Hi, I am having problem looking through IPC and finding according to ISO spec what should the paste height requirement vs stencil. EXAMPLE: The stencil is 5mils thick and according to the specs we have on paste height measurement the max height to be acceptable is 1.5mils higher than the stencil and the lowest limit is .5mils lower than the stencil. That means if the paste measurement height is reading 6.5mils is acceptable but if it is 7mils then it is too high and fail. What is the industry standard on this issue? Thanks and appreciate any assistance onthis matter.
The paste height is what you establish it as. You're probably using a laser measurement system, and these depend on where your reference datum is. Some will measure from a silkscreen, a trace, or solder mask. These add height, not to mention HASL plating, because your 0 point is now about a mil or 2 over. Your USL should generally be around 1.5 mils over your foil thickness, so for a 5 mil stencil, 6.5 mils is about right as your target measureent. Then you go about .5 to 1 mil over that. Lower spec limit should be around 1.5 mils less than your target, so a reading of 5 mils for a 5 mil stencil is still within spec.