Proper joint preparation is important to obtain good quality brazed or soldered joints. There are three critical steps to ensure properly prepared joints:

Cleanliness
Parts should be clean and bright prior to joining. Contaminants such as oils and grease, excess polishing compounds, or excessive oxidation of the metals being joined can impair the flow of the filler metal and lead to poor-quality joints. Most applictions will require only a light cleaning in a soap solution or a degreaser to remove surface oils and greases. Acid dipping or pickling may be required for heavily oxidized parts.

Joint Fit-Up
Proper joint fit-up (gap size and alignment) will have a big impact on the appearance and quality of the finished joint. Sloppy fit-up will show, no matter how well the brazing or soldering operation is performed. Where possible, joining surfaces should be flat, smooth and parallel to each other. Ideal gap sizes for joining will generally be 0.002"-0.005". (This varies somewhat based upon the filler metal used, and the heating method used). Ensuring that the joining surfaces are parallel to each other produces higher-quality, and less visable joints.

Fixturing
Fixturing of one form or another is often used to align joining surfaces. It is important to ensure that the desired gap and alignment is maintained throughout the heating process.
Fixturing can be accomplished in a number of ways, including tack welding, the use of mechanical fixtures (clamps, etc), graphite boards (typically for furnace brazing), third-hands, wire-wrapping, or the simple expedient of laying one piece atop another and being very, very careful. Regardless of the fixturing method used, keeping the parts aligned and minimize, or preferably elimine, any possibility of movement during the heating process is essential.

Joint preparation is a little bit more critical when using paste than when using wrought forms of solder. When using wrought forms, you can always add another chip or bit of wire if the joint isn't filled properly, which is a great thing if you have rough, or uneven joints. With paste (while you could still add a chip or wire, it defeats the point), what you start with is what you have, so it is more important to have a soundly constructed joint to work with.

After ensuring that the joint is properly prepared for soldering the paste can be applied. Paste is always applied before heating. Because the paste has a liquid component, trying to apply it to a preheated piece (like we would when using chip or wire solder) won't work. At best it will result in messy splattering. At worst, the paste will bounce right off the part.

Applying From Syringes

Paste packaged in syringes or cartridges is the easiest to apply. Attach an appropriate sized dispensing needle to the syringe by pushing the needle on and turning it clockwise until secure. Okai supplies a general-purpose dispensing needle with every syringe of solder that is suitable for most dispensing applications. Smaller and larger needles are also available upon request.

The easiest, and most efficient, way to apply the paste from a syringe is to dispense it directly on to the joint. Simply place the tip of the syringe where you want the paste, and depress the plunger until a suitable amount of paste comes out. A little press of the syringe tip can move the paste around slightly if needed, and help it adhere to the assembly.

Applying From Jars

While applying paste from a syringe may be easier, plenty of jewelers prefer the flexibility of applying paste manually from a jar using a variety of tools. Applying paste this way gives a better tactile feel for the jeweler, and can produce a greater variety of creative results.

Paste can be applied using a solder pick, or a tooth pick. Using the tip of the pick, a small amount of solder can be scooped out of the jar, and the pick can be used to daub on the paste where it is needed. The pick can also be used to 'smear' the paste around for broader coverage.

For the ultimate short cut, when attaching posts the post can be dipped into the paste until the desired amount is on the end of it, then held in place while soldered.

For jewelers who like the practice of balling solder on the end of a solder pick, this technique can also be used with solder paste. Simply scrape up the desired amount of solder paste on the tip of the pick. Heat the pick gently with a torch to dry the paste. The dried paste can be applied just like using balled chip or wire solder. Alternately, go one step further and continue heating the paste until it completely melts on the tip of the solder pick into a ball of solder, and use it like you would traditionally balled solder.

Paste can also be applied with a brush. Okai manufactures paste sytems designed to be brushable like a paint. Standard paste systems can also be thinned to a brushable consistency if desired. The creative possibilities are endless when brushing on paste.

Where Do I Put It?

For a solder to flow effectively through a joint, it must be in contact with the joint, so the first rule of placing the solder is to make sure it is touching the actual joint. It is not necessary to put the solder in the joint (ie, directly between the two pieces being joined). While this can be done, in most cases, putting the solder between the two sides of a joint can cause an uneven gap, which is more difficult to solder. Better to make a nice, tight joint, and put the solder on top of it.

For most small joints, a dot of paste at a single location should be enough to do the job. For larger joints or long solder runs, more applications are usually necessary.

Solder can be applied in one continuous bead along the entire joint, but this technique almost always uses more solder than necessary (and could lead to excessive finishing after soldering). It is usually better to apply several small dots, or dashes of solder long the length of the joint. The solder will melt, and flow together to fill the joint when done properly. There's a bit of trial in error in this approach, but with a little practice, you'll soon get the feel for how far apart paste deposits need to be.

It's important to keep in mind the 'footprint' of the paste deposit. Wherever you place the paste, there is likely to be some residual solder on the surface after you finish. This means if you're not careful in how you place your solder, you could find a bit of discoloration where the big messy deposit was. Smaller deposits, spaced out along the joint are usually the answer. Also, keep an eye on surface textures on the parts. Solder loves to wich through tight spaces due to capillary action, and it will happily run across textured surfaces if given the chance--with unpleasant results for the appearance of the texture. If you can not avoid placing solder on a textured surface, consider using a stop-off to prevent solder flow.

How Much Do I Use?

Of course we'd like to say, "Lots and Lots!". The reality is, however, that most people use too much solder. For top quality joints, less is often more.

Having said that, it's difficult to give a guideline that covers all situations since there are so many variables at play in each application. Generally speaking, if you're used to using chip solder, then a paste solder deposit that appears to be the same size as the chip solder you would select is a good place to start. (The paste deposit will be about the same size as the chip, but will usually be a bit 'higher'--since the chip is flat. When the binder and the flux burn off in the paste, roughly the same amount of solder alloy will remain). If you're used to using cut pieces of wire, then the paste deposit should be about 15% bigger than the wire piece you'd select.

This is definitely a matter of trial and error, and one of preference. Everyone will be a little bit different. Practice first with some scraps. It won't take long to get a feel for how much you need to produce an excellent solder joint.

It's essential to make sure the parts being joined are heated evenly and up to the proper soldering temperature before melting the solder. Using the torch to force the solder to melt before the parts are up to temperature will produce a weak joint, or no joint at all.

The best way to ensure that the parts are at the right temperature is to heat the parts, and NOT the solder. This holds true no matter which form of solder you're using, but it's particularly important when using paste solder.

Steady (not fast) torch movement is key. Play the flame back and forth across all of the components of the joint. If one part is larger than another, then more heat needs to be put on that part. Where possible, avoid aiming the flame directly at the solder paste, and focus on the metals instead.

One of the great things about solder paste is that it gives excellent visual cues as to what temperature the parts are at (assuming you're not heating the paste directly).

As the solder alloy begins to melt and flow, you can focus the torch more directly on the joint, and use it to add extra heat to those areas where you want to pull the solder.