This article is not about the making of PCBs, that’s PCB manufacturers’ job. It’s about how to avoid pitfalls and across barriers associated with incomplete designs files, and finally make PCBs that work. If you are a pro, this article is of little help for you, but if you are a beginner, like I was, that suddenly get an assignment to make some PCBs someone else designed a long time ago, this article could save you a lot of time and pain.
Depends on how lucky you are, you may get few of these: populated PCB samples, some schematics, BOM lists, Pick and place files, Gerber files. One thing is certain, no one can guarantee you that any of these input are complete or up to date. Following that, here’s the biggest lesson you can learn from this article if you decide to not read further: Assume errors are everywhere, make your own judgements based on multiple sources of information.
For those who are reading further, I am listing 4 big lessons I learnt personally and mostly painfull on making PCBs out of incomplete design files:
- First thing first, confirm with your customer which version of information are more correct. When your start talking with your customer about the assignment, he may give you BOM lists, Gerber files, Schematics (most likely printed version), and other files. The first question to get answer from the customer is which version of information you should trust the most. Because if every source of information are consistant with each other and complete, your customer most likely will directly ask a PCB manufactuer to produce the PCBs, instead of bothering with you. So if you get the assignment, you will meet problems here and there hidden on the way. Typical problems that I met in such projects include: No PCB design files, The schematic is not matching the sample, The model names and the values are confilcting for some components, Some model names are not matching the sample, Sample board is not fully functional, Surface mounting parts are marked as through hole parts, and so on. So the first information you should get from your customer is which version of information is the latest, most reliable, according to the best of their knowledge. That will be the baseline of your further try and error process.
- If you are going to outsource SMT assembly, your supplier will ask for a pick and place file. But if you only have Gerber files, not the design files, it is still possible to generate the pick and place file with some software tools. The easiest method I normally use is first use Kicad Gerber viewer to export a PCB layout file from the Gerber files first, then generate the pick and place file from this PCB layout file. Another good feature of this software is that you can do some basic editing directly on the gerber files. For example, for the last project I was working on, I need to move some component resignators, and increase the soldering pad size of some components. These are all possible direcly in the graphic interface in Kicad Gerber viewer.
- The final deliverables are not the PCB assembly, but more the expected behavior of the PCB. It should be ideally specified before the project starts. Practically, it will become a discussed result between you and your customer. Because the customer don’t have a full understanding of the design, they only know what the PCB do externally, maybe from a user’s point of view. What are the technical requirements will be finalized by close communication between the customer and your technical team. The final deliverables include both the electronic, mechanical, or even environmental aspect of demands that needs to be specified. The electronic part include basic parameters of the circuit including output amplitude, linearity, noise level, distortion, phase, frequency response, etc. The mechanical part is often ignored by an electronic engineer that causes headache afterwards. One of the project I was working on that has a big delay because the only heat sink on the market that we can find that matches the size of the one used on the sample board doesn’t have space for screw holes. Those screw holes are used to fix the heat sink to the base plate of the device. Later we find all the transistors our supplier mounted on the PCBs need to be removed because there is a flaw in the PCB cutout that if the transistors are mounted vertically, the back of them can’t attach to the plane of the heat sink properly. In addition, since the heat sink will be mounted on the base plate, so is the PCB board, then the height of the transistors are constrained by the relative position of the heat sink and PCB. In the end, we have to remove all the transistors mounted, clean the soldering pads, mount the new transistors onto the heat sink first, insert the transistor pins to the PCB, land the heat sink at the mounting position on the base plate, then solder the transistors to fix their positions. That is a lot of work to solve one mechanical problem. So it is important for electronic engineers to keep in mind the mechanical constrains of the project. The root cause of this problem for us is that we were sent only sample PCBs by the customer, and assumed make more of those PCBs working electrically is our task. The problem only floats to the surface when we asked for a whole set of the device to work as a reference for testing of the boards we made.
- Deal with the nightmares. Work on someone else’s design has many nightmares. You may need to do trial and error on some components because you don’t have a model number for them. You may realize the parts you bought according the BOM list you get from the customer looks totally different from the sample boards. In these scenarios, the biggest lesson I learnt painfully, as I mentioned in the beginning should help. By combining the information from different sources, you get to the target faster. For example, assume you have a component without a model name. What is the package marking on top of the chip? How many pins it has? What isthe pins spacing and component body size? What function is it likely to be according to the schematics? Input output voltage measurement? Oscilloscope waveform type. All those inforamtion combined can narrow down your choices and reduce the trial and error cycles. The worst of all nightmares is that a lot of comonents in the BOM are out of stock, discontinued, or obsolete. In my experience if a part is out of stock or discontinued in major european suppliers like Farnell, Conrad, RS components, and Mouser, there is still a good chance you can find them in Chinese market. If you understand Chinese, Taobao.com would be ideal for searching for non-stock and discontinued parts. But if one part is unavailable everywhere, then you have to dig into its datasheet, try to find the key parameters that you should match with an alternative, then search for those alternatives. Such key parameters will likely including Pin out, power rating, noise level, linear range, etc. Sometimes, you have to go deep with the schematic to understand the function of this component in the circuit first to grasp what requirements it should met. It is hard work, but there is no easier way.