I'm currently thinking about a Kickstarter project and if it succeeds, I'll need a robot to solder the PCB board. At first I was looking at a Dobot but I can't find more information on using it for soldering except for one video.
Umm - just - no.
I know there are a lot of tutorials out there to build your own robotic arm, but could I use it for soldering? Because I'm not so good at soldering and there will need to be around 20.000 pins soldered, I would like to use a robot for this, but I'm still searching for the right solution.
20k pins per board? Or for the overall production run? Regardless, anything you could likely afford will not have the precision or longevity to do what you want.
When I build a robot arm, should I just attach a soldering iron to it to solder all the pins and program it so that it will solder in the right place? Are servos accurate enough for this task? The PCB I'm soldering on will only be 50 x 50 mm so the arm doesn't need to be long, but accurate.
Again - anything you can buy won't work; any robot that will work will be (likely - since you are planning a kickstarter) out of your price range.
Does anyone has advice on this or any experience? It would be great to hear your opinion on this.
I'm not going to pretend to have experience, but I can tell you that for production, robots are not (typically) used for point-to-point soldering. You'll see them used for parts placement, as well as transport and handling in a factory - but the actual soldering is handled on a batch basic using either reflow soldering techniques, or some other kind of technique (old school thru-hole is done using wave soldering, for instance).
But don't try to reinvent the wheel here - you'll likely eat up a ton of money and fail in the process if you do, especially since you have no experience with this.
Instead - design the board; verify your prototypes, send it out to a boardhouse to be made, get those prototypes soldered "by hand" (whether this means you and a soldering station of some sort, or a toaster oven, or something like that) at home. Get them debugged. Get your design perfected. Get your "bill of materials" (BOM) together (ie - all the parts involved).
Long before all of this is completed - you'll need or want to look into "board stuffing" services. These are similar to places which create low-run PCBs - but generally not only do they take a PCB definition file and make a PCB, but they can also "stuff" the board (ie - populate and solder) it with parts - either from an in-stock system, or from a set of parts you order in the quantity needed for the run. What you get back in the mail will be a set of fully populated PCBs for the product you have in mind. Some of these places can even custom create enclosures and also offer other services. Think of them as a "turnkey manufacturer" for low-run electronics goods.
These do exist which will do small runs (generally around 100 pieces minimum). Once fully geared up and going, they can pump out the products fast. The difficult part is getting to this level (it will be a lot of back-and-forth foot work - you might even have to travel to where they are to assist). But once set up, the process can run for as long as needed. They'll even usually keep your information on file in case you want a future new run.
A company of this nature, for instance, that serves very large clients - is a company out of Shenzhen, China called Foxconn. Known originally for their motherboard manufacturing, they have long since branched out into making other electronic goods. One of their (or the?) largest clients? Apple. They manufacture to Apple's specification the iPhone, iPad, and other such devices. Apple isn't their only client, either. However, they do no do small runs - but there are more than a few smaller similar companies which do offer such a service. Just note, though, that even for these small 100 piece runs, for instance, the price can get seemingly hairy. Expect a minimum of 5 figures on the bill (unless things have come down drastically since I last looked into such services).
If you go this route, you will get a quality product that can be passed on to your supporters without needing to worry about variations in the product's quality, plus you'll get it done relatively quickly without causing major headaches trying to hand solder or setup a system to do so (which very well will have bugs of its own).
That's my understanding of how you would properly do it - and why. Do some research of your own; look into how other successful Kickstarters have been able to get their product to market (perhaps even talk to the people involved if you can - it will be good advice to learn from, and if your product is novel or successful, they might even give you some great advice or point you towards people or other resources which can help you).