OSH Park, for instance, uses a premium FR408 substrate for their four-layer service, which yields excellent RF performance. Luckily, many services have chosen their offerings well. With hobbyist-oriented services, however, you get their standard four-layer stackup. If you’re ordering a large number of PCBs directly from the manufacturer, you can customize the stackup, changing the spacing and copper thicknesses to suit your design. There are some very compelling reasons to do this, which we’ll explore in a bit. If you’ve never worked with four-layer PCBs before, you might assume the extra two inner copper layers were equally spaced within, but they’re typically much closer to the outer layers. A typical example is shown in the figure: 1.4 mil (aka 1 oz) for the copper and 60 mils for the core. The particular arrangement of the layers within a board is known as a “stackup.” Two-layer stackups are simple: there’s copper of a specified thickness on each side of a core material, most often FR4 glass-reinforced epoxy laminate. The obvious difference between two-layer and four-layer PCBs is two extra layers of copper. How Do They Stack Up? Example Stackups (data from OSH Park) If you’ve never considered one for any of your designs, you may be pleasantly surprised at what little extra cost is involved for all the benefits you gain. So, let’s take a look at designing four-layer PCBs. Each of the previous shifts has brought easier design and construction as well as improved performance, and the same will be true as four layers becomes more commonplace. I think this will inevitably increase, as has been the case with all the previous technologies: the advanced eventually becomes the mainstream. Today, the “advanced” hobbyist may be using four-layer boards, although the four-layer adoption rate is still relatively low – OSH Park produces around 90% two-layer and 10% four-layer, for instance. Eventually, this gave way to the aggregating PCB services we have now with full two-layer boards, complete with soldermask and silkscreen. I later saw the rise of “bare bones” fabricated PCBs: professionally made fixed size boards with plated-through holes, but no soldermask or silkscreen. By the time I started etching my own PCBs, the advanced hobbyists were on to double-sided home-etched boards - the only type not pictured above, because I couldn’t find the one successful example I ever created. At that time, etching even single-sided boards was for “advanced” hobbyists. This was not the case in my youth, when I first acquired an interest in electronics. Many readers are certainly familiar with the process for home-etching of PCBs: it’s considered very straightforward, if a little involved, today.
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