Understanding Soldering – Part 1: The PCB Soldering Process

It’s often easy to take the basics for granted. In college, I had a lab where we had to solder two wires to a speaker and almost no one in the class knew how to do it. This kind of steep learning curve is even more prevalent in board design, where you have to learn the jargon to even begin to understand what a tutorial is talking about. This series will give you a deep dive into the basics of soldering so you can understand how important it is in the manufacture of a good PCBA. The basics of soldering are the same whether you’re connecting wires to a speaker or SMT components to a PCB; you’ll need solder, flux, and heat. When manufacturing production volumes your CM isn’t going to use some solder wire and an iron; instead, they’ll likely use solder reflow or wave soldering depending on the types of components in your design. Understanding these processes and principles behind soldering will help you design to avoid the various defects that can be difficult to trace and kill a design when it’s time for production.

What is Soldering?

Soldering is the process of connecting two metals together by fusing them with a different element, solder. The process is distinguishable from welding in that only the fused metal melts, while the two connecting metals remain undeformed. With this technique, a strong but reversible connection can be made between electrical conductors, if necessary for component replacement or board repair.

Solder is an alloy typically composed of tin and lead; however, other elements; such as silver are also used. Due to concerns over the toxicity of lead, unleaded or lead-free solder is becoming more common. The primary difference between the two is their melting points, with lead-free soldering usually having a melting point 50 C higher than leaded solder. Unless regulations require you to use lead-free solder, like in the European Union, leaded solder will generally be simpler to use and has a lower risk of defects. Exposing board components to such high temperatures can reduce their useful lifetime and cause problems when your board is in the field.

The other main ingredient in soldering is flux. Flux can be broadly defined as a cleaning agent, which removes oxidation from the metal surfaces to be soldered. If you’ve used a handheld soldering iron and solder wire the smoke you see when you melt the wire is the flux burning off. You’ll learn more about the differences between flux and solder in the next blog in this series.

Solder Reflow vs. Wave Soldering

When it’s time for your CM to manufacture your board they’re almost certainly going to use either solder reflow or wave soldering. These days solder reflow is the most common process, but wave soldering is still sometimes used for certain boards. The deciding factor is often the number of SMT components or through-hole components a board uses.

  • Solder Reflow - This process is mostly used for boards with a lot of SMT components. In reflow, a solder paste consisting of tiny solder balls suspended in flux is applied with a stencil to a PCB. SMT components are then placed on the pads where the solder paste acts as a bonding agent and holds them on during heating. The boards are then sent through an oven where the solder melts, then cools and forms the electrical connections for the PCBA. For double-sided boards in this process, one side is done first then the PCB is flipped and the other side is soldered.
  • Wave Soldering - In this technique, the PCB is sprayed with flux and then sent through a wave (or multiple waves) of solder. This technique is generally better for through-hole components as the solder will fill their vias more and make a better connection. For double-sided boards, SMT components are often glued to the bottom and through-hole components are mounted on top.
  • Selective Soldering - This technique is a more precise alternative to wave soldering. Newer systems can utilize CAD data directly to accurately position a laser that provides more flexibility and consistent high-quality solder joints.

Be sure to talk with your CM to see which method they recommend for your board, and ask them about any DFM guidelines they have for that process. Now it’s time to see why it’s important to learn all about soldering.

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Soldering Defects

PCB defects from soldering can be difficult to find and fix. Some of the more common defects are tombstoning, bridging, and excess solder.

  • Tombstoning - This happens when a component only connects to one pad and lifts away from the other with no connection. This defect can occur due to improper pad design. For example, if one SMT component has two pads and one is significantly larger than the other it will heat up more quickly. In solder reflow when the pad heats more quickly the solder there will melt first, bonding that pad and causing the component to rise on the other side and fail to connect.
  • Bridging - This defect is when two pads are connected (bridged) by solder. It often occurs when pads are placed too close together, or there isn’t enough room in between pads and their solder mask.
  • Cold Solder Joint (Excess Solder) - This situation is when the solder does not melt completely. The result is excess solder or too much solder on a pad or lead. This can happen when leads are too long, if too much solder is applied, or if the solder did not wet properly between a component lead and the pad. Extra solder can also increase the impedance of a connection.

The first step to avoiding all of these defects is to delve deeper into soldering and understand how the process works from beginning to end. Now you know a bit about the basics of soldering; solder, flux, and heat. We’ve also gone over solder reflow and wave soldering as the two main processes used today for mass production of PCBAs. The real key to successful board design and production is having a CM who can help you with DFM, like Tempo Automation.

Tempo‘s Custom PCB Manufacturing Service
  • ISO-9001, IPC-600 and IPC-610 commitment to quality certifications.
  • Accurate quote in less than 1 day.
  • Performs entire turnkey process in as fast as 3 days.
  • Emphasizes DFM to eliminate time-consuming back-and-forth design corrections.
  • Sources components from the most reputable suppliers in the industry to reduce procurement time.
  • Performs multiple automated inspections during assembly to ensure PCB quality for prototyping.
  • Provides support throughout the PCB manufacturing process, beginning with design.
  • Smooth transition from prototyping to production.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on CAD files or how to incorporate your design into a CAD format, contact us.

In the next installation of this series, you’ll learn about the building blocks of soldering, solder, and flux, in detail.

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