Choose a PCB Manufacturer
This is by no means an attempt to provide a comprehensive list of fab houses that you might want to use for a low-quantity order; that information is readily available via Google. Instead, I want to offer some suggestions based on my knowledge and experience. I’m not going to comment on the quality of the boards produced by a given manufacturer because I’ve never in my life received a PCB that I would describe as poorly manufactured (I don’t know if this is because I’m lucky or because I have a knack for identifying fab houses that are serious about quality). I think it’s safe to assume that all of the following fab houses will deliver a fully functional PCB.Oshpark
The primary advantage is the price, which is excellent. Oshpark orders are subject to constraints that, depending on your circumstances, could be completely unacceptable, completely insignificant, or anywhere in between. The lead time is also quite long, but again, if you’re not in a hurry this is a non-issue. I really appreciate their user interface: it’s intuitive and straightforward, and it provides immediate visual feedback combined with informative explanations. For example:
Oshpark accepts KiCad and EAGLE project files.
Advanced Circuits
In contrast to Oshpark, Advanced Circuits is a longtime industry leader that offers the full range of advanced features and purchasing options. Nevertheless, they are not opposed to low-quantity orders, and they have special deals for two- and four-layer boards that conform to certain restrictions. Two things that set Advanced Circuits apart, at least in my mind, are the FreeDFM file check and their free CAD software, called PCB Artist. FreeDFM checks your Gerber files for issues that could delay or prevent proper manufacturing; some things are even automatically corrected.Here’s an example from the Advanced Circuits website:
PCB Artist is a fully featured CAD tool that is completely free. The “compromise” here is that you have to manufacture the board through Advanced Circuits. If I understand correctly, the software will generate standard Gerber files (which can be sent to any fab house) after you have made the initial order with Advanced Circuits. Overall the restriction seems fairly reasonable.
Sunstone Circuits
This manufacturer is also in the “longtime industry leader” category, but they encourage small orders and offer a low-cost prototype ordering option. Unlike Oshpark, which has a minimum time-to-shipment of five business days (and that applies only to two-layer boards), Sunstone can ship your prototypes the next day, or maybe even the same day, if for some reason you’re in a desperate rush. Like Advanced Circuits, Sunstone offers a completely free CAD tool (called PCB123) that seems to be quite impressive. I’ve never tried it, but the website claims that it has been used by corporations and institutions as prestigious as Intel, Honeywell, and the United States Naval Research Laboratory. If you have any experience with Sunstone or PCB123, please leave a comment and let us know what you think.Choose an Assembly Method
After so much time and effort invested in designing, checking, and manufacturing a printed circuit board, it can be depressing to realize that you have succeeded in arriving at the most difficult part of the production process. This is not always the case, but nowadays—i.e., the age of small (if not minuscule), densely packed surface-mount components that sometimes do not even have protruding leads—the issue of physically attaching parts to the board can present the greatest challenge to low-quantity PCB fabrication.You have four options: professional assembly, DIY reflow, hot-air-gun soldering, and hand soldering (i.e., with a soldering iron). I’m not going to discuss hand soldering because in most cases it will be difficult, impractical, or downright impossible (though the soldering iron can certainly be useful for minor rework tasks). Let’s take a look at the other three options.
Professional Assembly
The primary obstacle here is cost. Automated assembly technology is mature and highly reliable, but the process is not economically adapted to low-quantity orders. It also requires additional manufacturing data:- BOM information: The assembly house needs information that enables them to either order the parts or organize the parts that you provide.
- Solder mask: You need to submit Gerber files that identify the areas of the board (e.g., pads for IC pins) that must receive solder during the process of solder-paste deposition. (Solder paste is the type of solder used in reflow assembly.) This can be a bit complicated because in some cases you need to create solder-mask divisions that divide one large pad into multiple smaller rectangles of solder paste (see this article for more information).
- Placement data: This includes spatial coordinates and rotation for every part on the board. The machine cannot place components if it doesn’t know which ones go where and how they are oriented.
I know of only one company (MacroFab) that can perform low-quantity automated assembly at a reasonable cost. If you’ve found any other way to obtain similar services at a similar price, please let us know in the comments.
DIY Reflow
This approach is surprisingly feasible. The general idea is that you deposit the solder paste, place the components, bake the PCB in a repurposed toaster oven, and voilà . AAC already has quite a bit of information on this topic:Manually depositing solder paste onto tiny, closely spaced pads is not easy; you may want to consider using a stencil, which is a flat object with openings corresponding to the solder-paste locations. Stencil-based DIY reflow is discussed in this article. The following image shows the excellent solder-paste deposition achieved by an AAC contributor with the help of a low-cost polyimide stencil.
Hot-Air-Gun Soldering
The only difference between hot-air-gun assembly and DIY reflow is the heat source, which is a good reason to become proficient in depositing solder paste and accurately placing components—once you’ve completed those steps, you can choose between reflow and hot air according to other factors. Hot air is convenient and highly effective for smaller boards. Reflow requires a more elaborate setup, but it heats the entire board evenly and gives you more control over the temperature profile. A reflow profile is a graphical representation of how the temperature should increase and decrease during the reflow procedure; for example:
One question that comes to mind is the proper hot-air temperature. I haven’t found authoritative information on choosing temperatures for hot-air soldering, so you might have to do some experimentation. This is the most specific information that I have at the moment: Recently I assembled a board using low-temperature bismuth-based solder that has a melting point of 138°C. I used a hot-air temperature of 180°C to warm up the board, then I increased to 270°C for the actual soldering phase.
You can read more about hot-air-gun soldering here, and this article (scroll down to the “Manufacturing Considerations” section) briefly discusses bismuth-based solder (which I highly recommend).
What assembly methods have worked best for you? Share your experiences in the comments below.
No comments:
Post a Comment