Solder for microwave assemblies

Updated June 22, 2005

Soldering is more of a material science than anything else, and an excellent example of a case where "what you don't know will hurt you". There are experts in this field, we don't claim to be in that category. But in the case of microwave engineering, or any other electronics industry, a rudimentary knowledge of soldering issues is essential to success.

Further down this page we provide a solder chart based on information provided by Indium Corporation's web site. They were kind enough to send us an update on February 12, 2004 (thanks Rick!) but we haven't checked our list against the new list yet. Indium is based in Utica, New York. If you ever visit Indium Corporation, be sure to take the tour of Utica's crown jewel, the Matt Brewing Company (anyone else remember Utica Club's Schultz and Dooley talking beer stein commercials? Those Dudes were cool but are no longer politically correct!) Check out the "Application Notes" section on Indium Corporation's web site--there's more information there than you ever knew existed about solders and solder pastes. Where did they get the idea for the name of their Soldering101 application note? We wish we had thought of that! Indium Corporation is to solder as Xerox is to copiers... without the bankruptcy rumors!

As on other Microwaves101 web pages, here is a clickable index to this page on soldering:

Soldering terminology

Some basic solders

Indium Corporation's solder chart

Soldering terminology

Below some solder-related terminology is defined; studying it will help you achieve microwave good-old-boy status in your assembly shop. We must acknowledge Indium Corporations's web site again here, providing us with excellent reference material.

Alloys
An alloy is a mixture of two or more elemental metals. Its melting temperature (surprise!) is often lower than the melting temperature of its ingredients.

Fusible alloys
Solders that have liquidus temperatures below room temperature (kind of like mercury, but maybe not as poisonous to fish) are called fusible alloys. These are not often used in microwave engineering but we left them in the solder table for completeness.

Intermetallics
intermetallics are layers of different materials that form when you join dissimilar metals together using welding, brazing or soldering. Sometimes a good thing, but sometimes a bad thing.

Welding
Welding is a very high temperature process, usually for joining "like" metals. Both surfaces must be slightly melted to form the bond, and the filler material (such as a welding rod in an arc welder, or the wire in a wire-welder) is composed of a material of similar melting point to the bonded surfaces. Industrial welding requires wearing special goggles because materials glow brightly at weld temperatures. Fluxes or forming gases (such as inert argon) are essential to preventing oxidation during the welding operation.

Brazing
Brazing is a lower temperature process than welding, where intermetallics are formed to create a bond, but still over 350 degrees C. The filler material typically has a much lower melting point than the bonded surfaces. A typical braze filler material is bronze. You may hear the word "braze" when very high temperature solders such as gold-germanium are used to bond feedthroughs into a hermetic housing, its eutectic is 356 degrees C.

Soldering
Soldering is a lower temperature process than brazing. Only the filler material, most often an alloy, is melted to create the bond. Some intermetallics may be present in the bond as molecules of the bonded surfaces leach off into the joint, which in some cases serves to weaken the joint rather than strengthen it. For example, when soldering gold surfaces using tin-based solder, you have to ask your plating shop to restrict the gold thickness to perhaps 15 to 30 micro-inches, or your joint may fail after many temperature cycles.

Liquidus
Liquidus refers to the temperature at which a solder melts.

Solidus
Solidus refers to the temperature that a solder re-solidifies.

Eutectic
A eutectic alloy is one which melts and solidifies at the same temperature. If the liquidus temperature in the solder table below ends in "E" the solder is eutectic.

Percentage IACS
The solder chart below references the conductivity of each material to that of copper, which is 1.72 micro-ohm centimeters.

Wetting and sweating
Wetting refers to the quality of a solder joint. In a good solder joint, the solder will appear shiny (like it is wet) as opposed to dull. A bad solder joint is often referred to as a "cold joint". Sweating is a slang term for soldering.

Solder paste, versus sheet, versus wire form
Solder can be procured in many forms. In paste form, the solder alloy is powdered and suspended in a flux. The paste can then be stenciled onto a printed circuit board, components placed over the paste, and the board sent through a reflow furnace to effect the solder joint.

Sheet solder, also called ribbon or foil, is available in different thicknesses, and is used in applications where a large area joint must be obtained, such as when a substrate is solders to a backplane or housing. When a sheet is cut to a particular shape of the item being soldered, the piece of solder is called a preform. For high-volume jobs, you can have solder vendors deliver preforms, saving you hours of "fun" with an Exacto knife.

Wire solder is used in manual "wiring jobs", and typical wire solders contain a core of flux so that you have one less mess on your lab bench.

Lead content
The EU (European Union) has advised manufacturers that all electronics assemblies must be lead-free by January 2006. Why? Because lead in our environment has erased billions of IQ points from the present generations of homo sapiens. What does this mean? There is a ton of money being spent to develop cheap lead-free solders. And the price of alternative materials may skyrocket on the commodities market...

Fluxes
Flux is a material that is used to remove oxides from the metal surfaces you are joining. Oxides are your enemy in obtaining a good joint, you will never achieve a good joint without clean, oxide-free surfaces. There are many types of fluxes, you should consult a soldering expert on which one is right for your application.

Forming gases
Like fluxes, forming gasses are used to prevent oxidation during soldering operations, especially if a "fluxless" joint is required (as is often the case in micro-electronics). Flux might not kill your integrated circuit right away, but some of it may end up trapped and cause your part to fail later.

Some basic solders you might encounter in your lab

PbSn 60/40 widely available (you'll find it at Radio Shack) and is used for many "wiring jobs". It comes in a variety of diameters, and has low liquidus and solidus temperatures (188C and 183C). It usually has a flux core, so anyone can make a good joint with little training. Not suitable for microelectronics, since it needs flux.

Indalloy #7 is a great low-temperature solder.

Au/Sn 80/20 is a gold-based eutectic solder used in many microelectronic applications. It does not need flux, but performs best under a forming gas such as nitrogen or hydrogen. Forming gases are used to displace oxygen so that no oxidation occurs during the soldering process. Be careful around hydrogen remember what happened to the Hindenburg!

One last piece of advice: remember to shut off that soldering iron when you go home, so you don't burn the place down!

Indium Corporation's solder chart

* #106 is compatible with copper and nickel