Stencil printing | Squeegee printing pressureFp squeegee speedVp stencil deposit solder paste print gap alignment pcb-board seal force Fs separation speed Vs separation distance Ls board support
Stencil printing | Squeegee printing Print pressure Fp 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 speeds Rule of thumb :Fp = 0,2 kg* per cm squeegee blade length Fp 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 *( 1 kg ± 10 N ) Too low : - un sufficient paste - paste smearing on stencil top
Vp Stencil printing | Squeegee printing Print speed Vp Adjust printing speed according to production flow. In the range 20 mm/s - 150 mm/s, higher speed needs higher 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™) Too slow :- no rolling - smearing, when Fp is not lowered - excessive paste (too much) * stencils that bear residue from cleaning agents on the top (solder paste) side can have sliding solder paste even with correct settings
Scooping Stencil printing | Squeegee printing Squeegees 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° To minimise damage set pressure as low as possible Squeegee damage :Check squeegee visually Check ifthe squeegee scratches the stencilCheck 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 60°
1. Chemically etched : low cost not for fine pitch 3. Electro-formed : chemical built up Ni very accurate, no debris, good for fine pitch expensive Stencil printing | Squeegee printing Stencil | types According to the production process following stencil types can be distinguished : 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 Blocking of fine pitch apertures: check stencil under microscope for damage
Stencil printing | Squeegee printing Stencil | material stainless steel : different qualities acceptable life-time Laser cut or chemically etched 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
Stencil printing | Squeegee printing Stencil | apertures Paste deposit : 90 x 360 Aperture reduction : usually 10%Avoid bridging on fine pitchAllows slight misalignment Pad :100 x 400 Rounded corners avoid paste blocking r = 100-300µm smallest aperture (mm) Stencil Thickness (µm)1 200-250 0.6 150-200 0.5 150-180 0.4 120-150 0.3 100-150 0.2 75-100 Special designs can avoid solder defects i.e. solder beading
Stencil printing | Squeegee printing Stencil | pcb board Finishing: The flatness of the surface of the finishing is important to get an equal amount of solder paste all over the board. Type flatness remarks HASL bad very high shelf life, withstands heatOSP good limited shelf life, sensitive to heatNiAu good limited shelf life, withstands heatElectro less Sn good limited shelf life, sensitive to heatElectro less Pd good limited shelf life, withstands heatRosin coating good very high shelf life, dirty boards
good alignment 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) smearing bad alignment • Solder mask thicker than finishing (OSP,NiAu) more solder paste equal thickness (best case)
Stencil printing | Squeegee printing Stencil | separation Speed Vs 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 effectand un-sufficient solder paste. Vs The correct separation speed can avoid solder paste sticking in the aperture holesof 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 higherseparation speed setting will give the cleanest separation. Trial and error
Ls Stencil printing | Squeegee printing Stencil | separation Distance Ls 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.
Vertical misalignment Stencil printing | Squeegee printing Stencil | seal force Fs / print gap 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. 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.
fiducial 1 Φ alignment pad 2 fiducial 2 Stencil printing | Squeegee printing Stencil | alignment 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. alignment pad 1 Y X
Stencil printing | Squeegee printing Stencil | deposit 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. deposit Y X
Y X Stencil printing | Squeegee printing Stencil | board support 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.