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I am trying to make an LCD shield from my Arduino and am having problems trying to got the solder to stick to the Freetronics protoshield PCB. I have cleaned it as best I can.
I am using a decent Proxxon soldering bolt with solder that has worked good for me in the past. How can I get it to stick?
It won't bond to the PCB, only the wires.
Heat! (One word answer)
A classic reason solder won't stick to something is because you're not getting it hot enough. My interns come to me with this problem all the time.
Make sure the tip of the iron is nice and shiny. Touch some solder on it, and it should melt almost instantly.
Put a nice little blob of solder on the tip of the iron.
Press the blob of solder into the metal to be soldered.
Initially the solder won't be too keen, but when the metal reaches the right temperature, the solder will suddenly be attracted to it, and you'll see it move slightly.
Now that the pad has reached temperature, you can touch the solder anywhere on the pad and it should melt almost instantly. I often add solder this way so I know I'm adding it to a nice hot pad.
Hugo
Flux! (One word answer)
[Appendix:]
Cleaning the board with, say, alcohol or Windex won't be enough-you're not really worried about finger greases here. What you're worried about is oxidation. Solder bonds to metal, but not metal oxides. Gold doesn't oxidize readily, but other compounds found on circuit boards, like copper and nickel, do so readily. I assume that your solder has a flux component, but stubborn joints often need a little more flux than what is embedded in your solder (usually <5%, probably closer to 2%). A flux pen should be a part of every soldering tool set.
The ProtoShield product uses a gold-based plating system, and claims features like "The PCB surface is gold-plated for maximum durability" and "Gold plated PCB: solders easily and very resistant to corrosion." However, that's not the whole story. Gold is great because it doesn't oxidize readily, but it has the problem that it dissolves in solder, and then reacts to form compounds mainly AuSn4, which weaken the solder joint.
Joint weakness is a problem primarily for processes which put the gold on in thick layers - "Immersion gold" - because there's more gold to dissolve and form those intermetallics which weaken the joint. This would be a concern if you were using surface mount connectors, or something which did not go through the board, but the sheer bulk of your solder joint should make this less problematic. If you're reworking the joint, make sure you have it quite clean, as the residue is damaging to your joint.
The dissolving property is a problem for plating processes which put the gold in a very, very thin layer - "ENIG", or electroless nickel immersion gold - which means that all of the gold dissolves in less than one second, and you're really soldering to the thicker nickel plating below, and using the gold to protect the nickel, like galvanized steel. This is gaining popularity in big production systems, which rarely need to do rework, and the reduced need for flux from plating with gold makes the process easier. The gold will be gone if you try to do rework, so you'll need to use flux with your solder on the nickel surface (which isn't that big a deal). I'm guessing that this is your problem.
There has already been some great advice given so I'll provide some extra info about the surface finish on the ProtoShields (and all the other PCBs that Freetronics has done so far) and the reasoning behind the decision. Unfortunately there's no one "best" PCB surface finish and all finishes have both good and bad points, so it's a matter of making a decision based on trade-offs that are most appropriate for the intended use.
Our PCBs use an "ENIG" surface finish, as @reemrevnivek guessed. That's "Electroless Nickel Immersion Gold", and consists of an underlying layer of nickel with a thin layer of gold over the top. The gold layer is very thin and not intended to provide the main structure of the track, it just acts as a protective coating for the nickel to prevent it tarnishing before it's soldered. Gold is extremely resistant to corrosion so ENIG has several upsides: it can be touched with bare fingers without tarnishing, has a very long shelf life, and the pads / tracks are very flat and square-edged (important for fine-pitch SMD). One downside is that it takes a bit more solder to complete a joint because the surface hasn't yet been pre-tinned, and because there isn't an existing layer of solder to melt against the iron and increase the initial contact area (hence increasing the rate of heat transfer) it can take an extra second or so to get the joint hot.
The most common surface finish you see on PCBs is called "HASL", or "Hot Air Solder Levelling". HASL boards are dipped in molten solder and then the excess is cleaned off using hot-air knives to leave behind the thinnest possible layer of solder. The solder itself then protects the underlying track from corrosion while making it extremely easy to solder, because the whole pad is pre-tinned. It's generally the cheapest finish available and a great choice for general-purpose boards. One downside to HASL is that even after the hot-air knife has cleaned off as much excess as possible, the meniscus of the solder will still cause the edges of pads to be slightly rounded. That makes surface-mount parts not sit down quite as flat as they would on an ENIG board.
So you'd expect that for a board like a prototyping shield the obvious solution would be to go for HASL. But there's a catch. We're trying to stick to RoHS-compliance as much as possible, which would mean we couldn't use regular HASL: it would have to be lead-free HASL. Lead-free solder has a higher melting temperature than regular solder so if we went with lead-free HASL it'd be a pain for customers who don't have lead-free equipment. We'd probably end up with lots of complaints from customers using regular solder and soldering irons who have trouble getting the lead-free solder hot enough.
Another possible finish is "immersion silver", and it provides an amazingly good finish but has a terrible shelf life. For boards intended for machine assembly immediately after manufacture, silver is a great option. The problem is that it tarnishes quickly and is adversely affected by touch, so it's no good for boards that are intended for distribution to hobbyists for (potentially) long storage and hand assembly.
In the end we settled on ENIG because of the benefits of long shelf life, tarnish resistance, RoHS-compliance, and easy soldering compared to other surface finishes. My personal experience with it has been excellent and I haven't come across any particular problems so far, but of course I'm not an expert on PCB manufacturing and I'm very happy to take advice on better ways to do things.
Over the last 6 months I've had a little over 2000 ENIG-finished PCBs fabricated, of which just over 1000 were assembled by pick-n-place machines, about 600 were shipped as bare boards to customers, and several hundred of the rest were hand-assembled by me personally using either reflow-soldering in an oven or using a soldering iron. Out of all those boards this is the first time I've ever heard of any problems soldering to them, so it's possible you were just unlucky enough to receive a board with a flawed finish. If there's something wrong with the finishing process from our PCB manufacturer I'd really like to know about it, so if you're still having problems with that board I'd really appreciate it if you could post it to me so I can inspect it. I'll cover the postage and of course I'll send you replacements free of charge. You can contact me personally at [email protected] to make the arrangements.
Hand soldering. Heated surfaces must be clean to reduce surface tension.
The tips should be cleaned daily with solder and sponge, keep tip on tight.
Surface, if oxidized, must be cleaned (abrasive) and fluxed.
If a large ground plane is connected to a solder joint or heat sink or shield.... then more heat is needed to reach the melting point.
Applying small solder to the tip to make surface liquid reduces thermal resistance and speeds up component heating, and then apply more solder as required quickly.
General purpose irons are 15-25 W.
Shields or large ground planes need more power or mass to tip to heat up the surface faster.
