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Improve your printing

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  1. Improve your printing Richard Boyle

  2. Printing • Most important part of the manufacturing process • The most common method of applying solder paste • May be stencil or screen • May be fully automated or manual • Must be consistent

  3. Electro- Laser Chemical Hybrid cut etched formed Cost $$$ $$$$$ $$$$? $$$$$$ Accuracy OK Accurate OK / Accurate Accurate Min. Aperture 0.225 0.150 0.150 0.150 at 0.15 thick (0.4 pitch?) Aperture Shape Material Stainless steel Nickel Stainless steel Stainless steel Brass? Fiducials Good Good Good Position? Stencil features

  4. Sandcastles…

  5. Which method to select? • “All methods can give bad results”(SMART Group, UK) • Coarse pitch to 0.7 mm QFPs Chem etch • Fine pitch, 0.65 mm and less Laser cut • Many apertures or very thin Electroform

  6. T L W Stencil thickness • For complete paste release • L x W > 0.662x(L+W)xT • W >1.5 T

  7. T D L W For small squares and circles • Area of aperture openings • Square apertures = L x W = W2 • Round apertures = pD24 • Area of aperture walls • Square apertures = 2 (L+W)T = 4WT • Circular apertures = pDT

  8. T D L W So for good release for BGAs, 0201s… • For squares W2 = W > 0.66 4WT 4T • For circles pD2 = D > 0.66 4pDT 4T

  9. Hence suitable stencil thickness • Coarse pitch 0.150 - 0.200 mm • 0.5 - 0.65 mm QFPs 0.125 - 0.150 mm • 0.4 mm QFPs 0.100 - 0.125 mm • 0.3 mm QFPs 0.075 - 0.100 mm • BGAs 0.125 - 0.150 mm • CSPs 0.075 - 0.100 mm

  10. Paste deposit Land Stencil Gasket seal Aperture width • 10 - 20% smaller than lands

  11. Squirt-outs or mid-chip balls

  12. Aperture designs • 10 - 20% smaller than lands “Home plates”, etc

  13. 0.5 mm BGAs • Pad > 0.5 mm dia? • Aperture = pad dia • Pad =/< 0.5 mm dia? • Aperture = dia but square shape

  14. Suggestions for pads & apertures Pitch n/a n/a 0.5 0.4 1.25 1.00 0.50 Pad lxw 0.65 x 0.5 0.4 x 0.25 2.3 x 0.3 1.5 x 0.23 0.65 dia 0.38 dia 0.30 dia Aperture lxw 0.60 x 0.45 0.38 x 0.28 2.2 x 0.25 1.4 x 0.18 0.65 dia 0.38 x 0.38 0.30 x 0.30 Thickness 0.125 - 0.150 0.125 0.125 - 0.150 0.125 0.150 0.125 0.125 Device 0402 0201 QFP QFP BGA FP BGA CSP

  15. 5 0.2 Also…. • Divide large apertures to stop scavenging by flexible squeegees and enclosed head

  16. Print Speed • Print speed is mainly dependent on paste type • Also dependent on print frequency, temperature, age of paste, pitch and squeegees • Excessive speeds may lead to “over-shearing” of paste causing flux bleed & smudged prints • Low speeds may lead to poor print quality and slow squeegee drop-off

  17. Full Paste Viscosity Profile

  18. Flux Bleed

  19. Pressure • Excessive pressure may lead to flux bleed • Excessive pressure may lead to “scooping” • Low pressure will lead to smearing of paste on the stencil leading to poor print definition and drying of paste

  20. High Pressure “Scooping” Flux bleed

  21. Low pressure “Smearing” on stencil “Dog ears”

  22. Board Support • High pressure should not be used to overcome problems due to poor board support • This will result in poor print quality and stencil damage • Put in more support to ensure good contact between board & stencil

  23. Dedicated tooling

  24. Dedicated vacuum tooling

  25. Form Flex advanced tooling • Supports PCB, components and stencil

  26. Tooling; Simple support pillars

  27. ­ ¯ ­ ¯ Print direction Print direction Board support Insufficientsupport Insufficientsupport? “Excesssupport” 1 worn squeegee

  28. Poor Board Support Stencil Paste left on stencil

  29. Damaged Squeegees

  30. Print Gap • Printing is usually “on contact” ie: print gap = 0 • This should reduce print “smudging” and flux bleed • Thick solder resist or inks may prevent true “on contact “ printing • If printer is not correctly calibrated the print gap may be less than 0 which will result in poor prints and stencil damage

  31. Pad surface HASL Gold on nickel Organic coating Resist Thinner than pads Board considerations

  32. Vias, notation and bar codes

  33. Understencil wiping • In a perfect world where your printer has a 100% gasket between the stencil and the board. All of the boards are perfectly flat, with flat pads and the resist is correctly placed and of the correct thickness. Then you should never have to use understencil wiping. • How ever this being the real world you will have to use it.

  34. Understencil wiping • This is used to clean of flux medium and paste from the under side of the stencil. This builds up if paste is squeezed out under the stencil during the print process. It also helps to remove paste starting to block the appertures in the stencil. • It works by having a roll of cloth that wipes this material from the stencil.

  35. Understencil wiping • It works best by using a combination of wiping and a vacuum. • First use a wet wipe, this helps to remove paste and loosen dried on material. It will not work if the solvent level is low or the application tube is blocked so this much be checked at the start of each shift. Also the correct solvent should be used. If it evapourates too fast then the paste could dry, if it is too slow or too much solvent is applied then this could stay in the appertures causing problems with subsequent prints • A dry wipe helps to remove solvent • A vac process sucks powder and paste out of the appertures.

  36. Poor understencil wiping

  37. Which leads to this

  38. What is a Solder Paste?

  39. Solder Paste Powder Particles 0.5mm Flux Medium

  40. Solder Powder • Available in most solder alloys • Formed by Atomisation • Available in a variety of sizes

  41. Powder Production (Old Technology) – stopped mid 2003 Molten Alloy Spray nozzle N2 N2 Oversize Sieves (turboscreens) Useful Powder Fines

  42. Powder Production (ultrasonics) Molten Alloy N2 N2 Ultrasonic assisted atomiser Oversize Sieves (ultrasonic assisted) Useful Powder Fines

  43. Powder atomisation comparison a) Volume %

  44. Powder atomisation comparisonb) number%

  45. Assembly defects mass production Existing solder Manufacturing failure rate Refined powder size Factory feedback Fine pitch printing 0.4mm BGA Inconsistent volumes Low Volumes Existing solder Consistent print volumes New refined powder solder

  46. What does the flux medium do ? • Flux ensures successful solder joint is formed between component and pad. • Suspends powder • Provides correct rheology and tackiness • Cleans surfaces • Removes oxide from solder powder • Protects surfaces • Leaves safe (or removable) residues

  47. Shield & Component movement • CuNiZn shield placed on pcb and reflowed • Movemented with paste A on both shields (inset and main view) – no movement for paste B Paste A Paste B