A Word About Solder

Solder is the fusible metal, with a melting temperature below 450 C (842 F), used to join metals in the soldering process.  Soldering is distinguished from brazing which uses filler metal with higher melting temperatures, and from welding which involves melting the base metals together.  When soldering, sufficient heat is applied to the parts to be joined to cause the solder to melt and be drawn into the joint by capillary action.  The actual joining is accomplished by wetting action.  Soldering is an ancient technique that has been used for nearly as long as humans have been making things out of metal.

Normal Electronic Soldering

Historically, probably the most popular manually applied solder for electronics has been rosin core 60% tin / 40% lead (Sn60Pb40) with a melting range of 183-190C (361-374F); the lower figure being the "solidus" temperature, and the higher figure being the "liquidus" temperature.  It is suitable for all surface well logging electronics, and for downhole tools that will not be run much over 150C (~300F).  Solder begins to weaken below its melting point, so it is best to stay well below the melting point for an actual maximum working temperature.  While Sn60Pb40 works very well for hand soldering, we have always preferred the eutectic alloy of tin and lead.

In metallurgy, there is a special kind of alloy referred to as "eutectic".  Eutectic alloys exhibit no plastic range upon melting, and the melting point is lower than that of any other alloy composed of the same constituents in different proportions.  Stated otherwise, a eutectic alloy has coinciding liquidus and solidus temperatures, exhibiting a true melting point as is seen with pure metals, contrasted with the melting range seen with non-eutectic alloys.  This allows quicker wetting as the solder heats up, and quicker setup as the solder cools.  A non-eutectic alloy must remain still as the temperature drops through the liquidus and solidus temperatures, as any differential movement during the plastic phase may result in cracks, giving an unreliable joint.  63% tin / 37% lead (Sn63Pb37) is the eutectic alloy of tin and lead and has a melting point of 183C (361F), with no melting range as with Sn60Pb40.  Sn63Pb37 was historically used extensively in printed circuit board (PCB) assembly applications, and we think it is easier to use in hand soldering applications as well.  Try some 63/37 and you may never go back to 60/40.  .025 /.028 inch (about 22 AWG) or .032 inch (about 20 AWG) are good compromise solder wire diameters.  Soldering iron tip temperatures are most commonly kept between 315-371C (600-700F) for Sn63Pb37 or Sn60Pb40 solders.

Traditional High Temperature Soldering

Downhole electronics must frequently be able to withstand temperatures in excess of 150C (~300F).  To keep the solder from melting and components from literally falling off circuit boards, high temperature alloy solders are necessary.  The bad old high melting point (HMP) solders were composed of 5% tin / 95% lead (Sn05Pb95) with a melting range of 301-314C (574-597F) or 10% tin / 90% lead (Sn10Pb90) with a melting range of 268-302C (514-576F).  A better HMP solder is now commonly used for high temperature downhole applications, an alloy of 5% tin / 93.5% lead / 1.5% silver (Sn05Pb93.5Ag1.5) with a melting range of 296-301C (565-574F).  The eutectic alloy of tin, lead, and silver is 5% tin / 92.5% lead / 2.5% silver (Sn05Pb92.5Ag2.5) with a melting point of 280C (536F), a very high melting point for a eutectic solder.

Lead-Free High Temperature Soldering

SnAg Solders  In lead free solders, 96% tin / 4% silver (Sn96Ag04) with a melting range of 221-229C (430-444F), and 95% tin / 5% silver (Sn95Ag05) with a melting range of 221-245C (430-473F), have been used in downhole tools to maybe 200C (~400F).  The eutectic alloy of tin and silver is 96.5% tin / 3.5% silver (Sn96.5Ag3.5) with a melting point of 221C (430F), but it is not as commonly available as the 4% silver formulation.  Silver bearing solders produce high joint strength (the 5% silver version is the strongest) and an empty wallet.  Kester makes a 96.3% tin / 3.7% silver (Sn96.3Ag3.7), which we often use for downhole applications (mostly because we can actually obtain it).  The straight tin-silver alloys are not stock items for most vendors, and can be difficult to obtain in many cases without large minimum orders.

SAC Solders  While the addition of small amounts of copper to tin-silver solder alloys imparts some usually desirable properties including reduced solubility of copper, better wetting, and a lower melting point, the reduced melting point is undesirable for high temperature applications.  There are a number of alloys in this "SAC" family with the true eutectic composition in the range of x% tin / 3.5-3.8% silver / 0.7-1% copper.  The SAC solders are used for both reflow and wave soldering.  The 96.5% tin / 3% silver / 0.5% copper (Sn96.5Ag03Cu0.5 alloy, also referred to as SAC305, has emerged as an industry standard mostly because it is the alloy with the least silver of the SAC bunch.  SAC305 is therefore readily available at typically less cost than the copper-free tin-silver solder alloys discussed above.  SAC305 a/k/a Sn96.5Ag03Cu0.5 has a melting range of 217-220C (422-428F), so can usually be used in place of one of the copper-free tin-silver alloys in downhole tool applications.  Soldering iron tip temperatures are most commonly kept between 371-427C (700-800F) for the lead-free alloys.

SnCu Solders  Sn100C was developed by a Japanese company, and is a tin-copper alloy with the addition of dopants, including nickel; Kester's clone of this material is called K100.  Sn100C / K100 has a melting point of 227C (440F).  It is being promoted as an alternative to SAC for wave soldering, but wire solders are also available for hand soldering.  For hand soldering, Sn100C / K100 is reported to perform a little worse than the silver based lead-free alternatives.

SnSb, etc. Solders  A more economical lead free solder is 95% tin / 5% antimony (Sn95Sb05) with a melting range of 232-240C (450-464F), but there have been reports of "cold" joint problems with this material in well logging electronics applications, especially in photomultiplier tube (pmt) voltage divider strings.  Finally, Kester makes SAF-A-LLOY comprised of 97% tin / 0.2% copper / 0.8% silver / 2% antimony (Sn97Cu.2Ag.8Sb2), but it does not flow as well as the other alloys discussed in this paragraph (it handles more like Sn50Pb50 than Sn60Pb40 or Sn63Pb37).

Eventually, the environmental / health regulators will probably mandate lead free solders for all applications in the US (solders containing lead are already banned for plumbing where Kester's SAF-A-ALLOY is now commonly used because its behavior mimics the old 50% tin / 50% lead plumbing solder of decades past).  Solders containing lead are already banned in the European Union (EU) and other parts of the world for electronics applications.

Some High Temperature Soldering Notes

HMP solder is hard to work with under perfect conditions, and a real nightmare when repairing old dirty tools.  One trick is to feed Sn63Pb37 or Sn60Pb40 into the old joint while desoldering a defective component; clean all the mixed solder from the board; clean the surface extra well, then resolder the new component with HMP solder.  "Cold" solder joints are very easy to produce with HMP, and during the boom days, certain manufacturers (Comprobe to be blunt, but not only Comprobe) used unskilled fabrication personnel who produced many bad solder joints.  Old logging tools constructed with HMP sometimes exhibit a peculiar cracking of the solder, especially in high voltage circuits; it is difficult to effectively repair this problem.

It is critically important to avoid contaminating lead-free high temperature solder joints with lead from conventional solders.  Even a small amount of contamination from older component leads tinned with tin-lead alloys or printed circuit boards plated with same can markedly lower the melting point of a lead-free solder joint.  To avoid problems from lead contamination reducing the melting point of lead-free solder joints, at least one major logging company historically dipped tin-lead coated leaded component leads into a succession of solder pots containing pure tin.  This labor intensive practice would usually no longer be necessary because most newer devices are now certified to be free of lead to comply with the EU RoHS initiative.


In the soldering process, the primary purpose of flux is to prevent oxidation of the base and filler metals.  The solders discussed herein attach well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. Rosin based flux is nearly inert at room temperature, but becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides.  Secondarily, flux facilitates wetting in the soldering process.

Activated rosin (RA) flux cored wire solder has been the electronics industry standard for decades; Kester 44, Multicore 362, or Multicore 370 are good selections.  Rosin flux is also available in a highly activated version for difficult soldering (Kester 88), in a mildly activated (RMA) version for sensitive applications (Kester 285), and in a non-activated (R) version (Kester Plastic).  Some technicians (this techie included) prefer the "multicore" technology with five tiny individual cores of flux, but there is probably little actual advantage over single core technology.

Kester has introduced its 48 RA flux, an improved descendent of the old workhorse 44 flux.  48 RA flux was developed especially for the flux cored lead-free alloys, but is being made available in the lead-bearing alloys as well.  Kester 48 provides a higher level of activity than 44, and at the same time reduces splattering by over 50%.  48 RA is represented as a no-clean flux, and in fact produces a transparent and nearly colorless residue as compared to the traditional amber appearance of 44 flux residue.  Multicore Crystal 502 is reported to be roughly equivalent to Kester 44.

The amount of flux in the core of wire solder is expressed as a weight percentage.  There are three common "standard" percentages:  3.3% or regular, sometimes called large (Kester No. 66), 2.2% or medium (Kester No. 58), and 1.1% or small (Kester No. 50).  Obviously the larger the flux core, the more flux is available for hard to solder applications; and the smaller the flux core, the less residue is left that may require cleaning.  3.3% flux core solder is probably the best choice for repair and rework of logging electronics, especially downhole tool printed circuit boards.

Rosin fluxes can be cleaned up nicely with Pure Grain Alcohol (PGA), but avoid the Completely Denatured Alcohol (CDA) typically sold through retail outlets in the United States since it contains nasty denaturing additives.  See our Cleaning Secrets Revealed page for more information on flux cleaners.  1,1,1 Trichloroethane was a superior flux cleaner, but has unfortunately been banned since 1996.  Only highly activated rosin flux residue must be removed; removal of regular RA or RMA flux residue is not required for most applications except perhaps in downhole tools which will be operated at elevated temperatures, though esthetics are usually improved with cleaning.

Pressure from the environmental regulators has caused the development of a wide variety of new flux products.  Much research has gone into the development of "no-clean" and water washable flux systems.  But activated rosin (RA) flux cored wire solder remains the best choice for repair and rework (until they take it away from us).

Wire Solder Designation Coding

A shorthand code is sometimes used by manufacturers to describe solders.  For wire solders, the first letter is always a "W".  The next two or three letters describe the flux type as in RA for fully activated rosin, or RMA for mildly activated rosin.  A "P" is then followed by a number, "1" for a 1.1% core, "2" for a 2.2% core, or "3" for a 3.3% core.  For example, a solder marked "WRMAP3" or "WRMAP-3" would be a wire solder with a mildly activated rosin core comprising 3.3% of the solder weight.

Desoldering and Resoldering

Solder should not be reused when doing repair work.  Some of the base metal ordinarily dissolves into the solder during the soldering process.  If the solder solutional capacity for the base metal has been achieved it will no longer properly bond with the base metal.  This can result in a brittle or "cold" solder joint, often with a crystalline appearance.

To the extent practicable, the old solder should be removed from a joint prior to resoldering using desoldering wick or with a manual or powered vacuum desoldering apparatus.  Desoldering wick contains abundant flux to facilitate solder removal and joint clean-up.  The cleaned joint will remain tinned, and will accept the new solder readily.  As discussed above, this cleaning procedure is especially critical if the joint will be resoldered with lead-free solder, and high temperature operation is intended.

Happy Soldering!

See Bob Baer's Temperature Chart (pdf File).

Manufacturer Links

Kester                Multicore                Alpha Metals                Canfield/Bow

FTC Disclosure: Neither AnaLog Services, Inc. nor the author has an economic interest in any of the companies or products discussed above, and no monetary compensation was received.  Free samples were received from various manufacturers.  None of the manufacturers / distributors was aware this page would be written.

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Last 10-20-10