic corrosion as primary root cause
Reasons Behind Galvanic (two-metal) Corrosion Inferior quality Al bondpads with inconsistent passivation layer thickness Degradation primarily occurs in the gold wire bond/aluminum bond pad system Propensity to increase in presence of water and oxygen media: Corrosion byproducts may be aluminum hydroxide
Process Alternatives (after extensive analysis) Use of alternate robust combination such as Al-Al or Al-Cu with consistent passivation layer: Not feasible since customer receives substrates from third-party sources Al-Au offers distinct advantages over other combinations Switch to a water-free (solvent-based) process: Not feasible since customer had already purchased conveyorized spray-inair inline cleaner Alternate ph neutral cleaning agent with superior inhibition package: Capable of overcoming the galvanic corrosion problem Accounts even for inferior quality parts that may be used on the manufacturing floor
Process Alternatives (after extensive analysis) Alternative micro phase ph neutral cleaning agent was evaluated Superior inhibition package: Resolved all the corrosion-related problems Excellent cleaning and compatibility results with a wide variety of devices and components o Extensive testing conducted to validate the findings Met customer requirements in terms of functional reliability after inline wash process: Process was successfully implemented at customer site
Results Gold wire bonded to the Al bondpad showing no signs of corrosion DuPont QQ550 dielectric encapsulate material fully compatible
Outline Introduction The Issue Test Plan and Result Initial Conclusion Final Recommendation
Final Recommendation Not all ph neutral cleaning agents are similar Novel micro phase ph neutral cleaning agents can offer efficient cleaning process with reduced process and operating costs Extensive testing is highly recommended!! Cleaning agents with superior inhibition package is required to overcome reliability issues
Thank You! Questions? Ravi Parthasarathy, M.S.Ch.E. Senior Application Engineer ZESTRON Americas r.parthasarathy@zestronusa.com