For the past year now we have been printing adhesive with no real problems. Our printing ranges from 0603's to SOIC using a variation of aperture sizes and metal thickness foils. The crucial parameters are :
aperture sizes : These obviously depend on the component to be glued, pad spacing is crucial as to big of an aperture will contaminate the solder pads, will give a greater stand-off and cosmetically look wrong, and to small of an aperture will give you problems with poor release from your stencil leading to insufficient's and missing components.A general guide line would be ; 12-16 thou diameter for 0402 16-20 " " " 0603 20-24 " " " 0805 40-47 " " " 1206 40-43 " " " mini melfs 40 " " " sot 23 Like I said the above mentioned is only a guideline. One advantage of printing adhesive is that all screen apertures can be manufactured in all shapes and sizes. The way forward would be to develop a test screen with a range of aperture shapes and sizes and see what best suits your process not forgetting the stress and flex test.
Glue selection : Again important,whatever you select ensure it has good green strength. Many glue manufacturers out there will try to sell you a glue which will be suitable for dispensing as well as printing, dont fall for this sales pitch and ask for one which is specifically designed for printing.
Print parameters : Critical. Make sure your under screen cleaner is switched off. Your speed, print gap and separation speed will govern your glue volume and whether you have peaks on your dots. Different print cycles can also be used.
Profile : Make sure your product sees 150 deg between 60 -90 secs.
I hope the above will set you on your way. Trust me this is not rocket science, If you can print solder paste then YOU can print adhesive. Please do not hesitate to contact me via email if you nedd further help or assistance.
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