PCB assembly costs ( ... keeping them down)

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PCB assembly costs ( ... keeping them down)

Postby bjmuld » January 27th, 2011, 10:00 am

I've got a few pcbs to make, and I would ask: "
How much does orientation of 1204, 0603 packages affect the cost of assembly? i.e. should they all be oriented either horizontally or vertically
What about silicon orientation?
Through-hole cap orientation?
Would components on both sides double costs? "

any other thoughts????
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Re: PCB assembly costs ( ... keeping them down)

Postby gadgetgangster » January 27th, 2011, 10:29 am

2-sided assembly will essentially double costs - it will have to run twice through the pick & place and reflow. You also need a second stencil & commercial assemblers don't mess around with mylar, they only use stainless steel. As for angle, it doesn't matter.

Best ways to cut assembly costs; reduce part count and number of parts, if you could keep the assembly 100% SMT, that would help. Also, simplifying the test procedure, if your assembler is also testing for you. Increasing qty is probably the easiest way.

Good Luck!
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Re: PCB assembly costs ( ... keeping them down)

Postby Evansbee » January 27th, 2011, 10:53 am

Are you looking to buy bare-boards and solder the parts yourself?

For a bare PCB things like orientation of parts doesn't really play into it. The thing that will kill you most is some formula related to number of layers and number of holes. Going all smt with big parts means that the process is pretty straightforward (you still have to drill+plate vias). I like sunstone for quick PCBs like this and I'll just solder and order parts from something like digikey.

If you're going to get the pick&place + reflow work done outside, as well, you're going to basically pay for number of DIFFERENT kinds of parts on the board due to the house having to use a variety of different feeders. In this case you REALLY want to commonize as much as possible.

-Ev
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Re: PCB assembly costs ( ... keeping them down)

Postby bjmuld » January 27th, 2011, 11:44 am

thanks gadget*.

The plan is to design for pick+place, but also keeping in mind that I'll be doing the first couple by hand... to test and verify myself. Low parts count is great, but somehow doing a Panasonic EXB or whatever (0603 size) resistor array by hand would seem near impossible, but that size seems like it's the most popular 8 element array out there (by a crude Digikey stock qty. comparison).

I suppose I'll be posting as I go. I feel I've got a good grasp on most aspects of design and fab processes, but I've yet to ride the bull.
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Re: PCB assembly costs ( ... keeping them down)

Postby SiliconFarmer » January 27th, 2011, 1:22 pm

You said "a few" boards. Is it really just a few?

Under 100 total? If so, be prepared for sticker shock. Gross rule of thumb: assembly will cost about what your parts cost, +/-50%. Doing anything you can to reduce parts count will help. Assembling them all yourself will become an attractive option after you see the assembly quote.

Is it 1000/month? Then you need to analyze what 8 individual (but same value) 0603 resistors will cost you vs one array of resistors. Either one is a single reel of parts, so assembly cost difference at those volumes is in the noise. I'd use an array if it saved board space, including routing.

For a "more than a few" quantity, have an assembly house (or an experienced manufacturing engineer) do a "design for manufacturability" (DFM) review of your first working prototype board design, and then plan to re-design your board (and re-prototype it!) based on their feedback to reduce cost. For 1000/month, you will also need a "design for test" review.
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Re: PCB assembly costs ( ... keeping them down)

Postby Ernie » January 27th, 2011, 2:19 pm

bjmuld: how are you going to reflow the solder?

I've done a few SMD boards myself, both single and double sided. Mine you I did these at and for my job, so I have a few toys to play with there. Here's how I assemble a few (dozen to 100) units by hand:

First off, solder: For SMD I get mine in tubes, comes as spherical drops of solder in a flux suspension inside a syringe tube. Using a fairly narrow boar tip and a dispenser (air source controllable via a foot switch) plus a microscope I'll dab solder to each pad. Use less then you think you need, only a tiny amount is needed. Our inspectors always told me I use too much.

Next off, device attach: Tweezers and again a microscope do the trick. I do have a hand held suction picker but never used it, tweezers do the job fast. A good assembly drawing in essential to get some sore of productivity. It always surprises me how far the surface tension of solder will twist a part into proper position, so when placing parts "close" is good enough.

Solder reflow: I have two devices for this. For small single sided units I have the neatest bench top unit, 3 parallelogram hot plates that form a half circle. Other half of the circle is a Formica top. In the center are 8 arms that sweep a circle, with a cam that lifts the arm at the end of the last hot plate. Its an open air 3 zone reflow device that does a really good job! My other machine is a 4" belt furnace, maybe 5' long. I'll use that for larger boards, or the second side of a double sided board.

Double sided boards need no special attention. Just flow one side, then flow the other. I'll usually put the part on a "boat," the general term we use for something for the board to ride on, like a blank piece of FR4. Surface tension of the solder keeps the parts on the bottom when flowing the top. I've also noticed some chain reflow ovens that only have a center heating element on the top, so the top side gets hotter then the bottom.

Hand soldering SMD is also possible, to the point where I actually prefer SMD parts for breadboards. For Rs & Cs I'll tin one end of the board, drop the solder for the part in tweezers, place part and flow one end. Then as the part is stuck in place the other end can be heated and solder applied. ICs can be done pin by pin, or with a "flood & wick" method where you flood solder on all pins then wick off the excess. I've had some luck doing

Breadboards? I like either good ole DIP pattern boards or simple plated thru holes. DIP pattern gives you nice buses while holes only give you the most options, and buses can be added with solid bus wire. 0805's and SOT-23's go down great on .1" holes. For SOICs and such search out adapter boards on Ebay. I get mine from some company in China for dimes a piece. I always keep a pile of Radio Shack breadboards on hand for "something quick."

Now when you finally toss this over the fence to production the job will stop dead unless you've added fiducials to the board. These are targets (2 minimum) on each panel and each circuit on the panel that the pick n place used for orientation. Small circles exposed from solder mask will suffice.
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Re: PCB assembly costs ( ... keeping them down)

Postby wellernumber7 » January 27th, 2011, 6:01 pm

"set up" costs can be killer.

Things like photo artwork for PCBs, or the stencil for silkscreening solder paste, or programming the pick and place machine, all add up.

Making 3 boards is often nearly the same cost as making a single board.

Single sided is quicker (only one paste, one pick and place, one reflow) and thus cheaper.

Orientation of components is handy if there's any visual inspection, but otherwise doesn't add any cost.

Obviously be aware that production machines need different things: maybe a pick and place machine likes 4 mm clear on 2 opposite edges of the board to be able to take it through the machine. Not having that 4 mm clear may mean that an operator has to manually place a few components.
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Re: PCB assembly costs ( ... keeping them down)

Postby wellernumber7 » January 27th, 2011, 6:05 pm

Ernie wrote:Double sided boards need no special attention. Just flow one side, then flow the other. I'll usually put the part on a "boat," the general term we use for something for the board to ride on, like a blank piece of FR4. Surface tension of the solder keeps the parts on the bottom when flowing the top. I've also noticed some chain reflow ovens that only have a center heating element on the top, so the top side gets hotter then the bottom.
[...]
Now when you finally toss this over the fence to production the job will stop dead unless you've added fiducials to the board. These are targets (2 minimum) on each panel and each circuit on the panel that the pick n place used for orientation. Small circles exposed from solder mask will suffice.


Obviously some heavy components can move or drop off when reflowing the second side, so watch out for those.

Also, nice clean fiducials are really helpful, not just for the pick and place machine but also for aligning the silkscreen machine.
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Re: PCB assembly costs ( ... keeping them down)

Postby gadgetgangster » January 27th, 2011, 7:11 pm

bjmuld wrote:thanks gadget*.

The plan is to design for pick+place, but also keeping in mind that I'll be doing the first couple by hand... to test and verify myself. Low parts count is great, but somehow doing a Panasonic EXB or whatever (0603 size) resistor array by hand would seem near impossible,


If you have space, discretes should be fine. I use an 0603 array for the Propeller Platform USB, when I prototyped them I would occasionally have problems.

But try to keep from doing a 2 assembly if you can - that does drive cost up. And setup costs are significant - it probably won't be worth it unless you want to do a few hundred units.
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Re: PCB assembly costs ( ... keeping them down)

Postby bjmuld » January 29th, 2011, 7:14 pm

Thanks, everyone for you help. There's a lot here to go o'er. I'll not be near the 1000/mo range, but more like 32-64/mo if i'm lucky. In-house hand-assembly seems like the no-brainer option here, but I'm hesitant to sell a board I reflowed in an EZ-Bake ;) I have seen excellent results, and I'm sure I could achieve them, but I'll have to wait and see how severe the sticker shock is first. Also, I know Aoyue has good stuff at reasonable prices, so maybe I'll look into that.
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