Stencil printing | Squeegee printing

pressure F p speed V p seal force F s alignment solder paste squeegee stencil pcb-board separation speed V s board support print gap deposit separation distance L s

To be set to a value on which the stencil surface is just clean from solder paste after a print stroke. Higher with higher print speedsprint speeds Too low :- un sufficient paste - paste smearing on stencil top Too high : - wearing out, damaging of squeegee and screen - permanent viscosity change of the solder paste because of friction generated heat - solder paste pressed between stencil and board: solder balling, bridging, smearing Stencil printing | Squeegee printing Print pressure F p Rule of thumb : Fp = 0,2 kg* per cm squeegee blade length * ( 1 kg ± 10 N ) FpFp

Adjust printing speed according to production flow. In the range 20 mm/s mm/s, higher speed needs higher pressure.pressure The solder paste must be rolling against the squeegee. If it is sliding* over the stencil, change the speed, either lower or higher. Too fast : - no rolling - un-sufficient paste - scooping (Puck Pack™) Stencil printing | Squeegee printing Print speed V p * stencils that bear residue from cleaning agents on the top (solder paste) side can have sliding solder paste even with correct settings VpVp Too slow : - no rolling - smearing, when F p is not lowered - excessive paste (too much)

Squeegees Squeegee damage : Check squeegee visually Check if the squeegee scratches the stencil Check if small lines of solder paste are left on the stencil Rubber squeegee : longer life-time of stencil tendency to ‘scoop’ paste out of aperture Stencil printing | Squeegee printing Most squeegees nowadays are made of stainless steel. Sometimes coated with a special layer to reduce wear out and improve solder paste drop-off. Squeegee angle to the stencil mostly 60° some printer manufacturers offer 45° 60° Scooping To minimise damage set pressure as low as possiblepressure

1. Chemically etched :low cost not for fine pitch 2. Laser cut : widely spread and used technique chemical etching step after laser cut advised to remove debris 7° angle to improve paste release Possible micro debris 3. Electro-formed : chemical built up Ni very accurate, no debris, good for fine pitch expensive Blocking of fine pitch apertures: check stencil under microscope for damage Stencil printing | Squeegee printing According to the production process following stencil types can be distinguished : Stencil | types

nickel : only for electro formation not a lot of experience yet Polymer: new technique, not a lot of experience yet laser cut only supposes to give better paste release Brass : lower wear-out resistance stainless steel : different qualities acceptable life-time Laser cut or chemically etched Stencil printing | Squeegee printing Stencil | material

Aperture reduction : usually 10% Avoid bridging on fine pitch Allows slight misalignment smallest aperture (mm) Stencil Thickness (µm) Rounded corners avoid paste blocking r = µm Special designs can avoid solder defects i.e. solder beading Stencil printing | Squeegee printing Stencil | apertures Pad :100 x 400 Paste deposit : 90 x 360

Finishing: The flatness of the surface of the finishing is important to get an equal amount of solder paste all over the board. Typeflatnessremarks HASLbadvery high shelf life, withstands heat OSPgoodlimited shelf life, sensitive to heat NiAugoodlimited shelf life, withstands heat Electro less Sngoodlimited shelf life, sensitive to heat Electro less Pdgoodlimited shelf life, withstands heat Rosin coatinggoodvery high shelf life, dirty boards Stencil printing | Squeegee printing Stencil | pcb board

Solder mask : The thickness of the solder mask is a parameter in the printing result. Also the ‘white print’ for component positioning can influence print result. Finishing thicker than solder mask (HASL) good alignment smearing bad alignment Solder mask thicker than finishing (OSP,NiAu) more solder paste equal thickness (best case) Stencil printing | Squeegee printing Stencil | pcb board

Because solder paste is tacky, it will tend to stick onto the sides of the stencil holes. It is advisable to set the separation speed to eliminate the dog-ear effect and un-sufficient solder paste. Stencil printing | Squeegee printing Stencil | separation Speed V s The correct separation speed can avoid solder paste sticking in the aperture holes of the stencil and therefore cleaning cycles can be set to longer intervals. According to the solder paste properties a lower or, on the other hand, a higher separation speed setting will give the cleanest separation. Trial and error VsVs

The separation distance is the distance covered at the separation speed while separating the pcb-board from the stencil. It should be at least the stencil thickness minus the print gap. A too large value slows down the total through-put speed of the printing process. Stencil printing | Squeegee printing Stencil | separation Distance L s LsLs

The force with which the board is pressed against the stencil has to be big enough to seal off the area that has to be printed. Too big force however can damage stencil and board. Vertical misalignment How : Check that there is no gap between the board and stencil. Print gap generally set to 0 a limited negative gap is allowed but it wears out the stencil more quickly. Max 100µm extra. Check that all areas of the board are touching the stencil equally. If this is not the case the vertical alignment of the conveyor or stencil can be wrong or the board is warped. After touching, a little bit more force or negative distance ( 100µm ) can be applied but not that much that the stencil is visually deformed. Stencil printing | Squeegee printing Stencil | seal force Fs / print gap

Due to different parameters in pcb manufacturing and stencil manufacturing combined with mechanical limitations of printing machines and vision systems, it is hard to get a 100% alignment of board and stencil. An aperture reduction of 10% helps to overcome these problems. Choice your fiducials or alignment pads diagonally in the far most corners to get the best compensation for X, Y and Φ. Stencil printing | Squeegee printing Stencil | alignment alignment pad 1 fiducial 1 fiducial 2 alignment pad 2 Y X Φ

The deposit is the amount of solder paste that remains on the pcb board after printing. It has to be well defined ( box like ) and constant in volume. The deposit is determined by parameters like : stencil release, print pressure, print gap, print speed, stencil thickness, board support and solder paste properties.stencil releaseprint pressureprint gapprint speed Stencil printing | Squeegee printing Stencil | deposit Y X deposit

Board support is one of the most important things in stencil printing. It influences or even eliminates most of the other parameters like : stencil release, print pressure, print gap, seal force, deposit. Make sure you have plenty and rigid support in all areas of the pcb board. Especially when thin boards are used. Use as much support as possible.stencil releaseprint pressureprint gapseal forcedeposit Stencil printing | Squeegee printing Stencil | board support Y X