What kind of consistency does solder paste for BGA parts have?

Factual Questions
1

Suppose you’re trying to build a prototype circuit board. You’ve picked the parts, you’ve got the layout, and you’ve ordered the board made from one of the shops that does them. So you’ve got the board and a baggie full of parts from digikey.

The way it’s currently done, as I understand it, is you send the schematic file to a piece of software that will create a plastic template for you. You have the template printed, giving you a piece of plastic with holes where the soldier goes. You lay the mask down on the board, paint on the solder paste, and lift it off.

So ok. I’ve not personally done this part because it sounds finicky. A slight misalignment, and the soldier paste is not aligned with the pads on the board. Reapply, and now the paste is bridging across pads, shorting it out.

The next part is more finicky still. You grab all the surface mount resistors and caps out of a bag, and with tweezes have to somehow put each one down where it goes. This sounds difficult - a slight bit of hand tremor on placement and won’t it smear the solder paste, creating unwanted shorts?

Actually installing the main IC for your board sounds even worse - nudge the chip slightly and you’ve just shorted 20 connections.

So do they make non smearing soldier paste that doesn’t move much on it’s own? Something that can only be applied at an elevated temperature, so that it clumps up on the board and you can just drop the chips down and shove them around until they are aligned?

The reason I ask is because I saw this nifty gadget, here. It apparently will print the solder paste onto your board for you, and you just have to drop the chips onto the board. It includes a heated surface that can apparently heat the board up enough to melt the solder, for a no fuss no muss prototype board.

Is this likely to work as easily as it sounds?

2

No. Your nifty gadget looks like just a rendering to me, and is apparently next to useless because it leaves you to place the parts, which is the hard bit.

My good friend does this stuff. The solder-paste is, as you say, applied through a thin mask- usually a stainless-steel plate photo-generated from the PCB data. This looks very hi-tech, but companies knock them out as pure routine.

The PCB is then put in our pick-and-place machine, which takes the components off bandoliers and plonks them down in the right spot. It’s an impressive and reliable bit of machinery, but more start-of-the-art than state-of-the-art. It runs under Ms-DOS. Then the PCB goes into the reflow oven.

The point I’m making is that the component placement is the hard part. With the tiny components used in the latest mobile phones, it is beyond the capabilities of a human with forceps and magnifying glass.

3

Wow, that things looks pretty nifty! Definitely a plus for the one-off prototypes and other hobby level electronics a lot like to do.

Now, if they could incorporate a vacuum tool for pick and place, that would be awesome. Then, you would be set.

Any ways, as was posted above, the hard part is the placement of components. That little guy makes little circuit boards which means you are looking at the smallest SMT/SMD parts to put down to maximize the board space. You’d at least need a 10x binocular scope, maybe a 30x, a vacuum pick tool (tweezers just don’t work at this point), and then have it do the reflow part. But, you’d want to keep the board on the device as you might have alignment issues as you remove and then replace. That complicates things.

For a small project with maybe 10-15 parts you might pull it off, with practice.

4

I’m assuming this is all for surface mount parts. Back in the stone age we’d never consider solder paste, we had to mine our own lead and rub two soldering irons together for the heat.

5

One thing that makes it a little easier is that the surface tension from the molten solder tends to pull the parts into proper alignment. So you don’t have to place them absolutely perfectly; if you get them close enough, the toaster does the rest.

6

Yes. That’s correct. The solder will pull the parts into alignment by surface tension. There are surprisingly few short circuits on surface mount boards compared to wave solder boards. The solder is attracted to the surface mount pads and will not adhere to the solder mask portions of the board, although miniature solder balls might form on the solder mask. These can be easily removed with an aqueous wash after the reflow process.

7

It’s fun to watch in action. The toaster oven slowly heats up–and when it reaches the melting point, all the components on the board just line up magically.

SMT at the larger sizes is just fine by hand even without special equipment. I can do 0603 components all day, and 0402 with a bit of finesse. Maybe in a few years my eyes will get bad enough that I need a scope, but for now I don’t have a problem in sufficient light.

I’d say that a solder paste machine would save 1/3 to 1/2 the time of building a board. Not the whole thing by any means, but significant enough to be worthwhile.

A consumer-level pick and place would be very interesting. I don’t think there’s any way to do it like a traditional machine with the giant reels. I think it would need a sophisticated camera system so that I can just dump the components out on a surface, and it will figure out what’s what optically.

8

Here is an example of reflowing a surface-mount board with just a crude heat gun. You can see the parts sort of snap into alignment as the solder melts and binds to the pads.

9

We do 0603 by hand if it’s only one or two boards, but it’s a slow business. In my view 0402 would be cruel and unusual punishment. For more than that we get the stencil made- they don’t cost much.

IMHO, there isn’t really a gap in the market between hand and stencil.

But you would have to select the components first to make sure the right ones were present, unless you’re planning on dumping a million components in a heap and letting the machinery sort it out. You have to have the pick as well as the place. In the olde dayes when I was involved with through-hole automation, the components were picked on one machine and made up into a custom bandolier which was fed to the insertion machine.

Our p&p machine may be a little old, but that don’t mean she’s slow, got a good flame coming from her smokestack… hang on, getting carried away here. One thing she does need is pretty hefty air compressor.

I don’t think we’re going to see consumer-level pick and place for a very long time, if ever.

10

Getting stencils made is another step, though–the goal of this unit is to do as much prototype-level stuff in-house as you can.

I dunno. I’m a hobbyist with a lot of disposable income. ~$1k to reduce a really boring part of the hobby sounds pretty good. I doubt I’m the only one in this situation.

That’s pretty much what I’m thinking, except with a bit of human intervention to make sure the components aren’t layered and the ICs are all right-size up. Just dump it all in a big tray and make sure it’s spread out. No need to align or rotate anything manually.

Wouldn’t work for everything, mind you, but even handling 90% of the stuff would be a big boon.

I think it depends on whether the computer vision part can be made cheap enough. But powerful processors are now cheap–see the Raspberry Pi 2. It easily has enough power to interface with a camera and do some basic computer vision stuff.

It doesn’t have to be that sophisticated, really. Start by identifying individual components in the pile. Group identical ones together. Provide a manual interface to match items with ones from the design file. No OCR needed, though for the resistors specifically it might be worthwhile, since they have consistent markings (unlike most other stuff).