Types of PCB Soldering and the Assembly Process

I remember when I first started working in a lab and had to occasionally solder wire leads onto metalized contacts. We were working with semiconductor materials, but the same materials can be used in PCB soldering, it’s all a matter of choosing the right blend to fit into the PCB manufacturing process.

The PCB manufacturing process involves multiple steps, spanning from bare board fabrication to assembly and packaging. As part of PCB assembly, there are different types of PCB solder used to mount components. Different solders have different mechanical characteristics, safety considerations, and disposal concerns that should be considered when planning for assembly. The transition to lead-free electronics is pushing the use of lead-based solder to the margins.

I won’t get into the lead-based vs. lead-free debate just yet as the internet has plenty to say about this. For now, let’s stick to looking at the different types of PCB soldering, specifically different materials and processes.

PCB Soldering Materials

There are many different kinds of solder available on the market, and picking the best type of solder might seem like a daunting task for the new designer or assembler. Solders are used to make electrical connections between metal contacts by allowing molten solder (which is a soft alloy) to form a eutectic that fuses as it cools. The blend of metals that makes up a soldered PCB will determine its mechanical strength after solidification, the required melting temperature, and any fumes put off during soldering. We can distinguish types of PCB soldering materials by core material, metallic constituents, and types of soldering flux.

Metallic Content

Lead solder blends are known as soft solders and are responsible for kicking off the electronics industry. They have a melting point of around 180-190 °C and shelf life of about 2 years. Common lead-based solder alloys include:

• 60/40 Sn/Pb
• 63/37 Sn/Pb
• 62/36/2 Sn/Pb/Ag

Other Sn/Pb ratios include 50/50, 30/70, and 10/90. Tin is primarily used as the base metal as it gives the alloy a lower melting point, and the lead inhibits the growth of tin whiskers. A higher tin concentration ensures the soldered joint has higher shear and tensile strength. The silver component in 62/36/2 Sn/Pb/Ag provides lower contact resistance and corrosion resistance. Note that there are other types of solder (indium, zinc alloy, etc.), but these are not used on PCBs as they are incompatible with the PCB manufacturing process.

Lead-free solders are gaining more popularity ever since the EU passed the Restriction of Hazardous Substances (RoHS) Directive, which restricts the use of lead in electronics. One problem with lead-free solders is that they are more likely to form tin whiskers. Conformal coatings are often used to prevent these tin whiskers from forming and to provide protection from humidity and corrosion.

Flux core solder is sold as a single reel and contains a reducing agent in the core. This reducing agent (I’ll discuss below) removes any oxide film on the metal contacts to ensure an electrical contact with high conductivity. The type of material contained in the core is another point to consider if you are soldering by hand.

Solder Core Material and Flux

Spools of solder or solder pastes will contain one of these types of materials to flux the metal contacts during soldering:

• Organic acid flux: An acid-based flux ensures aggressive removal of oxides from metal contacts as they are soldered. This flux is water soluble and requires residues be cleaned after soldering to prevent corrosion.
• Rosin flux: Rosin is a solid form of resin derived from conifers. Rosin flux residue will not cause corrosion, so it is used whenever it may be more difficult to remove residue from organic acid flux.
• Alcohol-based no-clean flux: These fluxes may be present in solder paste and may be mixed with rosin. They also evaporate quickly during heating and are less likely to splatter.
• Solid core solders: Some solder wire has a solid core and does not contain any flux, so flux needs to be applied by hand. This type of solder is useful for hand soldering as long as flux is available.

Once soldering is completed, flux used during soldering will leave behind some residues on the PCB surface that may appear tacky. These residues can be slightly conductive and contain salts, so they are normally cleaned after assembly. It is generally assumed that residues from no-clean flux can be left on the PCB after assembly, but it is well-known that this is not the case. No-clean flux should actually be cleaned with an alcohol solution or a cleaning spray.

No-clean flux residues need to removed from a PCBA for several reasons. The most important reason is that flux residues are highly hygroscopic, may be water soluble, and they contain salts. If water adsorbs into the residue, leakage current through the residue may drive corrosion on exposed conductors. A simple cleaning process following assembly prevents this.

PCB Soldering Processes

Today, the most common type of solder in PCBs is lead-free (Sn-Cu) rosin core solder. Unless your assembler is working a one-off board or you are assembling your own board, the PCBA will not be soldered by hand. Instead, it will go through an automated process:

• Wave soldering: Used for through-hole components
• Reflow soldering: Used for SMT components in a reflow oven
• Selective soldering: Used when a through-hole component might be damaged by high heat or is unsuitable for the wave and reflow processes
• Intrusive soldering: Used to apply solder paste to through-hole components, and solder joints are formed in through-holes during reflow

Flux/paste is first applied to the metal contacts on the board to reduce oxidation and even out the flow of molten solder, which strengthens the finished soldered joint on a PCB. Most designers will probably assume that you need to assemble parts with Pb-free leads using a Pb-free solder paste, but this is not a strict requirement. According to a panel of soldering experts, it’s not uncommon to mix these materials, although be aware that the final alloy you form may have mechanical properties that lie in-between the final Pb-based and Pb-free alloys.

Another type of soldering that involves a reflow process is intrusive soldering. In this process, solder paste is applied to through-hole components, and the assembly is reflowed or selectively soldered with an iron. When high throughput is required, the board will typically be passed through an oven to form the solder joint.

During intrusive soldering, the applied solder paste infiltrates the through-hole and bonds to the component lead. The process involved is quite simple:

1. Solder paste is applied to the holes using automated paste dispensing or through a stencil
2. The board is passed into reflow and solder joints form in a standard process
3. Flux residues from the paste are cleaned and removed from the surfaces

If a through-hole component will be intrusively soldered, then the PCB footprint will need to have a solder mask opening applied around the through-hole. Depending on the size of the lead and weld area, the required mask opening could be rather large. If intrusive soldering is requested for your assembly process, your assembler can recommend paste mask openings, or they can apply the required openings in your Gerbers. Whenever there are soldering process questions, make sure to contact your assembly house for guidance.

When you need to generate manufacturing deliverables for your board, including any required assembly steps and conformance to regulations, use the complete set of PCB design and production features in Altium. Once you generate your Gerber files and other fabrication files, you can quickly create assembly drawings and add annotations to specify your assembly requirements. It’s easy to specify the different types of PCB soldering materials you might want to use in your next PCBA.

FAQs

Which PCB soldering method is best for high-volume manufacturing?

Reflow soldering is the most commonly used method in large-volume PCB manufacturing because it provides consistent quality, supports automated assembly lines, and can be used with SMT-type components. Wave soldering remains the preferred method for through-hole printed circuit board applications. Selective soldering supports mixed-technology assemblies and enables manufacturers to maintain tighter process control.

What causes cold solder joints on a PCB?

Cold solder joints most often occur due to inadequate heat application, poor solder flow (i.e., the solder does not flow smoothly), contaminants on either surface, or physical movement during cooling. Weak solder joints increase resistance, which may cause intermittent failure of electronic components after installation. Evidence of this type of failure includes dull or cracked surfaces with no shiny finish, whereas normal soldering results in smooth, shiny finishes. Using proper temperature profiles, cleaning both surfaces before application, and controlling all aspects of the soldering process will reduce or eliminate cold solder joints.

How does selective soldering differ from wave soldering?

With selective soldering, you can target specific through-hole parts with a controllable solder nozzle. In contrast, wave soldering uses a wave motion to apply molten solder across the entire bottom side of the PCB. Therefore, selective soldering is better suited for higher accuracy and reduced thermal stress on sensitive parts due to its ability to support complex mixed-assembly designs; however, wave soldering is more productive and cost-effective when producing large quantities of simple through-hole assemblies.

Why is solder paste inspection important in PCB assembly?

Solder Paste Inspection identifies defects before assemblers place and reflow components. Defects that may occur due to inadequate paste application, bridging, or improper stencil placement can result in tombstoning, open circuits, or solder-short defects during the assembly process. Yield is improved, and costly rework is reduced when defects are caught earlier in production through automated SPI systems.

What factors affect PCB solder joint reliability?

Thermal cycling, component size, board type, solder alloy type, and assembly quality are among the factors that contribute to the reliability of a solder joint. Poor pad design, excessive mechanical stress on joints during manufacturing, and improper temperature profiles during reflow all contribute to shortening the lifespan of solder joints.