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Cleanliness And Lead Free Conformal Coating Processing
 

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.

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 Cleanliness And Lead Free Conformal Coating Processing Presentation Transcript

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