Thomas: From what you're telling us, you seem to be thinking correctly:
1 Paste should not spread appreciably in preheat 2 Something has changed, because you've not had this problem, until recently
Two things to consider:
1 Has any thing changed in your oven? Blowers are wipped, profiles have stepped, etc 2 You may want to qualify your paste suppliers for the product you are manufacturing. Paste suppliers go stupid periodiclly and it might be good to have base-line measures of paste performance. Below is a snip on slump measurement from our procedure.
4 Slump Test 4.1 Print paste on white ceramic substrate or microscope slides. Using the stencil described in paragraph 3 above. 4.2 Inspect the paste at X30 magnification. The shape of the deposit should be a "brick." Record the pitch that slumping became prevalent. 4.3 Bake the substrate at 150�C for 2 minutes. 4.4 Inspect the paste at X30 magnification. There should be no slumping. Record the pitch that slumping became prevalent.
Stencil description from 3:
3.1 Obtain a stencil with 15 to 20 rows of a series of 0.025 by 0.050 inch apertures spaced 0.050, 0.040, 0.025, 0.015, 0.010, 0.010, 0.025, 0.040, 0.050 inches apart. See Appendix 1. Source of supply: Metal Etching Technology.
Good luck
Dave F
reply »