Film Defects vs Solutions

Transcription

1 Technical Datasheets 技術資料 / TDS1504 PCA Conformal Coating Application Film Defects vs Solutions TOP5 / Common Errors in PCB Conformal Coatings TOP12 / Common Film Defects in General Coatings 티티앤에스주식회사 /TTnS Inc.

2 Component TDS1504( 英 ) TOP5 / Common Errors in PCB Conformal Coatings 1. Missing or Overrunning in Coating Coverage Accidental Missing and/or Overrunning in Coating Coverage 1. Coating Robot in Panic 2. Accidental Change of Coating Material Viscosity It really doesn t occur in the fully automated coating system that runs with entirely closed fluid circuit. 3. Nozzle Tip Blockage 4. Pneumatic Controls in Trouble Bottom Face Coating Inner Block After Coating Actual Coating Coverage Typical Coating Coverage (MDPS / Selective Coating) 2. Excessive Film Bubbles in Number, Size and/or Bridges on Conductors 1. Too Thick Material Viscosity and/or Film-build As viscous as 40-60(Cps) for film application, and as thick as DFT25-35(um) with Rubber Resin, DFT30-45(um) with Acrylic Resin and DFT60-90(um) with Silicone Resin are optimal. 2. Insufficient Solvent Flash-time Essential to have solvent flash-time properly in ambient prior to cure Generally 4~8(min) flash-time required according to applied materials 3. PCA Substrate Contamination Make sure the PCA surface entirely clean that oil-free, moisture-free, ionic residues-free and Si particulates-free 4. Improper Coating Parameters & Programming Technique Inadequate coating overlap, Z-coating height, coating speed and/or poor coating patterns 5. Extreme Workshop Temperature & Humidity The optimal coating environment in terms of temperature & humidity, 22 Temperature 26, RH30% Humidity RH50% Coating works in the circumstances of below 20 /RH25% and above 30 /RH70% prohibited 6. Coating Materials Contamination Mega-Bubbles & Bridges Make sure flushing of the Teflon fluid tube prior to coating material change Mega-Bubbles & Bridge Bridged, Mega-Bubble Ø900(um) I. Primary & Secondary-Bubbles IC-Chip Ⅱ. Secondary-Bubbles Ø400(um) D Ø1,000(um) (AECU / top face) Mega-Bubbles & Bridge Primary Bubbles Ø50 - Ø75(um) Bridged, Mega-Bubble Ø1,000(um)

3 IC-Chip IC-Chip TDS1504( 英 ) I. Secondary-Bubbles Ⅱ. Secondary-Bubbles 450um 600um Approx. Ø250(um) Approx. Ø200(um) Bridged, Mega-Bubble Ø750(um) IC-Chip AECU / bottom face / SR60 / NG A mega-bubble(ø750um) gets jammed in IC-leads. AECU / top face / AR50 / NG Visual-bubbles(Ø150~250um) get jammed in IC-leads. Ⅲ. Primary & Secondary-Bubbles Ⅳ. Secondary-Bubbles Non-bridged, Mega-Bubble Ø600(um) Thick Film Bridge Primary Bubbles Ø50-75(um) Approx. Ø300(um) AECU / top face / AR40 / NG Many of primary-bubbles observed on PSR base film. Visual to mega-bubbles shaped around components. AECU / bottom face / AR40 / NG Visual-bubbles with thick film-bridge shaped in small parts. Too thick film-bridge may cause an overstress in solder joints due to thermal expansion. Ⅴ. Primary & Secondary-Bubbles Primary Bubbles Ø50-75(um) IC-Chip Ⅵ. Secondary-Bubbles Bridged, Mega-Bubble Ø500(um) Bridged, Mega-Bubble Ø1,800(um) Visual Bubbles Ø (um) AECU / bottom face / SR60 / NG A mega & visual-bubbles shaped on the circuitry. AECU / top face / AR35 / NG A mega-bubble shaped over IC-leads.

4 Ⅶ. Secondary-Bubbles Ⅷ. Secondary-Bubbles Big-Bubble Ø450(um) Component Jammed in Non-conductive Ø350x300(um) Jammed in Non-conductive Ø350x450(um) MDPS / bottom face / SR60 / Acceptable Though many visual and even big-bubbles observed around small parts, it s acceptable since non-bridged. AECU / top face / AR50 / Acceptable Though number of big-bubbles jammed in small components, it is acceptable since non-conductive. Ⅸ. Secondary-Bubbles MDPS / bottom face / SR60 / Acceptable Many micro to visual size bubbles shaped around small components. Optimal Coating Film ( 良質見本品 ) Optimal Coating Film ( 良質見本品 ) IC-Leads Fine Gap: δ250(um) AECU / bottom face / RR25 / OK Optimal coating film with no-bubbles AECU / top face / AR30 / OK Optimal coating film with just a few micro-bubbles

5 3. Inadequate Coating Application in terms of, Applied Material, Thickness of Film and/or Curing Procedure Inadequate conformal coating material selection and/or thickness of film-build (mostly too thick and heavy) Inadequate coating material applying, curing and too thin or too thick film-build can be a root-cause of poor film uniformity, insufficient dielectric strength or overstress to the arrayed small components that result in coating failures. 1. Short of Knowledge and Poor Understanding about Conformal Coating Application 2. Inadequate Coating and/or Curing Process Technical Tips( 기술참고 ) T(inches)= 6.44x(V)x10-6 Proper Film Thickness Calculation / for high-voltage & high current circuitries < V is Maximum Voltage of the Circuit > From a contamination standpoint, the ability to dedicate equipment to particular coating types is important, as switching from one coating type to another can be an issue depending upon the materials. In order to minimize failures, the following questions should be asked prior to utilizing a conformal coating. 1) What are the through-put or usage requirements? 2) What is the end-use environment? 3) What application capabilities are available for automotive? - Need a selectively consistent precision layer with high-throughput & fine edge-definition: Answer) Film Dispensing - Need a consistent precision layer broadcast with high-throughput & fine edge-definition: Answer) Film Dispensing - Need a consistent layer partly selective or broadcast with no specification for film uniformity, throughput and edge-definition: Answer) Film Dispensing & Spiral Bead Dispensing - Need a consistent layer broadcast with undiluted thick material with no specification for film uniformity, throughput and edge-definition: Answer) Spiral Bead Dispensing 4) What is the end-use of the assembly (high-power, RF, digital, etc.)? - This will dictate some of the appropriate coating material and technical requirements. 5) What are the surfaces composed of? - Need investigation any particular parts of surface are with smooth residues, release agents, large porous, etc. This will dictate adhesion limitations.

6 Bubbles Form in primary PSR, secondary Conformal Coating Film Effect of the bubble Ref / NASA-STD A - Bubbles bridge on uncommon conductors, trapped moisture, contaminants may reduce insulation resistance or cause shorts and possible arcing. - Excessive film bubbles can badly affect the film adhesion, and result in the film de-lamination in the long run. - Drying and/or curing schedule can significantly affect bubbles. Secondary Bubbles Primary & Secondary bubbles coexist / 50(um) Ø 325(um) Primary Bubbles in PSR Ⅰ. Primary-Bubbles 25(um) Ø 75(um) Ⅰ. Primary-Bubbles 25(um) Ø 75(um) Ⅱ. Secondary-Bubbles 25(um) Ø 225(um) Flux Residues GAP : 750(um) AECU / Primary-Bubbles already exist prior to Conformal Coating Performance AECU / Primary-Bubbles already exist prior to Conformal Coating Performance AECU / Secondary-Bubbles form on the circuitry in the c-coating procedure

7 Bubbles Form in primary PSR, secondary Conformal Coating Film Ⅲ. Primary-Secondary Bubbles coexist 25(um) Ø 75(um) BAD Scratch PSR : Photo Solder Resistor MDPS / Primary-Secondary Bubbles coexist Primary Bubbles, Secondary Bubbles The Bubble Categorization 1. Super Micro-bubble: B Ø50, 2. Micro-bubble: Ø50 B Ø Visual-bubble: Ø150 B Ø500, 4. Mega-bubble: B Ø500 Mega-bubble observation over the film shall be estimated A Reject Terminologies of Long Term Reliability & Test 1. Loss of Adhesion, 2. Film De-Wetting, 3. Film Bubble & Void 4. Film Bridge, 5. Scrap Bridge & Debris, 6. Film Wrinkle 7. Film Blistering, 8. Film Cracking, 9. Film Discoloration 10. Film Peeling-off, 11. Film Scratch, 12. Film Degradation 13. Film Edge & Point Coverage, 14. Film DFT / WFT

8 Remaining Bubbles as time goes-by (Bubbles Form & Burst in the Film) PHOTO#4 4.0(min) Stay After Dispensing PHOTO#1 1.0(min) Stay After Dispensing Ⅲ Bubbles as time goes-by 1B31 Ⅱ PHOTO#3 3.0(min) Stay After Dispensing PHOTO#2 2.0(min) Stay After Dispensing

9 4. Most Common Five, Film Defects in Conformal Coating 4-1. De-Wetting (Non-Wetting) During the conformal coating process, numerous de-wetting areas are observed as shown right. MDPS / bottom De-Wetting caused by Silicone Contaminant The critical parameter to the success of a conformal coating is its ability to adhere to the surface it is applied to. Since most coatings are liquids, the coating must be thick(viscous) enough that it remains on the PWB surface as a uniform film but not too thick or have too high a surface tension to cause voids. The wetting is influenced by changes in the composition of the coating and compatibility with the PWB surface. (solder mask, board or components) De-Wetting due to Silicone Contamination 1. Improper Cleaning PWB Surface Contaminants Usually the contaminants can consist of trapped moisture, rosin or water-soluble flux residues, oil and grease, fingerprints, etc. 2. The Presence of A Silicone-based Adhesive on PWB prior to Conformal Coating Process 3. Oil Contamination of Pneumatic Lines 4. Accidental Changes in the composition of the Coating and Compatibility with the PWB Surface 5. Release-Agents designed to free plastic parts from the Molds De-Wetting due to Ionic Residues 4-2. Orange Peeling It exhibits dimpled coating film-surface, like the skin of an orange. 1. Improper Dispensing-gun Adjustment and/or Techniques 2. Extreme Workshop Temperature and/or Humidity : That dries out the coating material droplets too quickly before they can flow and level properly. 3. Too Thick Applied Material Orange-Peeling / A Typical Film 4. Insufficient Flash-time and/or Improper Drying Condition 4-3. Runs It exhibits heavy application of dispensing material that fails to adhere uniformly to the surface 1. Excessive Film Thickness and/or Inadequate Material Formulation 2. Insufficient Flash-time and/or Improper Drying Condition 3. Coating Materials, or PWB Surface Too Cold Runs / Not to exceed 5% over PWB surface area (NASA-STD A) Runs / A Typical Film

10 4-4. Fish Eyes It exhibits small crater-like openings in the finish. 1. Improper Surface Cleaning and/or Preparation : Usually involving deposits of silicone or wax on the old surface 2. Effects of a finish with embedded Silicone that is non-removable by solvent wiping 3. Oil Contamination of Air Lines Fish Eyes / A Typical Film Fish Eyes / Not to exceed 5% over PWB surface area (Ref. NASA-STD A) 4-5. Pin Holing Tiny holes exhibit in the coating film-surface. 1. Improper Surface Cleaning and/or Preparation : Usually the result of trapped solvents, air or moisture 2. Moisture or Oil Contamination of Air Lines 3. Coating Application is Too Wet 4. Insufficient Flash-time and/or Improper Drying Condition Pin Holes / A Typical Film 5. Inadequate Workshop Cleanliness and Workmanship in Pre & Post Coating Procedure Inadequate Pre-cleaning of PWB, Poor SMT/Coating Workshop Cleanliness (Lower degree of ISO CLASS#8) and Surroundings, Poor Workmanship and/or Quality Control will easily generate such kind coating film defects of - Non-Wetting: Estimated that a kind of missing in coating coverage caused by contamination / A Fail - Orange-Peeling: A severe orange-peeling appearance on the film surface needs to be reworked while a general one which is estimated not-that-severe visibly can be acceptable. - Runs: Acceptable once the Runs occupies as many as 5% or less over the entire coating space - Fish-Eyes: Acceptable once the Fish-Eyes occupies as many as 5% or less over the entire coating space - Pin-Holing: Acceptable once the Pin-Holing occupies as many as 5% or less over the entire coating space in the process of conformal coating and beyond. In addition, it may come some coating associated defects of - Bridged Scraps on conductive zone: May be a root-cause for short circuit / A Fail - Severe Debris: Need to improve the workmanship and SMT surroundings since it can be a cause for losing insulation resistance and results in a short circuit after all / Conditionally Pass - Deep Scratches on conductive zone: May be a root-cause for short circuit / A Fail in the process of primary PSR coating through post conformal coating. Each of the following photos in the next page shows a concrete case-example.

11 Ⅰ. A Scrap-Bridge on Conductors Ⅱ. Debris and Scraps Secondary-Bubbles Non-bridged, Visual-Bubbles Bridged, Wire Scrap on Conductors Inadequate Works Cleanliness & QC / No Good Poor workshop cleanliness and/or workmanship This style wire-scrap can cause a severe short-circuit. Inadequate Works Cleanliness & QC / No Good Poor workshop cleanliness and/or workmanship Severe debris / scraps in the film can cause a loss of adhesion and dielectric-strength. Ⅲ. A Scrap-Bridge on Conductors Ⅳ. A Wire-Scrap & PSR Delamination PSR Primer Film Delaminated Nearly Bridged-Scrap on Primary Bubbles Secondary Bubble Inadequate Works Cleanliness & QC / No Good Poor workshop cleanliness and/or workmanship This style wire-scrap can cause a severe short-circuit. Inadequate Works Cleanliness & QC / No Good Poor workshop cleanliness and/or workmanship This style wire-scrap can cause a severe short-circuit. Ⅴ. A Scratch in Conductive Zone De-Wetting by Silicone Contaminant Deep Scratch in Conductors Inadequate Workmanship & QC / No Good Poor PWB pre-cleaning and workmanship This style scratch can cause a severe short-circuit.

12 TOP12 / Common Film Defects in General Coatings CRACKING A series of deep cracks, resembling cracks in a dry pond exhibit. 1. Excessive Film Thickness : That magnifies normal stresses/strains in the film 2. Poorly Mixed Materials 3. Insufficient Flash-time CRACKS / A Typical Film ORANGE-PEELING It exhibits dimpled coating film-surface, like the skin of an orange. 1. Improper Dispensing-gun Adjustment and/or Techniques 2. Extreme Workshop Temperature and/or Humidity : That dries out the coating material droplets too quickly before they can flow and level properly. 3. Too Thick Applied Material 4. Insufficient Flash-time and/or Improper Drying Condition ORANGE-PEEL / A Typical Film PINHOLING Tiny holes exhibit in the coating film-surface. 1. Improper Surface Cleaning and/or Preparation : Usually the result of trapped solvents, air or moisture 2. Moisture or Oil Contamination of Air Lines 3. Coating Application is Too Wet 4. Insufficient Flash-time and/or Improper Drying Condition PINHOLES / A Typical Film RUNS It exhibits heavy application of dispensing material that fails to adhere uniformly to the surface 1. Excessive Film Thickness and/or Inadequate Material Formulation 2. Insufficient Flash-time and/or Improper Drying Condition 3. Coating Materials, or PCA Surface Too Cold RUNS / A Typical Film

13 FISH EYES It exhibits small crater-like openings in the finish. 1. Improper Surface Cleaning and/or Preparation : Usually involving deposits of silicone or wax on the old surface 2. Effects of a finish with embedded Silicone that is non-removable by solvent wiping 3. Oil Contamination of Air Lines FISH EYES / A Typical Film BLISTERING Bubbles/pimples appear in the film, sometimes months after applying. 1. Improper Surface Cleaning and/or Preparation : Usually dirt on the surface can act as a sponge and hold moisture. As temperature or atmospheric pressure increases, the moisture expands and builds-up pressure, resulting in blisters. 2. Excessive Film Thickness : Trapped air, moisture & solvents escaping in the undercoats may blister. 3. Contamination of Air Lines with Oil, Water & Dirt BLISTERS / A Typical Film SOLVENT POPPING Tiny holes exhibit in the coating film-surface. 1. Too Fast Solvents Evaporation 2. Excessive Film Thickness : Trapped solvents in the undercoats that later escape SOLVENT POP / A Typical Film PEELING (De-lamination) Loss of adhesion between the coating film and substrate 1. Improper Surface Cleaning and/or Preparation : Dust or other surface contaminants preventing the film from coming into proper contact with the substrate. 2. Use of Incorrect (mismatched) Primer/Sealer, or Substrate/Sealer PEELING(Delamination) / A Typical Film

14 WET SPOTS It exhibits spots of various sizes that are discolored, slow drying, or both. 1. Improper Surface Cleaning and/or Preparation 2. Improper Drying and/or Excessive Undercoats Film-build 3. Contaminated Undercoats WET SPOTS / A Typical Film SWELLING (Wrinkling) Surface distortions exhibit while the coating materials applied or drying. 1. Improper Drying : Too fast forced drying-off the topcoats causing the undercoats to soften 2. Excessive Topcoats over Insufficient Undercoats Flash : Piling on heavy & wet coats, which don't allow the wet undercoats to release their solvents and set-up at the same rate as the topcoats. 3. Incompatible Combination of Materials 4. Improper Workshop Temperature, or Drafts SWELLING(Wrinkling) / A Typical Film CRAZING Fine splits, small cracks, or crowfeet, that completely checker an area by an irregular manner. 1. Too Cold Workshop and/or Extreme Humidity : Literally causing the original material to shatter under the softening action of the solvents being applied. CRAZING / A Typical Film LIFTING Surface distortion or shriveling exhibits while the topcoat is being applied or drying. 1. Incompatible Materials (Coating Layers, Top vs Under) : Solvents in the new topcoat attack the old surface. 2. Insufficient Flash-time or Drying between Coats 3. Improper Surface Cleaning and/or Preparation : Ensure the surface is completely dry before applying coats. LIFTING / A Typical Film