Applying Solder Paste By Hand
A quick and inexpensive way to apply solder paste to your prototype boards
While the normal way of applying solder paste to your boards is with a metal stencil (or perhaps a cheaper plastic stencil for one off prototypes), if your board only has a couple dozen components or you are pressed for time, it may end up being more time consuming than it's worth to have a real stencil made. Thankfully, solder paste stencils aren't the only feasible way to accurately apply solder paste to you board. With only a few inexpensive pieces of equipment (and a bit of patience) you can just as easily apply the solder paste by hand, as you'll see below.
Most professionally produced PCBs have (or should have!) a layer of solder mask on them (sometimes called solder resist). This solder mask naturally repels the solder when it reflows, and keeps the paste where it should be when you put it in the reflow oven. Thanks to this layer of solder mask, solder paste is remarkably forgiving stuff. As long as you don't apply too much paste (particularly with 0.5mm fine pitch components), the solder mask usually prevents the solder from joining and forming 'bridges' even if it wasn't placed with perfect machine accuracy. Once it reflows or 'melts', it will naturally (within reason) tend to remain 'on' the pads.
If solder mask takes care of 90% of our paste placement problems, though, you still need to apply the right amount of paste. The problem is that if you add too much solder paste, you'll still end up with bridges since when the paste 'reflows' it will be forced to join together with the paste on the neighbouring pads, simply because it has nowhere else to go (meaning you'll need to pull out the soldering iron to fix the bridges). The key to success when manually applying solder paste is applying just enough paste to establish the bond between the components and the pads on the PCB, but not so much that it seeps over and forms bridges with neighbouring pads. The good thing with solder paste, though, is that if you really muck something up, you can just wipe the pads clean (or the entire board for that matter) and start all over again.
Which brings us to the question of 'how' to apply the paste in the right position and in the right amount? Thankfully, all you really need is the right type of solder paste (actually, the right packaging for solder paste), an inexpensive dispensing tip, and ideally some magnification and a bit of patience.
Solder paste typically comes in two main packages: in flat, round 'jars' (which is what you probably want when working with stencils), or in 'syringes', the latter being what we want for applying paste without a stencil. It's important to note that the difference isn't just the packaging. Solder paste in syringes often has smaller solder 'beads' than the paste bought in jars, since it needs to easily flow through a much smaller area (the needle tip). As such, if you can only buy one type of paste, you should probably purchase paste in a syringe, since it can be used for both stencil and no-stencil applications.
Helpful Hint: Solder paste typically has a shelf-life of about 6 months, and should be kept refrigerated. Preferrably not somewhere that you keep your food, though, since there are some yucky things that go into the manufacturing of solder paste. If you have a small bar fridge that you can repurpose for storing things like solder paste or other chemicals/substances you might use (for electronics purposes, of course!), you can just stick the paste in a sealed ziplock bag, and mark the date it was manufactured and opened.
I personally tend to start with a 0.43mm plastic dispensing tip (store), simply because it's a decent general purpose size. It's not so small that you can't force the small beads of solder out of it, and it's not so large that you have a really difficult time controlling the flow. Some people might find blunt metal syringe tips (store) easier to use (similar to the type you see at the doctor's office, but without the sharp end to allow it to pierce your skin). It's really a personal preference. Personally, though, I have a preference for the plastic tips because:
- I find it takes a lot less pressure to start the flow of paste with the 'conical' plastic tips
- I find them a bit safer, having a 4 year old child and a dog around when I'm working from home
- I feel less creepy than if I were to leave a dozen discarded syringe tips sitting on my desk when an absolute stranger walks in. :-)
Assuming you can find a good source for decent quality solder paste in a syringe (which is a lot harder than it should be outside of North America), you'll be unpleasantly surprised when that package arrives and you have no way to get the paste out of the tube. That's because the syringes are typically used in automated dispensers (with their own pressure-delivery systems), meaning there is no real need for a manual 'plunger' since it would probably just end up in the garbage anyway. What that means is that you will also need to purchase an appropriately sized plunger (10cc is a standard size, for reference sake). Unfortunately, they can be a bit of a headache to find, which is why we've decided to sell them ourselves. Thankfully, you really only need one of these since they are reusable.
Once you have all the equipment you need (all 5 € or so of it, minus the solder paste and the PCB), it's time to start pasting! If this is the first time you've tried to apply paste manually, it might take a few minutes to get a feel for the syringe and the plunger, and to know how much pressure you have to apply to get the right amount of paste, etc. It's more intuitive than scientific, so the best way to learn is simply by trying it out. And don't worry: if you apply too much paste, miss the pad entirely, or make some other mistake, just wipe the board clean with some paper towel or an alcohol soaked Q-Tip and start over again. (You'll want to keep some paper towel laying around anyway since you'll occassionaly need to wipe the tip to remove excess paste.)
To help you get started, you can see an image of some empty pads below, and the same pads with solder paste applied to them with the 0.43mm plastic tip (store). This is a decent amount of solder paste to apply (though you'll need to experiment to find the amount that works for you and the components you choose). You'll notice that you may tend to get little 'mountain peaks' with the syringe tip. It's not something you can always avoid, but try to make sure you don't let the little trailing bits of paste go all over the place, even if it will almost certainly stick to the pad when reflowed. I've found that holding the tip almost 90° to the board gives me the best results, since I can clearly see the paste coming out under the microscope, and can simply lift the syringe 'up' to avoid spreading any remaining paste anywhere unwanted. Again, practice makes perfect, and you should quickly get the hang of it. I'd also recommend securely fastening your board down to the workspace so that it doesn't shift while you're working, and so that you have both hands free should you need them.
A Note on Microscopes: Personally, I tend to do all my soldering and component placement under a large microscope (whether manually soldering with a traditional soldering iron, or applying solder paste with a syringe by hand). This isn't necessary, but I have so-so vision, and it doesn't seem to get any easier as the years go by. Not everyone can easily justify or afford it when starting out, but it can be a great long-term investment in terms of inspection, component placement, and soldering. If you do start looking for a microscope, I'd recommend getting one on a boom stand (for added flexibility), and if you can afford to add a few euros/dollars, you might want to consider a trinocular (as opposed to binocular) stereo microscope since it will allow you to easily add a camera later if you want to record/photograph your work. I'd also recommend a minimum working distance of around 10cm/4", which can often be doubled to 20cm/8" with special 0.5x adapters. You need to get your hands in there and also don't want anything splashing on the expensive lenses, so the more space you have the better. If you can't afford to get a professional microscope, don't worry, though. A simple 10 € set of 'helping-hands' can go a long way as well!
Common Problems (and solutions)
Sooner or later you'll make some mistakes when adding the paste, such as smearing it with a finger, or placing a component and smearing the paste because your hand budged a bit, etc. This is easy to clean up, though. Simply take a decent quality Q-Tip, drop a little bit of alcohol on it, and wipe the solder paste away. The alcohol should quickly evaporate, and you should be able to easily remove the misplaced paste (as seen below). It's important to find decent quality and 'tightly packed' Q-Tips, though, since you don't want to leave loose strands of cotton on your board!
Another common problem you are likely to encounter when placing your components, is that you'll notice that there isn't enough solder paste on a particular pad or pin. The solution is obvious: simply add more paste to the pin. Depending on the type of component, you may not even need to remove it from the board.
Manually applying solder paste has two big advantages: it's relatively quick (compared to waiting around for a professionally made stencil to arrive in the mail, or creating one yourself), and it's cheap. In fact, it's almost certainly the cheapest way you'll find to get the job done. And with a bit of practice, it works surpinsingly well! If you don't have the means (or the time) to have stencils made (and you don't want to/can't 'hand-solder' your board), manually applying paste remains an ideal option for one time prototypes or proof-of-concept boards, where you can afford to carefully place the paste on each pad since you'll only be making one or two boards anyway.
That said, it's definately a bit less accurate than using a well made stencil, and a whole lot slower (especially if you plan on making more than one board at a time or your board has a lot of pads!). Those definately aren't disqualifying factors, but you're the one who will need to decide what your priorities are, and choose the most appropriate method for your situation. But if you're simply curious to try soldering your boards with solder paste (as an alternative to hand-soldering), this is definately the cheapest and most accessible way to get started.
We've put together a very simple video showing how easy it is to apply solder paste by hand. In this particular case we're using a 0.26mm dispensing tip to apply paste to the pads of an 0603 resistor, a 1206 LED, and the structural support pads on a USB Mini-B connector. This was recorded under a microscope at 10x magnification, which is the same magnification we generally use for all of our soldering or SMT rework:
If you happen to get any bridges on fine pitch parts like QFPs, you can simply apply a bit of flux to the appropriate pads and touch them gently with a general-purpose soldering iron tip. The solder mask on the board should naturally cause the solder to seperate. If you have too much solder (as is the case in the last bridge in this video), simply pick up a bit of solder by touching the pads as normal, clean the tip well, and try again. We typically use Kester #2331-zx water soluble flux pens ourselves, since they work well and you can easily wash the remaining flux residue off with warm water and few gentle strokes with a soft toothbrush.
Why not just hand-solder your board with an iron?
If you own a decent iron/soldering station you could definately do that. It might even be a little bit quicker! The big advantage that solder paste has over hand-soldering your components with an iron is that it's far easier to fix your mistakes with solder paste (just remove the specific component and the paste and start over again). Of course, if you make a mistake with a soldering iron, you can still use a 'hot air reflow system' to remove the components, but it's a bit more aggressive on the PCB and components, and you can easily lift the pads off the board if you're inexperienced at it or if you simply aren't careful. Manually applying soldering paste isn't quicker, but it is a bit more forgiving, especially if you have poor manual dexterity or less than steady hands. There are also certain components that are just easier to solder with solder paste because the pins are in difficult to access places. Traditional manual soldering (with an iron) definately has it's place, and you really need to learn how to do it properly, but like in any other domain ... you need to choose the right tool for the right job. A bit of time and experience will help you figure out which of those tools is best suited to whatever job you might happen to have at hand.