Clean vs. No-clean Solder Process

Reading time ( words)

IPC, which was founded in 1957 as the Institute for Printed Circuits, is the trade association responsible for standardizing the assembly and production requirements of electronic equipment and assemblies.

Although IPC suggests clear guidelines, agreeing on the cleanliness (or otherwise) of a PCB assembly can often be a subjective and even contentious subject within the electronics manufacturing industry.

If you’ve chosen to outsource your assemblies, how do you decide what to specify to your EMS partner?

Defining "Clean" in PCB Assembly

Firstly, it’s probably worth defining what "clean" actually means. I think we're on safe ground to assume that lumps of unwanted debris or particulate matter—mysterious white residues or corrosion of any kind—are obviously not allowed.

Such things not only make the assembly look untidy, but they can also damage the functionality or long-term reliability of the product. Most of the time, when we’re talking about "cleanliness," we're referring to flux residues.

It is possible, to an extent, to measure cleanliness. For example, once an assembly has been cleaned, you can measure the residues in the cleaning process cycles to ensure the desired maximum has been achieved. However, for many—perhaps most—electronics applications, it’s often more about the cosmetic finish or other process considerations.

As you might imagine, when we start to talk about solders, fluxes, and cleaning agents, there is quite a bit of chemistry going on. But for now, let's focus on the practical issues surrounding whether an assembly should be cleaned or not.

To Clean or Not to Clean?

Why clean in the first place? Obviously, there are cosmetic considerations to keep in mind. If a no-clean flux is used during the SMT process, the assembly should look perfectly fine, as most of the flux gets burned off during the process.

However, if there is through-hole soldering afterwards—particularly hand soldering—it can show small areas of flux that make it all look a bit untidy due to the extra flux that is typically applied and the reduction in both heat and time associated with hand soldering compared to SMT ovens.

Perhaps most importantly, any residues must not interfere with visual inspection. If there is anything left obscuring solder joints, for example, then it must be cleaned off either by hand or by using one of the automated cleaning machines available.

If the product is to be tested using test probes, such as in-circuit test or flying probe, then flux can cause unreliability if the probes can’t access their test points on the PCB. This is usually fine if a pin-testable flux is used, but any excesses may need to be cleaned off.

If the assembly is to have a conformal coating, then all flux residues must be removed before application, so the PCBA will have to go through a cleaning process.

Are there any situations where you might choose not to clean? Well, yes. Some fluxes are designed to be cleaned off, but some aren't. Again, neatly side-stepping the chemistry here if the flux is designed to stay on the PCB then trying to clean it off generally isn't a good idea.

There is also an argument that the flux can provide a harmless protective layer over the solder joints and that removing it can expose the PCB finish to problems later.

Naturally, there are cost implications too. Cleaning is an additional process, so it's going to have a value attached to it. And there is also the impact on the environment with additional water or chemical supplies needed, which then need to be disposed of correctly.

It's also worth bearing in mind that certain types of components simply can't be cleaned due to the risk of the cleaning agent getting inside the body of the device and causing damage.

If cleaning is required, it can also overly complicate the build process—for example, having to fit components after the main clean, then individually cleaning those components, or by adding extra drying stages.

Again, any extra processes are going to mean additional cost.

Which Cleaning Method is Best?

What are your options when it comes to cleaning? Here are three:

  1. Localized cleaning: Can be achieved with a brush and cleaning agent such as IPA. However, care must be taken to ensure the residue is cleaned off and not simply spread thinly across the assembly.
  2. Automated solutions: There are some that use water (very similar to a dishwasher) or other cleaning agents, depending on the flux used. These cleaners tend to be relatively small in terms of physical footprint and the waste can usually be disposed of down existing on-site drains.
  3. Ultrasonic cleaners: These tend to be larger in size, cost more, and due to the chemicals used, will require specialist handling of the waste that is produced to ensure it is disposed of safely. There are, however, certain components that can't be cleaned using this equipment.

Many electronics manufacturing services (EMS) providers now standardize on a no-clean process. If you are unsure, or you are weighing up the pros and cons, then it’s always best to take advice from your EMS partner. They know what your requirements are—and they'll have the experience, equipment, skills, and processes in place to meet your demands.

Russell Poppe is the director of technology at JJS Manufacturing.



Suggested Items

IPC Standards Development: Business Challenges and an Inside View

10/28/2020 | Graham Naisbitt, Gen3 Systems
With increasing frequency, standardising the standards, such as ISO 9201, imposes certain rules that must be met to ensure “fair play” amongst the supply chain. There will be those familiar with hearing about “false positives/negatives” and “never trust the salesman,” so mitigating these is no easy task. However, there is the chance for each 5-30 Task Group to review industry requirements and set out the work program for the ensuing period. With that in mind, much of what follows is based on comments we learn about from our industry around the world, many of whom are not yet IPC members. Yes, this is a membership recruitment drive, unashamedly, as well as a search for volunteers willing to help create the standards of tomorrow.

How to Benefit From Robotic Soldering Processes

10/28/2020 | Pete Starkey, I-Connect007
Webinars are in vogue! But in Pete Starkey's experience, the master of the technical webinar with many years’ experience of delivering first-rate events is Bob Willis—electronics assembly specialist, soldering expert, and provider of training and consultancy in electronics manufacture. Here, Pete recaps Bob’s presentation on the upcoming robotic soldering experience.

90% Solder Dross Recovery: Eliminate Waste, Save Money

10/20/2020 | Nolan Johnson, I-Connect007
The impact of 90% solder dross recovery can have a substantial impact on reducing waste and increasing cost savings. Nolan Johnson recently spoke with Jay Hardin, MS2® product manager at P. Kay Metal, about the process and benefits of efficient solder dross recovery, as well as product roadmaps, the soldering machine market, and raw materials supply chain for solder alloy manufacturing. As Jay says, “For many companies, the savings are in the thousands or millions” of dollars.

Copyright © 2020 I-Connect007. All rights reserved.