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AUTOMATIC REFERENCE PRINT HEIGHT DETECTION FOR SOLDER PASTE SCREEN PRINTERS

IP.com Disclosure Number: IPCOM000008789D
Original Publication Date: 1998-Sep-01
Included in the Prior Art Database: 2002-Jul-12
Document File: 3 page(s) / 117K

Publishing Venue

Motorola

Related People

Andrew Cornelius: AUTHOR

Abstract

One specific problem associated with printing solder paste onto printed circuit board pads is repeatedly maintaining a gasket between the stencil through which the paste is deposited on the pad itself. All stencils are not the same, different manu- facturers use different "glues" which bond the sten- cil to the frame that surrounds it. It is the glue thick- ness variation that is the cause of the problem from set-up to set-up or simply from print to print. If a gasket is not formed, paste can be allowed to bleed outside the preferred printable area and cause sol- dering defects such as shorts (solder bridges) on tine pitch components and solder balling. One other effect of not having an adequate stencil to pad gas- ket is a variable separation speed i.e. as the squeegee wipes across the stencil rolling the paste into the apertures the stencil will separate from the board in an uncontrolled manner, this may lead to paste release issues and show itself as "dog earring" etc.

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MOTOROLA Technical Developments

AUTOMATIC REFERENCE PRINT HEIGHT DETECTION FOR SOLDER PASTE SCREEN PRINTERS

by Andrew Cornelius

INTRODUCTION DETERMINING "ZERO SNAP OFF"

  One specific problem associated with printing solder paste onto printed circuit board pads is repeatedly maintaining a gasket between the stencil through which the paste is deposited on the pad itself. All stencils are not the same, different manu- facturers use different "glues" which bond the sten- cil to the frame that surrounds it. It is the glue thick- ness variation that is the cause of the problem from set-up to set-up or simply from print to print. If a gasket is not formed, paste can be allowed to bleed outside the preferred printable area and cause sol- dering defects such as shorts (solder bridges) on tine pitch components and solder balling. One other effect of not having an adequate stencil to pad gas- ket is a variable separation speed i.e. as the squeegee wipes across the stencil rolling the paste into the apertures the stencil will separate from the board in an uncontrolled manner, this may lead to paste release issues and show itself as "dog earring" etc.

  What is clearly necessary is a device that will detect the exact position of the stencil each and every time on all screen printing equipment. This would mean that each and every time a stencil is inserted into the machine, the machine with the cor- rect associated control system in place, would detect where the stencil is and therefore be in a position to drive the printed circuit board the required distance to gasket fully with the stencil. This is the point of zero snap-off and from this point the post print sepa- ration can be controlled at a given rate, eliminating another variable from the process.

  In order to demonstrate a working solution to the problem, I have to refer to one particular screen printer design and perhaps one of the most com- monly used screen printers in the industry.

  Figure 1 demonstrates this screen printer's oper- ating principle with no machine calibration. This set-up will leave a large print gap which is undesir- able to the process.

  A method of determining the stencil height or the exact point at which the stencil lies within the printer is to sense the position of the stencil by use of a microswitch or similar sensor strategically posi- tioned within the machine.

  Figu...