PCB Manufacturers: the reflow of truth

How can you check the quality of a PCB If you only had one choice? assemble it!

PCB Solder Paste being applied on a PCB via steel stencil from PCBway – Picture from Francesco Gritti

Whenever you have everything ready for the production of your PCB and have your manufacturer dialed in, maybe as from the previous article (PCB Manufacturers: does the choice matter?) you’re now ready to test the PCB. How would you know if it is good or not and when to commit to the manufacturer?

How do you define quality PCB-wise?

Skipping the whole differentiation between various manufacturing classes, PCB quality is practically defined in a few key parameters, most of which can be tested while performing the assembly process, which, as you may have guessed, is arguably one of the fastest ways to test the reliability and quality of a board.

What are the key factors to look at? What should I do

The key checks include many little things, as well as some peculiar ones. This list is not exhaustive, but should be able to give you an idea of how many ways are there for things to go wrong, and how to prevent them:

  • Take a ruler and actually measure your PCB dimensions BEFORE you assemble it or try to install it to avoid increasing the probability of errors.
  • If you have it at hand, take a caliper and check the inner diameter of all the holes you were planning to put a screw in. Especially the plated ones.
  • If the PCB is barebone with no components on it, take some alcohol and synthetic cloth and clean it accurately. It may happen that the PCB has salts and other residues from the manufacturing process, or even detergent residues, that mixed with the solder paste will really cause issues to BGA components or very big SMD pads. If you have a manufacturer, this step will be performed by them before and after cleaning (for the after cleaning, this is not true as it depends on the moisture exposure level of the components you are mounting).
PCB Manufacturing dirt residue example – Photo by Nadim Conti
Post-reflow water-free cleaning foams can also be very effective in cleaning pre-reflow – Photo by Nadim Conti
  • Take a coin and try to scratch the silk screen away. Is it easy? if you can do so fairly effortlessly that’s a clear sign of low silk-to-solder adhesion. One may not think of this as a problem, but in environments where temperature changes and vibrations are present, your board will eventually just turn green, with your silk screen turning into dust, which is something I saw more than once, even where ionizing radiation was not a concern. This problem is especially true when dealing with flexible PCBs as the one in the picture below. The loss of silk screen is so evident the coin was not even needed to remove the silkscreen, this becomes especially easy to check after or during assembly as the high temperatures generally increase the magnitude of this problem. The solution was, again, to talk with the manufacturer and explain exactly what you want, or change the manufacturer in the first place if talking is not an option.
Flexible PCB manufactured for the LHCb Upstream Tracker Experiment at CERN – Photo by Nadim Conti
  • Verify all the edges, holes, cuts, and dents in the PCB. Every single sign present on the PCB before the assembly process begins must be considered. As an example, local burns or rough edges should be checked as they may be a future source of delamination or contamination in the middle layers of a PCB.
PCB with CO2 laser cut induced local burns – Photo by Nadim Conti
Badly routed PCB. FR4 splinters are present and are a sign of too high routing speed or bad tooling – Photo by OSV
Nicely routed PCB edge showing no issues – Photo by Nadim Conti
  • Look for any kind of discoloration or bubble or strange color on the solder mask. To continue with the provocation, if you try to assemble a board with clear discolorations that are due to air, moisture, or dirt trapped inside your PCB, you’ll either be lucky or with a blown-up PCB pretty soon. Unfortunately, these kinds of details are hard to check for many manufacturers, therefore PCBs may still be shipped with issues such as bubbles in them. To be sure, if a PCB has one, unless it is a class-1 PCB or a prototype, discard it or repair it.
Delamination bubbles showing places where layers are not well bonded between each other – photo by Nadim Conti
  • Once assembled, try to shear and pull-remove components. You may have soldered your components in the most impeccable way, but lamination damage can occur even during the manufacturing process of the PCB itself, there’s no need for a reflow to make it appear in certain cases. Therefore, the only way to check if the PCB was well made, especially if you have connectors on it, is to try to solder a solid, wide tin soldering wire to a pad, and try to rip it away by measuring your force at least in a sensible way. If the wire breaks at the solder joint or somewhere else, then the PCB could be considered good. Additionally, try to solder a copper wire and repeat. This is extremely important for reliability as it may happen that, down the line, after a few connector insertions and detachments, your PCB is going to fail, badly.
USB solder pads ripped away during a shier test – Nadim Conti
PCB with badly bonded layers with a trace completely detached away from the pull test – Nadim Conti

Are there cheap manufacturers that check the list?

To write this post I had the opportunity to try to test and assemble some boards manufactured by PCBway, which is a very fast and cheap option to get your PCBs done but can also manage to deliver high-complexity projects we’ll see in the future.

For our test, we ordered a 4-Layer PCB with steel-made stencils with a standard assembly frame for a total cost of around 200 USD.

Steel-made framed stencil from PCBway on the right, and the boards to be soldered on the left – Photo by Francesco Gritti

Out of 50 PCBs, 6 showed silk screen issues, the others were completely fine, which probably indicates that, at least for our economic-standard production, the silk-screen check is not something performed on their side. Nevertheless, the overall silk-screen quality was satisfying.

14/Sept/2022: I would also like to add that, building a relationship with the manufacturers also means talking about these issues. In particular, once noticed the silk-screen problem, PCBway offered either a refund or to manufacture those boards again. This is exactly what you should look for in a manufacturer, the willingness to fix issues.

The stencil was rigid enough to apply the paste evenly, but close attention should be made to the material used. Even though it is specified that the cheap production batches do not use high-temperature grade (TG) FR4, we had quite some issues with the FR4 softening down during the reflow and heating process, indicating that the TG was probably not respected, which is not a sign of a PCB that can be used for an extended time in high-temperature environments, nor for a project that will last decades. Anyhow, for a cheap prototype like ours, this was almost to be expected.

PCBs after reflow. One SMD device was not attached to show the exposed pads and check their separation – Photo by Francesco Gritti

After the second reflow process and a bit of cleaning, the boards were still holding well, showing no issues of delamination nor inner layer damages, meaning a well-controlled manufacturing process on PCBway‘s end.

Fully assembled boards manufactured by PCBway – Photo by Francesco Gritti

In conclusion, we had fun and were satisfied with the product quality which, most importantly, matched our expectations. For a Manufacturer like PCBway it should be clear that one cannot order class-3 PCB having a project in their cart using their standard service, but as long as everything is coherent, checked, and discussed there are no obstacles ahead. In this regard, environmental concerns aside, PCBway is an excellent choice for fast & cheap PCB manufacturing.

After two articles it should definitely be clear then that a specific PCB manufacturer is not super important as long as your expectations match your actions, and as long you and the manufacturer are communicating openly, possibly even before the design process begins.

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