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SMT Solder Paste Problems Troubleshooting

Print speed and Squeegee pressure are inter-related

Factors effecting speed

Temperature

Cold = slow as pastes are very temperature sensitive

Squeegee Material

Polymer is faster than metal

Poly is simply a better squeegee (windshield wipers)

Poly wears very quickly, not common today

Sharpness

Not all metal squeegees are equal

Plating of Transition Automation is round and dull

Angle

60 degree is faster than 45 degree

Watch out for MPM; long blades = low angles with high pressures

Various Metal Blade Characteristics


Stock DEK

E-Blade

Print-Perfect

DuraGlide

Ultraprint

Supplier

DEK

AMTX

(PhotoStencil)

Transition  Automation

Photo Stencil

Alpha Metals

Material

Spring Stainless

Nickel

Spring Steel

Spring Steel

Spring Steel

Forming Method

Machined

Electroplated

Machined

Etched

Machined

Coating

None

None

“Permalex”  Teflon/nickel

Titanium Nitride

Nickel plate

Body Thickness

7.5 mils

10 mils

9.5 mils

12 mils

17 mils

Tip Thickness (bottom  0.1”)

7.5 mils

10 mils

10 mils

11.5 mils

17 mils

Relative Sharpness**

6

8

4

7

4

Stencil Roughness

Rougher better for roll

Rougher requires more pressure for clean wipe

Etched or lasered foil is more rougher than EFAB

Paste factors

Higher viscosity usually equates to slower print speed and higher pressure.

Higher tackiness usually equates to slower print speed and higher pressure.

Squeegee pressure take away message

“Apply minimum pressure to get a clean wipe, (no solder powder) around the apertures”

This is independent of all of the other variables effecting squeegee pressure

Stencil Issues

Not all stencils release equally well

EFAB is best

Laser is OK, with electro polish or nickel plate is better

Chemical etch is worst

Stencil wiping not totally effective

Automated better than manual

Coined solder can remain and cause more needed wiping

Sticking to squeegee

Paste viscosity too high

Solids too high

Paste got hot in storage or shipment

Insufficient paste on the stencil

Start with 15gr/cm of squeegee length

Squeegee lift incorrect

Too high after stroke

0.5” to 0.75” is OK

Too slow

Fast (>10mm/s) helps shear paste and leave on stencil

Pause between prints too long

Paste rheology goes back to rest viscosity

Short stroke

AOI too slow

Gasketing is critical for good paste release

Aperture must be smaller than pad in both length and width

Critical for fine pitch, small components

Legend near fine pitch is NG

Bar code or other labels near fine pitch is NG

Mask should be < pad height except over traces

HASL can be a hassle


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