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Cleanliness And Lead Free Conformal Coating Processing

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Conformal coating reliability is a major issue with lead free processing. The factors that influence reliabiltiy include the cleanliness of the board and the proesses used.

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Cleanliness And Lead Free Conformal Coating Processing

  1. 1. Lead Free Coating, Cleanliness and the Conformal Coating Process
  2. 2. Outline <ul><li>What is cleanliness? </li></ul><ul><li>Considerations for lead free processing </li></ul><ul><li>Why is cleanliness important for conformal coating? </li></ul><ul><li>How can we measure cleanliness? </li></ul><ul><li>How can we ensure cleanliness? </li></ul>
  3. 3. What is cleanliness? <ul><li>Cleanliness will determine: </li></ul><ul><li>Application performance </li></ul><ul><li>Conformal Coating finish </li></ul><ul><li>Long term circuit reliability </li></ul>
  4. 4. What is Cleanliness? Imagine your bare printed circuit board Plating / Etch Salts Flux Residues Grease / finger salts Oils Soya Sauce
  5. 5. No-clean Flux Residues Grease / finger salts Hand cream Oils (eg. Wave, Silicone ) What is Cleanliness? Imagine your assembled printed circuit board
  6. 6. <ul><li>In a perfect world, substrates would be completely clean before coating </li></ul><ul><li>Can be as many as 13 chemical processes in manufacture </li></ul><ul><li>Interaction between residues? </li></ul>What is Cleanliness?
  7. 7. <ul><li>Different residues have different effects: </li></ul><ul><li>Ionic (flux-residues) – corrosion </li></ul><ul><li>Non-ionic (greases, oils etc.) - coating issues such as de-wetting, delamination </li></ul>Why is Cleanliness Important?
  8. 8. <ul><li>E.g. No Clean flux residues, finger salts </li></ul><ul><li>Osmosis leads to Blistering… leads to Corrosion! </li></ul>The Effect of Ionic Contamination
  9. 9. <ul><li>What is Osmosis? </li></ul><ul><li>“ pumping action” caused by: </li></ul><ul><li>a contaminant solubalising and re-crystallising between the coating film and substrate, as moist air moves in and out. </li></ul><ul><li>The contaminant is usually a salt and each induces different osmotic pressure which may be extreme. </li></ul>Conformal Coating
  10. 10. Contaminant left on a coated board at 25 o C 50% RH
  11. 11. Board exposed to Humidity
  12. 12. Moisture has “found” contaminant, and a blister has formed through osmotic pressure
  13. 13. The soluble contaminate has now formed a conductive fluid with the moisture
  14. 14. A dendrite is forming between tracks
  15. 15. <ul><li>Dendrites are a problem! </li></ul><ul><li>Grow and blow phenomenon </li></ul><ul><li>Estimated root cause of 25% of returns </li></ul>Conformal Coating
  16. 16. Corrosion On Pin Delamination Of Coating Coating Dissolving & Cracking In Circled Area
  17. 17. Non-ionic contamination <ul><li>Major Problems seen during conformal coating application: </li></ul><ul><li>Conformal Coating de-wetting </li></ul><ul><li>Lack of adhesion of conformal coating </li></ul><ul><li>Delamination of conformal coating </li></ul>
  18. 18. Where do residues come from? <ul><li>Board fabrication </li></ul><ul><li>Components </li></ul><ul><li>Assembly equipment </li></ul><ul><li>Soldering processes </li></ul><ul><li>Operator handling </li></ul><ul><li>Incorrect cleaning </li></ul>
  19. 19. So you’re told to use no clean <ul><li>In the past to produce Hi-Reliability boards without cleaning would not have been considered. It can be now, if :- </li></ul><ul><li>Incoming boards and components inspected for cleanliness. </li></ul><ul><li>All operators doing in-process handling should wear gloves. </li></ul><ul><li>“ low residue” no clean flux/paste is used. </li></ul>
  20. 20. So you’re told to use no clean <ul><li>The soldering window must be fine tuned and the system controlled. </li></ul><ul><li>Low residue wire/flux used for rework. </li></ul><ul><li>Process controls are put in place. </li></ul><ul><li>The process is validated. </li></ul>
  21. 21. SMT with lead free solders <ul><li>Key variables summary </li></ul><ul><li>Melting temperature of alloy </li></ul><ul><li>Flux chemistry - activation, temperature effects </li></ul><ul><li>Wetting and surface tension properties of the alloy </li></ul><ul><li>Solder balling and bridging potential increases </li></ul><ul><li>Component / board reliability </li></ul><ul><li>Compatible rework / repair </li></ul><ul><li>Compatible wave, selective soldering process </li></ul>
  22. 22. Compare SnPb and SnAgCu <ul><li>Tin-Lead 183°C </li></ul><ul><li>Tin-Silver-Copper 217°- 220 ° C </li></ul>
  23. 23. New Solder Paste chemistries <ul><li>New activators </li></ul><ul><li>New resins </li></ul><ul><li>New gelling agents </li></ul><ul><li>Better surfactants </li></ul><ul><li>Additives to prevent oxidation </li></ul><ul><li>Alloy specific fluxes </li></ul><ul><li>Give rheological character </li></ul><ul><li>Aid stencil release </li></ul><ul><li>Hold components prior to reflow </li></ul><ul><li>Remove oxides from the surfaces to be joined </li></ul><ul><li>Protect against further oxidation </li></ul><ul><li>Reduce surface tension in the soldering process </li></ul>
  24. 24. Lead free and Tin Whiskers <ul><li>What are they ? </li></ul><ul><li>Single crystal grow from electroplated tin coatings. </li></ul><ul><li>Needle like, spiral, modules, mounds </li></ul><ul><li>Diameter approximately 3-5um </li></ul><ul><li>Various lengths, shape from micrometer up to mm’s </li></ul><ul><li>A form of metallurgical stress release </li></ul>
  25. 25. SEM Photograph 150 and 900 magnification, 3 months aging Tin Whisker on Pure SnPb
  26. 26. Sn Plated Chip Caps
  27. 27. Lead free and Dendrites <ul><li>High Sn & Ag alloys promote dendritic growth </li></ul>
  28. 28. Lead free and Dendrites <ul><li>Field Failure due to dendritic growth </li></ul>
  29. 29. Summary Lead free and Reliability <ul><li>High Sn & Ag alloys are prone to dendritic and tin whisker growth. </li></ul><ul><li>Conformal coating reduces the problem. </li></ul><ul><li>All lead free fluxes are higher activation than Pb alloy solders. </li></ul><ul><li>Higher flux residue volumes. </li></ul><ul><li>Lead free & VOC free wave solder flux is 3 x solids content. </li></ul><ul><li>Less compatible than leaded solder residues. </li></ul><ul><li>More requirement to test compatibility. </li></ul><ul><li>UV40 adds mechanical stability to lead free alloys and is our hardest coating. </li></ul>
  30. 30. Further Information <ul><li>Website </li></ul><ul><li>www.conformalcoating.co.uk </li></ul><ul><li>FAQs </li></ul><ul><li>http://www.conformalcoating.co.uk/FAQs.php </li></ul><ul><li>Technical Documents and bulletins </li></ul><ul><li>http://www.conformalcoating.co.uk/technicalbulletins.php </li></ul>

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