Hi Azman,
I used to work for hp and then for Jabil Circuit as Manufacturing
Engineer in Italy.
I changed position, but I didn't forget my background.

My suggestions are:
-start troubleshooting your process from the bottom up.
-try to see a sort of patterns in the defects you have got (board
position, frequency of the problem, date and time of happening)
-do not focus only on paste parameters, as some colleques said, it
seems the paste is fine.
-keep the viscosity consistent throughout the day (control humidity and
temperature INSIDE the screen printer)
-check for dog ears on your paste bricks. They tell you a lot on
printing process, one for all, that  there is some problem in
separating the paste from stencil.
-check for alloy presence under the stencil, right on the edge of the
apertures, if there is, then you don't have good gasketing.
-last but not least, check for complanarity parts-pcb on your pick and
place machine, component height, machine setting.
-is there only one pick&place machine? only one head to place parts? or
is there more? Do the shorts come from the same lane?
-look under a microscope how the parts sit on the paste brick, Is the
paste squeezed under the component leads? How are the leads VS the
paste bricks, centered?

- viscosity is important, metal loading in the paste is not second to
anything:  It is not the flux that withstand the weight of the parts.
-I might ask you about oven, but my experience is that it  plays
important role ONLY if everything else is under control.

Hope it will help

Ivan



Il giorno 25/giu/05, alle 11:36, Norazman Othman ha scritto:

> Thank Le Thanh Tung,
>
> Ok here are some parameters as you requested..
> Smallest fine pitch: 20mils
> Stencil design: Laser cut, 5mils of thickness, 10% reduction for fine
> pitch
> IC
> Printer: DEK288
> Solder paste: Kester EM907
>
>
> Best Regards,
> Azman
>
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