All About Solder

Are you using the right solder?

Solder is an eutectic metal alloy wire with a melting point low enough to be melted with a soldering iron. An eutectic alloy melts at a lower temperature than the individual metals used to produce it.

Lead Solder

Conventional solder is made from Tin (Sn) and Lead (Pb), despite the heath issues associated with lead it is still the preferred type of solder for many electronics enthusiasts. Although lead solder can not be used in products destined for sale, lead solder is still produced and easily obtainable for use in electronics. The melting temperature of Tin-Lead solder is approximately 190°C.

Tin has a lower melting point than lead, so more tin means a lower melting point. Most common lead-based solder you’ll find will be 60Sn/40Pb (60% tin, 40% lead). There’s some other minor variations you’re likely to see, such as 63Sn/37Pb, but for general hobbyist purposes I have used 60/40 for years with no issue.


As well as lead based solder a variety of other alloys have long been used in solder. Until recently however these were mostly used for specialist applications. Since the introduction of the RoHS (Reduction of Hazardous Substances) regulations manufacturers have been required to use solder that does not contain lead. As a result Lead-Free solder has replaced conventional solder throughout industry. Lead-free solder is a bit harder to work with because it doesn’t flow as nicely or at as low a temperature as its lead-based equivalent. The difference will not stop you from successfully soldering something, and in general lead-free and lead-based solders are both useable. As the melting temperature is 30-40°C greater than conventional solder, if you have a temperature controlled iron you will need to turn it up or a basic iron will require a longer dwell time.


Whilst often overlooked, the flux used in the soldering process has a great impact on joint quality. Flux is the chemical cleaning agent added to solder to ensure a good joint. Most solder will be manufactured according to the IPC specification J-STD-004 designator system for soldering fluxes. There are in fact two revisions of the specification A&B, most solders are classified using the A revision, however this is not overly significant for general use.

Flux CompositionFlux/Flux Residue Activity LevelsHalide Content
RO (Rosin)L (Low)0 Halide-free
RE (Resin)M (Medium)1 Contains Halides
OR (Organic)H (High)
IN (Inorganic)

The Flux activity level has implications on how corrosive the flux is and therefore determines if any residue can be left or must be cleaned from the circuit board.

  • L – Not corrosive (cleaning not required)
  • M – Moderately corrosive (cleaning not required but recommended)
  • H – Corrosive (must be cleaned)

The 0 and 1 indicate the absence or presence of halides, respectively. The use of ionic halides in a no-clean flux creates a greater long-term reliability risk due to accelerated corrosion. This however is unlikely to be a consideration for DIY circuits and is of no consequence with conventional rosin flux lead solder.

Rosin or Resin

Rosin and resin based flux are the most common types used in hobby electronics. The terms rosin and resin are often used interchangeably but they are not the same. Rosin is a naturally occurring substance being a plant based product (made from pine) and are therefore subject to more natural variation than resins. Resin is either produced from a modified rosin or completely synthetic materials, resins are commonly used in newer flux formulations due to their more consistent performance.  However not all resin flux based solder is the same as when more than one rosin/resin are combined, the IPC classification is based on the larger constituent. Therefore a solder classified as resin based may contain up-to 49% rosin as a cost saving measure.

Rosin flux is acidic when heated to a liquid, but when it cools it become solid and inert. Since it is inert, despite looking messy it can be left on a PCB without harming the circuit. However there are a few circumstances where you will need to remove the residue.

  • If the circuit will heat up to the point where the rosin will become liquid it will start eating away at the connection.
  • High frequency radio and precision measurement equipment where the board Surface Insulation Resistance (SIR) is critical.

Resin based solders offer better thermal stability and are typically modified to make them easier to clean or leave virtually no residues. Resin based flux offers a more consistent performance and they are not significantly more expensive than ordinary rosin making them a good choice for the hobbyist.

Organic Flux

Organic acid fluxes are stronger than rosin fluxes and clean the oxides off much quicker. The flux residue must be removed as the residue is conductive and corrosive, however they are usually water soluble so can be cleaned with water. Some leave a dry powdery residue that can be brushed off. This type of solder tends to be a little more expensive than resin flux solders but are a good choice for people who are sensitive to rosin (colophony) fumes.

Inorganic Acid Flux

Inorganic acid flux should not be used for electronic assembly work or electrical work. Where used it requires complete cleaning after use to remove all of the corrosive residue from the surfaces which will weaken or destroy the solder joint if left in place.

Choosing Solder

The most common basic solder contains ROL1 flux. Being a rosin based low activity flux the surfaces to be soldered must be very clean and fresh. This type of solder will work perfectly well with newly manufactured circuit boards, components and freshly stripped wire that have no dirt or oxidisation. Components and boards should be cleaned and particularly if not pre-tinned may require rubbing with an abrasive to remove oxidisation. Sometimes it may be necessary to use additional flux where problems occur, especially in low cost solder where the rosin quality can vary enormously. Some solder manufacturers will label this type of solder as R (non- activated Rosin). For this reason this type of solder is better suited to experienced users rather than novices.

If you don’t want to have to go to quite so much trouble in making sure everything is spotlessly clean and deoxidised, then a better option is to go for a ROM1 or REM1 solder as the flux is mildly activated, it will work better in less than ideal circumstances while not having to worry about removing all the residue to avoid corrosion as would be necessary with a ROL1 or REL1 solder (sometimes labelled RA). Some solder manufacturers will label ROM1/REM1 solder as RMA (Rosin Mildly Activated). Solder with REM1 flux would make the ideal solder for an inexperienced user or where components have been kept in storage.

Low-Solids/No-Clean fluxes are popular in manufacturing as they reduce costs by removing the requirement for a cleaning process. In general, no-clean fluxes have less active chemistry (2%-8% active solids) and are more challenging to solder with than rosin or water soluble types. If you are happy to clean residue from your circuit boards or leave it in place then using ordinary solder with (15%-45% active solids) will make soldering easier.

Wire Diameter

When considering what gauge (diameter) to choose, the thinner it is the quicker it melts and the easier it is to control how much solder you apply. While thinner solder makes it easier to avoid applying too much solder, it does mean you have to feed more for larger joints.

We recommend a 1.2mm (SWG : 18) solder for wire soldering and larger components like switches and connectors. But for the vast majority of jobs 0.7mm (SWG : 22) is the prefect size. Not only does it allow for good soldering on most through-hole circuit boards, it is also useful for small connectors and even some surface mount work. If you do a lot of surface mount work by hand (rather than use solder paste and an oven) then a 0.5mm (SWG: 25/24) can be useful.


In this YouTube video Paul Carlson provides a useful guide to solder.

The solder reels he shows are labeled R, RMA & RA, these are equivalent to ROL1/REL1, ROM1/REM1 & ROH1/REH1 respectively.

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