Soldering Basics

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New for February 2019: here's a link to an update on lead-free solders, from Aerospace Corporation, from 2011.  In case you were wondering, the issues with lead-free solders have never really gone away.  To put it in perspective, 88% of lead consumed in the United States is used in storage batteries, which are not subject to RoHS.  The lead content in solder represents 0.5% of lead consumption. Aerospace is operates a federally funded research and development center (FFRDC), so they essentially work for the US taxpayer. Learn about battery recycling n Wikipedia.

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!

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 Corporation's web site again here, providing us with excellent reference material.

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 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 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 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 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 refers to the temperature at which a solder melts.

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

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.  For a very nice slide deck all about this, check out "Lead-free Electronics Reliability - An Update" by Andrew Kostic of the Aerospace Coporation over on the NASA Electronics Parts and Packaging site. What does all 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...

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

Indalloy #
Temp. Liquidus C
Temp. Solidus C
Mechanical Properties
Mass Density (gr/cc)
Electrical Conductivity (% of IACS)
Thermal Conductivity (W/cm-C @85C)
Thermal Coefficient of Expansion (ppm/C @ 20C)
46L 8 7 61Ga
6.5       Alloy liquid @rm.temp, wets glass and quartz. Mercury replacement.
51 11E 11 62.5Ga
6.5       Alloy liquid @rm.temp, wets glass and quartz. Mercury replacement.
60 16E 16 75.5Ga
6.35       Alloy liquid @ room temp., wets glass and quartz. Mercury replacement.
117 47E 47 44.7Bi
9.16 4.5 0.15 25 General purpose fusible alloy.
136 58E 58 49Bi
9.01 2.43 0.1 23 General purpose fusible alloy. Wettability not outstanding. For use as solder.
19 60E 60 51In
7.88 3.3   22 Environmentally safe fusible alloy. Contains no lead or cadmium.
158 70E 70 50Bi
9.58 4 0.18 22 Lowest melting point solder.
162 72E 72 66.3In
7.99       Environmentally safe fusible alloy. Contains no lead or cadmium.
174 79E 79 57Bi
8.54       Environmentally safe fusible alloy. Contains no lead or cadmium.
8 93E 93 44In
7.46   0.36 24 General purpose fusible alloy.
42 96E 96 46Bi
8.99       Bismuth added to achieve a low melting point. Fair wettability.
224 108E 108 52.2In
7.27       Pb free solder, high dross due to Zn.
1E 118E 118 52In
7.3 11.7 0.34 20 Indium-tin eutectic with sharp melting point. Has good wettability on glass, quartz and many ceramics. Has good low temperature malleability and therefore compensates for some difference in coefficient of expansion when dissimilar materials are soldered.
255 124E 124 55.5Bi
10.44 4 0.04   General purpose fusible alloy.
281 138E 138 58Bi
8.56 4.5 0.19 15 Good low melting point solder for electronics assembly of for applications where Cd and Pb are to be avoided, and for thermo-electric applications. Shear rate sensitive.
290 143E 143 97In
7.38 23 0.73 22 Indium with silver added to improve strength. Has the wettability and low-temperature malleability of indium. Particularly useful for cryogenic applications.
181 145E 145 51.2Sn
8.45   0.35 24 Good general purpose solder in the medium temperature range. Maintains its creep strength well. Not applicable against gold.
2 154 149 80In
7.85 13 0.43 28 Especially useful for soldering against gold, as leaching is minimized. Good thermal fatigue properties. Compatible with Indalloy numbers 204, 205, 206, 7, 10 and 150 in step-soldering applications.
4 157MP   100In 7.31 24 0.86 29 Soft, ductile metal has good wettability on many surfaces including ceramics, glass and quartz. Deforms indefinately under load and has no tendency to become brittle, making it valuable for cryogenic applications.
97 163 144 43Pb
8.99       Good general purpose step soldering alloy.
9 167 154 70Sn
7.79 12.2 0.45 24 General purpose solder in 160 degrees C range with good physical properties.
204 175 165 70In
8.19 8.8 0.38 28 Indalloys #7, #10, #150, #204, #205 and #206 comprise a group of lead-indium solders designed to cover the temperature range of 165 degrees C to 275 degrees C. All have the minimum gold-leaching characteristics of lead-indium as well as good thermal fatigue properties.
Sn62 179E 179 62Sn
8.41 11.9 0.5 27 Good general purpose solder. Can be used on silver bearing surfaces to reduce scavenging.
205 181 173 60In
8.52 7 0.29 27 See Indalloy #204 above.
106 (Sn63)* 183E 183 63Sn
8.4 11.5 0.5 25 Standard eutectic tin-lead solder with wide application. Not recommended against silver and/or gold.
231 186 174 86.5Sn
7.36       Pb free solder. High dross.
227 187 175 77.2Sn
7.25 9.8 0.54 28 Pb free solder can be used as a replacement for 63Sn 37Pb, 62Sn 36Pb 2Ag and 60Sn 40Pb in that it has similar physical and mechanical properties.
201 199E 199 91Sn
7.27 15 0.61   Recommended for soldering to aluminum. Use flux #3.
7 210 184 50In
8.86 6 0.22 27 See Indalloy #204 above.
232 217E 217 93.6Sn
7.43       Patented Pb free solder.
121 221E 221 96.5Sn
7.36 16 0.33 30 Standard tin-silver eutectic solder with wide application when lead-based solders do not meet temperature, strength or safety requirements. Not recommended against gold-plated surfaces. Excellent for step soldering with #42, #106, #165 and #171.
206 231 197 60Pb
9.3 5.2 0.19 26 See Indalloy #204 above.
238 217E 217 90Sn
7.78       Lower temp eutectic, in the Au Sn system.
209 233MP   65Sn
7.8     36 Low temperature die attach alloy. Has very high tensile strength.
3 237 143 90In
7.54 22.1 0.67 15 Indium with silver added to improve strength. Has the wettability and low-temperature malleability of indium.
133 240 235 95Sn
7.25 11.9 0.28 31 Higher tensile strength solder than #121. Particular application is joining copper tubing in refigeration and potable water systems. Has good wettability with good long time sheer strength at 100 degreees C.
236 247 237 83Pb
10.35       Intermediate temp solder. Creep resistant.
233 255 245 85Pb
10.36 6     Intermediate temp solder. Creep resistant.
10 260 240 75Pb
9.97 4.6 0.18 26 See Indalloy #204 above.
150 275 260 81Pb
10.27 4.5 0.17 27 See Indalloy #204 above. Lowest In containing for good wettability.
182 280E 280 80Au
14.51   0.57 16 Eutectic die attach and package sealing.
151 296 287 92.5Pb
11.02 8.6   29 Similar to Indalloy #165.
228 299 267 88Pb
10.75 8.5 0.27 29 High temp solder.
159 302 275 90Pb
10.75 8.9 0.25 29 High temp solder for BGA applications.
237 304MP   93Pb
11.07       High temp solder.
165 309E 309 97.5Pb
11.28 6 0.23 30 Standard lead-silver-tin eutectic solder with wide application in semiconductor assembly.
164 310 300 92.5Pb
11.02 5.5 0.25 25 This alloy is very useful in the 300 degrees C range. It has particularly good thermal fatigue properties as well as the minimal gold leaching properties of indium-lead alloys. Often used in reducing atmospheres.
171 312 308 95Pb
11.06 8.8 0.23 30 Wide application in soldering.
239 313E 313 91Pb
11.05       High temp solder.
183 356E 356 88Au
14.67   0.44 13 Eutectic die attach.
178 485 451 82Au
14.9       Stiff alloy. Limited fabrications available.


* #106 is compatible with copper and nickel

Author : Unknown Editor