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Desktop Pick-&-Place Machine: An EETimes Community Project

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betajet
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Ariel Electronics Circuit Writer
betajet   6/19/2014 4:50:01 PM
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Regarding conductive epoxy, here's a repeat of a comment I made last year:

Around 1990 there was a promising attempt to use CNC technology to make circuit boards by Ariel Electronics (California).  They created a gadget called the Circuit Writer which extruded conductive plastic wires onto a substrate, basically a 2-D plotter with an extrusion head.  I actually visited Ariel and saw a Circuit Writer working.  I don't think the technology got anywhere, but maybe it was just ahead of its time and with newer 3D extrusions this could be done practically.  For more info, Google "ariel electronics circuit writer".

Thinking_J
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Re: Interesting
Thinking_J   6/19/2014 4:48:30 PM
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@Salbayeng.

Your comments show much experience. Agreed - conductive polymers are not going to replace solder for every application.

Like so much of life... the devil is in the details.

Cost: I can get both solder and conductive polymers cheaper than what you quoted... and what ever it is today, will be different tomorrow. Often the difference is based on where you are.

Shelf life: similar observations to above (we don't like to keep solder for more than 6 months and never reuse by placing excess back into storage)

Curing: agree.. some curing heat is generally required. But much lower temps than solder. Especially the higher temps of ROHS solders.

Smearing: correct can be easier to be corrected when using solder. But always a bad situation. Even solder balls create problems.

Wicking (surface tension): this depends on the pcb surface finish. OSP (organic surface protectant) type finish,  solder doesn't generally wet beyond where you put it. ENIG or HASL variations generally the solder will wet entire surface.
And, yes , the surface tension can re-align a component with solder. Some designs and facilities depend on this characteristic of solder. But if often this creates as many problems as it solves (tomb stoning, floating off center because on thermal issues, solder balls floating around in the assembly).  Better solution: just put the part where it belongs... and expect it to stay there. And don't bump it until it is cured or reflowed! I have seen articles by "experts" claiming the lack of wetting and automatic re-alignment with many new ROHS solders helps them reduce solder bridging! .. go figure...everyone seems to view this differently.

An item many have ignored: ROHS vs NON_ROHS compliance on components, specifically the surface finish of the connecting points. It is generally not recommended to mix soldering materials and component types. This is not an issue with conductive polymers.

 

Aeroengineer
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Re: Interesting (limiting uses of conductive polymers)
Aeroengineer   6/19/2014 4:12:03 PM
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Once again, thank you for the detailed information.  This provides interesting food for thought not just for this effort, but for efforts to manufacturer other PCBs that are in production.

Thinking_J
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Re: Interesting (limiting uses of conductive polymers)
Thinking_J   6/19/2014 3:52:11 PM
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Zeeglen: I understand your basic concern: thermal / electrical performance of conductive polymers comparted to solder (primary metal: tin .. assuming ROHS compliance).

You are correct .. conductive polymers are not appropriate for everything. But I was directing my thoughts to whom I thought would be using a table top machine with all the processing done at one station... for prototying or very small production runs.

Generally,

- Thermal conductivity:

Yep.. metal is better. But most applications don't approach the limits of either material.

- Resistance:

The differenences over 0.0005-0.001" thickness in the junction typically larger than 20 sq mil, are minor. Assuming use of quality conductive epoxy intended for this application. Unless you are sensitive to variations of less than 0.00001 ohms (you might be).. I don't think it will matter much. Many are not aware of the variations they may be experiencing with solder on current sense resistors. Good pcb layout methods are the primary concern for current sensing.

- Mechanical differences:

Many are not aware of the sensitivity of the some of the components to variations in connection process to the mechanical strength of the joints. Ceramic capacitors with high plate density (capactitance per volume) often demonstrate large deviations in quality due to this.

Example: the measured shear strength of a soldered X7R chip capacitor varies significantly with capacitance for a given size.
 I have measured a range of 2-17lbs for single location/size. While the same location/size with conductive epoxy showed very little standard deviation from 7 lbs of shear strength.

It was determined the capacitor's end platng (based on density) was the primary reason for the large std dev in the soldered joint performance. And the primary reason for this sensitivity to the quality of the capacitor end plating was exposure to higher temperatures and sensitivity to flux activation level (all mild).

Because of the physical space to work with and the minimum capacitance over temperature extremes and voltage required.... we had to work with capacitors that were (at the time) near the limits of how much capacitance we could get in a given volume (size).

I couldn't find a capacitor manufacturer that didn't display some form of this problem. I couldn't tolerate less than 5lbs shear strength on ANY product (avionics -55.+125C rating with high shock and vibration requirements). So, the ONLY solution at the time was to replace solder process with high temp rated conductive epoxy.

Connection junctions are typically a very small fraction of a square. (bulk resistance of a given material, generally specified in "squares" with a given thickness or resistance per cm)

Which is a very, very different animal compared to conductive epoxies vs copper used for traces (many squares) , often an issue when using bendable circuitry (copper vs conductive polymers).

 

Aeroengineer
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Re: Desktop pick and place
Aeroengineer   6/19/2014 1:27:46 PM
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Yes, Javier, it is started.  The idea is taking shape, and it should be a fun ride.  I am sure that we will tap you for some help on it ;)

Garcia-Lasheras
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Re: Desktop pick and place
Garcia-Lasheras   6/19/2014 12:31:17 PM
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Adam, I'm glad to see your desktop pick'n place machine is on a roll !!

About holding little boards, I've always seen these produced by building a grid panel with the appropriated total size.  I'm eager to see how do you plan to solve issue ;-)

Aeroengineer
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Re: Desktop pick and place
Aeroengineer   6/19/2014 12:08:02 PM
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I agree that DFN/QFN/QFP parts are a must.  I think that BGA's could be accomodated, though I think that the biggest issue will be the overall size and weight of the package.  I have given some initial thought into how to handle large (>100 pin QFP sized parts) packaged, though it is far from fully baked.

 

In the area of holding small PCB's, this is still very notional.  I need to do some research on this.  If you happen to have any links that might give some great solutions to small PCB holding, let me know and I will use them as research.

elizabethsimon
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Re: Desktop pick and place
elizabethsimon   6/19/2014 11:59:49 AM
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would be really nice if you could do DFN and QFN packages since theres a lot of newer parts that only come in those pacakges. BGA would be nice too but probably not practical.

Another consideration would be how to hold the PCB. Especially a small one.

I'm sure that you've given that some thought.

 

Aeroengineer
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Re: Desktop PnP--some inspiration
Aeroengineer   6/19/2014 11:58:25 AM
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Thanks, this is some great info. 

 

In some cases, stencils make a lot of sense, in other places, they do not.  I for example will turn a handful of designs in a short time period, and the board itself only costs me $1-5 for three copies.  The stencil will cost more than the board.  In this case, I can save a significant amount by having this integrated into a machine that can print the paste.

 

On the other hand, if I were trying to do a few hundred boards, then the case you presented might argue for going the stencil route as you can do this faster, though it still will require more workspace.

 

Perhaps we can do a trade and then we will post the results.

Mark.Kirkwold
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Desktop PnP--some inspiration
Mark.Kirkwold   6/19/2014 11:31:43 AM
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In my opinion, making and using a stencil is too easy to try to automate the solder-paste dispensing in such a small machine. See, for example, http://imajeenyus.com/electronics/20100109_solder_stencil/index.shtml

While you are at it, you should also have a look at

http://imajeenyus.com/electronics/20130504_pick_and_place/index.shtml

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