Hi Harlan, Q: Is there a relationship (weak or strong) between solids-content and viscosity or do I keep the solids-content a constant and simply ask for a lower viscosity solder-paste if I need it for (PIH)? A: Yes, but you need to determine whether that is the significant factor affecting your process. ( DOE of PIH: Refer to Journal of SMT 1995 PTH soldering with Solder paste reflow by Bill Barthel, EAC) If you try to do fine-pitch and Paste-In-Hole at the same time, I think the thickness of you stencil will have more impact to the amount of paste desposite than paste viscosity. Anyway, I think there are several factors that you need to look into at the same time. 1. Stencil Thickness 2. Stencil aperatures 3. PCB Application 4. Screen Print parameters 5. FPD lead pitch (20 mil???) Thanks Michael Yuen ---------- From: TechNet-request To: TechNet Cc: hgvince Subject: Re: ASSY: Solder Paste Viscosity Date: Wednesday, May 01, 1996 4:56PM Our experience is that there are "windows" of available viscosity for a given wt%; other factors have significant contribution by the way including the relative "size" of the particles, their shape, etc. as well as the thixotropic (thickeners) used. Overall a lower wt% paste (say 85%) can achieve a lower viscosity mix than a higher wt% (say 92%) but there is a large amount of overlap. Note also that even small increases in Wt% can make a big differnce in volume& solder (e.g. 85% by weight is ~42% by volume; 90% by weight is ~53% by volume: C. Lea, A Scientific Guide to SMT, Figure 6.7). I suggest that some of the "printability" attibutes that are "good" for fine pitch paste may work against you in paste-in-hole. I suggest contacting you're paste vendor re a appropriate "mix." ______________________________ Reply Separator _________________________________ Subject: ASSY: Solder Paste Viscosity Author: [log in to unmask] at esdigate Date: 5/1/96 3:21 PM I am trying to "optimize" a screen printing operation. This operation screens solder-paste for both SMD and Through-Hole(Paste-In-Hole). I am using a solder-paste that is 90% solids(Non-No-Clean) with a viscosity of 1M(cps)+-100K(cps). Whew!...Finally my question(s): I have found that a higher viscosity helps when doing fine-pitch but I may be hurting myself when doing Paste-In-Hole. Is there a relationship (weak or strong) between solids-content and viscosity or do I keep the solids-content a constant and simply ask for a lower viscosity solder-paste if I need it for (PIH)? Thanks Harlan [log in to unmask]