Soldering SMD commponents, simplest way to put solder paste on PCB

[sairfan1]

My hands are bit shaky and its quite hard for me to solder smd components with hand, I have reflow oven and I often get stencil with my PCB prints, but as you can understand its not possible for every prototype project.  I also tried with solder paste dispencer but mostly paste does not stick to pad,

Therefore I'm requesting you guys to share your experience

[Infraviolet]

If your hands shake, maybe you'd be best building some sort of semi-manual pick-and place. But the board underneath an x-y-z stage assembled from leadscrews (turn the ends by hand to advance in different directions) and strap initially a solder paste syringe to the stage, go round and drip the paste down. Then strap some sort of tweezers (elastic band or something wrapped round to hold them closed) or a "vacuum" picker pen to the stage and go round and placed down the components. Once they're in contact with paste blobs on the pads they'll stick just well enough not to wobble off while you transport it from your desk to the oven.

If your hands are less shaky I'd do the same process manually, paste syringe with a needle (plastic conical needles less likely to block than metal ones) held by hand rather than held on an x-y-z stage. Then hold the tweezers or vacuum pen* in your hand to place the components.

Please describe more your trouble with "paste does not stick to the pad" when using a dispensing needle. Photos perhaps? I've never had it. How much paste are you putting down, I'd tend for being a little excessive unless you're dealing with fine pitched components, you'd typically deposit a fair bit more paste per pad when needle dispensing as versus the tiny quantity of paste which gets put on a pad when using a stencil.

*I've always used tweezers, have never tried a vacuum pen, but I know they are an option

[jpanhalt]

I have used a steel pin that is about 0.8 mm (1/32") in diameter .  I ground the end to a slight, flat bottom cone.  It applied just enough paste for something as big as an SOT-23.  For much smaller parts, I can see adding individual dots of paste would be tedious if not more difficult.  I have old age jitters myself.  A beer beforehand may help.

[JustMeHere]

I use a syringe.  I'm a bit sloppy with my paste placement.  It doesn't matter a super amount.  The solder mask helps you here tremendously.

[Konkedout]

I have had "essential tremor" (unsteady hands) for my entire life and I am an electronic engineer.  I have been building electronics since I became an amateur radio operator as a kid in 1965.  These days if I build something it is 90% SMT.

Recently I have figured out that when I want to solder something difficult, I drink a beer a few minutes before doing the soldering.  Works great!  When I was a kid assembling my radio, my hands made it very frustrating and I wish now then that I could have had a beer then when I needed one.   I promise you that I am not an alcoholic.

I have never had or used solder paste.  I can understand its value for mass production (or maybe for BGA's?) but I do not know what it would enable me to do what I have not done with 63/37 tin/lead solder of different diameters, activated rosin flux (a flux pen), two soldering irons, a hot tweezer, and a cheap hot air tool. 

When I want to solder an IC with a grounded bellypad I design the board with a relatively large Plated Thru Hole in the center of the bellypad.  The IC bottom gets a coating of solder flux.  I will solder some of the other pads first to make sure that the IC is mounted with correct alignment.  Then I solder through the belly pad hole with a small tip of my iron and near maximimum tip temperature.

[jpanhalt]

Another beer drinker who has found the magic elixir.  Just don't have more than one or you will have steady hands but be soldering in the wrong parts. 

[EPAIII]

I use a syringe.  I'm a bit sloppy with my paste placement.  It doesn't matter a super amount.  The solder mask helps you here tremendously.

No need to overthink this.

If you squirt about the right amount of paste with a syringe, across a row of pads in one continuous noodle of paste with no effort to separate it and then place the part on top of that, when you heat it in an oven or with a hot air gun the paste will separate on it's own as it melts and the part will align itself to the pads via capillary action. It's actually fun to watch a board with parts that were set down at small, arbitrary angles to their correct orientation all just straighten themselves out and line up perfectly.

I use a toaster oven with one burnt out heating element as a reflow oven. I just put the board with paste and parts in cold and turn it on. Once everything melts I take it out. Yea, yea, the guys at TI and the other chip makers would faint because I am not following the correct temperature profile, but it works.

[Psi]

A syringe with dispensing needle and your thumb on plunger works fine to manually place dots around pads, however I know what you mean about solder not sticking to the pads. Ive seen that before myself.  It was the solder paste I was using, it just wasn't very well suited to this application. I tried a different brand and it worked fine.

I also had good results mixing in some extra gel/tacky flux into the solder paste before putting it into the syringe. Bit messy, but it worked.

[Microdoser]

Personally, for boards that I don't have a stencil for, I squeeze a good amount of flux over the pads, put a blob of solder onto my iron, then run the iron over the pads. This deposits pretty much the right amount onto the pads for 0603 components. For larger pads, it puts too much solder on (especially the pads in the middle of an IC) so I then dry my iron and repeatedly melt the solder on the pad and clean the iron which gradually lowers the amount of solder on the pad to the right amount. I then use hot air to melt the solder a bit before placing components. Clean the flux off using your favourite method.

No need for precision. No errant solder balls. After a little practice, you'll have a 100% success rate.

[Solder_Junkie]

It sounds like defective solder paste. The shelf life is quite short and storage temperature is important too.

While I appreciate you have shaky hands, I have given up using paste, as a hobby  constructor I find hand soldering is much easier, even down to 0603 size parts. I also use lead solder, it’s superior to lead free.

This YouTube video gives plenty of tips for anyone considering hand soldering SMD parts.
https://youtu.be/eZtPR8_x0nc?feature=shared

SJ

[sairfan1]

Yes, some manual PNP is a good appoach to deal with shaky hands.  I was already working on this direction have bought parts and building something that can help.
Regarding solder paste not sticking to the pads, i found that it got dry because it was bit old, I mixed little solder paste with it and it got better.

I want to know how you guys use the syringe and needle, after use some solder stays inside the needle that will block it, I tried to wash the needle but no luck, looks like on every soldering I need a new needle for it.

I'm also thinking to give it a try to 3D print stencile, though it will be a little thicker but see how it works including time and effort needed.

[LinuxHata]

The simple solution of solder paste not sticking to pads is to preheat the PCB.
I use PTC heater to pre-heat PCB to about 50C - solder paste then applies and distributes much better, so easier to put on proper amounts onto PCB.

[sairfan1]

The simple solution of solder paste not sticking to pads is to preheat the PCB.
I use PTC heater to pre-heat PCB to about 50C - solder paste then applies and distributes much better, so easier to put on proper amounts onto PCB.

wow sounds like a good idea.

[LinuxHata]

Yeah and these PTCs are great that they do not need thermocouple, PID controller, relay or adjustments - you just buy one designed for needed temperature, plug it in and it is all ok.

[Psi]

The simple solution of solder paste not sticking to pads is to preheat the PCB.
I use PTC heater to pre-heat PCB to about 50C - solder paste then applies and distributes much better, so easier to put on proper amounts onto PCB.

wow sounds like a good idea.

That works but it cools down pretty fast, unless you're able to paste it while it's sitting on the preheater.

[LinuxHata]

I keep it all time on, while applying the paste. This is why I selected PTC, that never gets hotter than 50C.

[alligatorblues]

To keep the component in place when soldering, i put a tiny piece of Gluetack or Funtack under it, and press it in place. It holds it tight.

[sairfan1]

Is there some simple way to clean syringe and needle after each use (solder dispensing)? I was thinking to leave in hot water for a while.

[julian1]

It's possible to buy small plastic conical nozzle applicators in bulk bags, that fit the screw- thread of regular syringes of solder paste. 
These are inexpensive - a few cents each, and come in bags, and work great. 
if you haven't soldered anything in a while, then the paste in the tip tends to dry/ loses viscosity.
But the applicator can just be swapped/replaced as needed, to get at the liquid paste in the syringe again.
Being plastic the tips tend to get munted when pressed against hot pcbs anyway, so replacing them is kind of needed.
The smallest nozzle applicators are the most flexible, because the end can just be snipped off to create larger diameters.   
One trick when dispensing is to rest the heel/base of the hand on the pcb for support and improve steadiness.

[twospoons]

Soak your needles in isopropyl alcohol to clean.  But the things are so cheap why bother? You can buy 1000 for sixty bucks NZ (~30 USD) on aliexpress.

[Psi]

Is there some simple way to clean syringe and needle after each use (solder dispensing)? I was thinking to leave in hot water for a while.

Seal up your syringe, with needle attached, in a sealed bag, then put it away in your cold storage fridge.
When you get it out again to use, 95% of the time you can just push hard on the plunger and clear out the solder paste that was sitting inside the needle and new paste starts to flow.  So cleaning it out isn't really needed. Only occasionally do you need to replace the needle.

Just be aware of the ball size of the paste you are using (T number) vs the size of needle you are using.
Small needles will clog a lot if you are using a paste with large ball sizes.

For general stuff like 0603. 0805, TQFP, SOIC only a single needle size is needed.
It's only when you start wanting to paste DFNs and 0402 and smaller that you start to get issues and need a smaller needle.
Manually pasting DFNs and 0402 can get a bit challenging, totally doable but takes a bit more time.

For DFNs i normally don't manually paste them, I pass a tinned iron tip over the entire footprint with some flux to get solder on all pads, then place the DFN on top with more flux and hot air it on that way,  but if doing this, you want to do it after after reflowing all other parts. You can't hit the PCB with hot air while it has paste on other pads or the heat will make a mess.

[Solder_Junkie]

You can clear solder paste from needles with enamelled copper wire.

SJ

[zapta]

I am often using the JLCPCB SMD service. It's very handy and reasonably priced.

Hints:
1. Design for JLCPCB (based on component availability)
2. Start the order upload with an empty cart. This will save you $2 for the PCB part.
3. Stick with 'basic' parts. They are significantly less expensive.
4. The minimum PCB order is 5 but there is an option to populate only 2 out of the five.
5. You can use their own EasyEDA, or third party cad such as Kicad.
6. If you use Kicad, use the Fabrication Toolkit plug in to generate the production files. It's very smart and can rotate/move parts to match JLCPCB reference.
7. If it's not urgent, pick an inexpensive shipping method (which is not necessarily the default one).

As for soldering, you can design small SMD to PTH PCBs that JLCPCB will populate for you and then solder them manually in project. E.g. have a PCB with a few common hard to solder ICS, have JLC populate them, and then cut them individually and use in project. Just an idea.

[Aspartame]

Apply flux on the board.
Put solder on every pad with a chisel tip, you can remove bridges with drag soldering if necessary.
Clean the board and apply fresh flux.
Place the components.
Use a decent hot plate like mhp30 at (190-240C).
Observe reflow and correct placement errors if any.
Reclean the board.