Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Paste Dispensing

John Storjohann

#4871

Paste Dispensing | 23 February, 2000

Anyone out there have any experience with paste dispensing equipment? With our high mix of baords and very low volume (< 10 of any one assembly) we are thinking of using dispensing as opposed to screen printing. We think the cost of and the maintenance aspect of stencils would be prohibitive. Any comments?

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John

#4872

Re: Paste Dispensing | 23 February, 2000

John, I'll try to take a crack at it. We manufacture our own boards, and have a high mix. We currently have BOTH a Camalot 2800 Dispenser and a DEK 260 Screen printer. We use the camalot for glue applications, and prototype paste. We use the printer for paste at higher volume. Dispensers such as the 2800 are good for 50+ mil work on hasl boards. Solder paste sold for dispensing is thinner, and has a higher flux content so it tends to slump more, so you might have problems with 20- mil parts bridging. Also, coated copper boards have some solderability issues. I think the dispensing paste is more expensive, and it definitely takes more time to dispense, but can usually keep up with a mid speed placement machine. Screens are expensive and are a pain in the a** to keep clean. You will also have problems with screens if your engineering department likes to change pad layouts on a weekly basis. Good luck

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John Sims

#4873

Re: Paste Dispensing | 23 February, 2000

John,

We have a Camalot 3800 that we have used to prototype double sided reflow boards. The flexibility a quick turn around work well for prototyping, but I wouldn't want to run production with it. The paste is 3 times the cost per unit volume of paste that you could print with. Also, the needle on the dispenser becomes clogged easily, so I generally have to watch every board go through. Also, it does slump easily and doesn't seem to be suitable for any fine pitch applications.

Good luck, John S.

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#4874

Re: Paste Dispensing | 23 February, 2000

John we paced through the same question recently.

Both automatic dispensers and printers can be used to deposit SMT adhesive or solder paste on substrates with a relatively high component density. Dispensers are superior to printers in terms of exact positioning and precise deposit volume control. New generation automatic dispensers can be re-programmed for a new product much faster than a printer can be setup. Dispensers do not require the lost time in developing a stencil for new or changed products. If the purchase decision is strictly based on capital, buyers generally choose printers over dispensers. A further comparison of dispensers and printers follows.

Factor|Dispenser|Printer || Solder paste waste (%)|4|30 Operator concerns|Very clean, no mess|Paste is messy |Straight forward machine clean-up|Stencil cleaning capability required ||Waste disposal requirements similar to other activities Deposition control|High control over amount paste|Difficult to control amount of paste ||Stepped stencils can be a nightmare ||Stencil apertures clog with paste |Coplaniarilty issues controllable|Coplaniarity issues make printing difficult |Paste rheology does not play a large role in dispensing|Rheology requires constant working of paste |Requires process control to prevent missing dots|Requires process control to prevent misprints, clogged apertures Flexibility|Allows frequent design changes|Stencil(s) required for each production stage Material handling|Nozzles require cleaning|Stencil cleaning capability required ||Stencils must be stored, maintained, and controlled Change-over time between products |Quick as loading another program|Stencil, vision system, printer, and substrate must be aligned Change over time between materials |Quick as loading another program|Stencil, vision system, printer, and substrate must be aligned |Change syringe and purge|Requires cleaned-up of previous material Solder paste cost|Expensive -400 to +500 mesh, negated somewhat by less waste|Less expensive New or revised product start-up|Couple of hours for programming|Hour plus to reduce and adjust apertures, create Gerbers, liaise with suppliers, write purchase orders la di da etc etc ||Stencil foils required for each side and version of product Specialized knowledge|Requires special knowledge to program machine|Operators without special skills can setup machine Speed|Slower, may loose 70% of tact on fast chip shooters|Can be put in front of and use fast chip shooters effectively

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