Solder Guide: An Engineer’s Guide to Solder

Solder Guide

Solder types, qualities, and uses can confuse anyone who has an interest in working on electronics. This solder guide will break down the different types, uses, and tips for the various kinds of solder. The importance of picking the right solder or solder paste cannot be understated. Poor solder joints and solder residue that is hard to clean can cause performance and reliability issues.

What is Solder

Soldering, like welding, is the fusing of two items through the use of an additional material. This material is called solder which is heated to create a metal connection between the PCB and the electronic components. Solder comes in reels with diameters from .25mm to over 5mm. Thinner solder allows for less solder material to be applied for more precise work but will be slower to apply. The different kinds of SMT and Through-Hole components that are being worked on will decide what solder diameters can be used. Solder paste is used primarily for SMT components and is applied and treated differently from solder wire. Leaded solder primarily consists of tin and lead with different ratios of the two. Secondary metals like copper, silver, or cadmium can also be present but at a much smaller concentration. The other type of solder is Lead-Free (RoHS) which uses tin and other metals like antimony, copper, and silver.

Solder Guide for Leaded vs Lead-Free (Wire Type)

Leaded solder has a percentage of 60/40 or 63/47 between tin and copper respectively. The benefit is that the melting temperature drops to 360-420 degrees versus pure tin’s 450 degree melting point. A higher lead concentration will lead to a higher melting point. More tin leads to stronger tensile and shear strength. Leaded solder wets better than Lead-Free and has been used in the industry for much longer. The resulting solder joints are shinier and easier to judge for quality. Negatively, lead itself can be hazardous and will make a PCB assembly non-recyclable. This solder can be found with acid or rosin cores which act as flux to give the solder better wetting performance and to keep the metal from re-oxidation. In electronics, rosin core is more widely used due to being non corrosive.

Lead-Free solder uses tin and other metals including antimony, copper, and silver to have a mix that generally is used in conjunction with “no clean” flux. The rise of Lead-Free solder stems from the EU where restrictions on the usage of lead in consumer products occurred. Most Lead-Free solder has a rosin or acid core where the center of the wire contains flux. Rosin flux is not corrosive and therefore is used in “no clean” assemblies. Acid core flux residue is corrosive and therefore must be removed after the soldering process. Due to the harsh reducing agents in the flux, Lead-Free solder has a shelf life and should be taken into account. Rosin- flux is used primarily in electronics due to it being much less corrosive than acid core. Lead-Free solder also has a high melting point that is generally 5 to 20 degrees higher than leaded solder. This means specialized solder tips and a solder machine that can reach high temperatures may be needed. The benefit of Lead-Free Solder, is that the assembly is recyclable and less hazardous.

Types of Solder Paste

Solder paste consists of a composition of powdered metal solder and flux. The flux acts as a medium that suspends the metal particle and allows for SMT components to stick to the PCB. A reflow oven is then used to melt the solder to create a solid connection between the two parts. Depending on the type of component, different compositions of metals will be used. Tin and leaded solder paste with a percent composition of 63 and 37 percent respectively is used for plastic components on FR-4 PCBs while tin and antimony is used when more tensile and shear strength is needed. Another way solder pastes are different are their metal particle sizes. Manufacturers will give specifications about the average and largest sized particles in micrometers. More factors that differ between pastes are the amount of solids in percent which will affect how strong and thick the ending solder joint is.

Types of Solder Paste Flux

Rosin-Based solder pastes have flux that is sourced from the extract of pine trees. Specialized solvents are used to clean rosin-based solder residue. An alkaline saponifier can also be used to turn the residue into rosin soap that can be rinsed off with deionized water. The benefit of rosin-based solder is that it is not acidic and therefore not corrosive. This helps with the longevity of electronics and will prevent etching of the copper traces.

Water-soluble solder pastes are easier to clean due to the flux being composed of gyycol and organic materials in comparison to rosin-based solder paste. Pressurized water can be used to clean the boards without the usage of saponifiers or special solvents.

No-clean solder pastes require almost no cleaning after being applied and heated. This type of flux leaves little residue that is non-corrosive. This saves costs in labor but the residue should still be looked over especially when using high clock speed circuits. Leftover flux can act as a conductor and can mess with how components act. To clean, water may not be enough. An organic saponifier with water along with higher water pressure and temperature is generally enough to clean. No-clean solder pastes can vary in flux and that will affect how easy or hard cleaning will be. Manufacturers will give recommendations of reflow profiles and should be used to ensure a good result with whichever solder paste is chosen.

Solder Guide to Application

Solder paste is used for SMT (Surface Mounted Technology) and works in conjunction with a reflow oven to create solder bonds between a PCB and it’s electronic components. The paste is sticky and can be applied in many ways. The most common ways would be jet printing, stencil printing, or syringe. The most accurate method is stencil printing which consists of creating a custom stencil for where the solder paste is needed for a PCB. The PCB is then lined up and solder paste is applied over the top of the stencil. Solder paste goes through the stencil and makes contact with the PCB SMT contacts.

Hope this solder guide was helpful in your understanding of the material. If you need further assistance on picking a solder type or issues with solder quality, feel free to reach out and our knowledgeable team will help out.


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