What kind of Solder Paste do they use on chipshooters that can handle fast moves - Page 1
Forum section: electronics / general

author: DigitalDeath
author_rank: Regular Contributor
timestamp: May 11, 2020, 10:21:25 am
content: I'm trying to figure out what kind of solder paste they use on fast board-moving pick and place machines like carousel chipshooters that can handle fast moves of the board with the parts not moving. I'm assuming it's a kind of sticky solder paste of some sort. Does anyone has info on that?Thanks,John.

author: Reckless
author_rank: Frequent Contributor
timestamp: May 12, 2020, 06:26:11 am
content: We use no clean SAC305 solder paste, pretty much any brand works. We switch between brands indium, aim, loctite. AIM seems to be a good brand for us. Ive been considering lead solder paste but have been told it dries out much faster making it harder to work with and harder to clean stencils. The issue is less the solder paste but stencil design.

author: DigitalDeath
author_rank: Regular Contributor
timestamp: May 12, 2020, 07:34:01 am
content: Thanks for the reply. When you say stencil design are you referring to the pin’s paste mask shape or other parameter like thickness? Would you mind elaborating a bit?John

author: SMTech
author_rank: Frequent Contributor
timestamp: May 12, 2020, 07:51:34 am
content: There is no special paste for different machine types. All paste is perfectly "sticky" enough to keep parts in place, even in high speed machines, also remember those moving table machines are now much rarer. The only modern moving table machine is a MyData everyone else only moves the head these days. Chipshooters don't place anything tall or otherwise massive so the risk of those parts going flying is pretty low anyway. Bigger parts of a flexible placer with moving table (i.e a MyData) move much slower when placing large parts, as do all machines.Stencil design is primarily about getting the right amount of paste on every joint to achieve proper wetting and good solder fillet, there are IPC standards that define what those look like including angle and coverage. Too little paste and you can get very poor wetting or "IMC" too much and you can get other problems like solder balls or tombstoning. You combat these by adjusting the pad-aperture ratio, aperture shape and stencil thickness, for really complex stencil you might even have multilevel stencils that are thinner in some parts than others. Of course if the footprint is already a very poor match for the device and then you apply an aperture shrink in your stencil layer, you can end up with a device that barely touches any solder which is far from ideal and is something I see from time to time, usually caused by people not reading datasheets or specifying one Tantalum case size in the BOM and another in their layout software for instance.

author: Reckless
author_rank: Frequent Contributor
timestamp: May 13, 2020, 08:15:49 am
content: Quote from: SMTech on May 12, 2020, 07:51:34 amWhat is the smallest thickness stencil you have seen? We recently moved from 6mm to 4mm and wondering if it can be causing manufacturing issues. We had some tombstoning and solder balls with 6mm. Our stuff is all chips, nothing big or complex.

author: SMTech
author_rank: Frequent Contributor
timestamp: May 13, 2020, 09:33:07 am
content: We don't do anything too exciting, but 100micron is the thinnest we have for 0.4mm pitch and a little BGA. Mostly tho' its 120/125 for typical 0.5mm pitch QFN/QFP & boring 0805. Older stencils from before 0.5mm became pretty much ubiquitous are all 150micron.For our own layouts we typically do 1:1 apertures, for one client in particular we homeplate their 0805 apertures or we get quite a lot of balls & tombstoning. For another client we leave them as is because they are a much bigger firm and they have already clearly worked out what works for them and built it into their process & libraries. There is some maths you can use, according to a venerable chart on my wall, your stencil wall:base ratio should be about 1.5:1 or less and critically the combined surface area of all 4 walls has to exceed that of the base. Our original stencil supplier had a whole folder about stencil design they gave their clients, one of several advantages to buying proper ones.