Many of our readers will find this information “old hat” or say, “that’s a given” but I think it’s time we addressed printed circuit boards in all of their flavors. Over time, they have been called different things: printed circuit boards (PCBs), printed wiring boards (PWBs), printed circuits, boards and cards. To add to a new buyer’s confusion, we can provide rigid boards, flexible boards, and rigid-flex. You can choose your color of solder mask (default is green), you can specify the number of layers, and you can even have “mouse bites”……huh?!
OK, let’s fall back to what a printed circuit board is, first. To get really basic, if you have dropped your phone, peered into the dark innards of your old TV, or taken apart your gaming console or PC, you’ve seen a Printed Circuit Board Assembly (PCBA) and what all those components (resistors, capacitors, diodes, etc) are mounted on is the PCB. Remember seeing those? Well, that “Bare Board” (see, we’re calling it something different again) is typically made up of multiple plies of epoxy-glass laminate material with copper foil bonded to the outside. That foil has been plated and etched to form a pattern of conductors (traces or lines, pads, which can be thru-hole or surface mount & plated-thru-holes or PTHs, which can be component or vias). The laminate acts as a non-conductive insulating base, while the copper pattern acts as a conductive interconnect for the components.
Printed circuit boards can be single sided (1-S), double-sided (2-S), or multi-layer (M/L). In a four-layer (4-L) board, for example, you have two external circuitry layers and two internal layers all laminated together. As you might surmise, a M/L is going to be more expensive (the more layers the higher the cost) than a 2-S, which is slightly more than a 1-S. M/Ls are almost always designed with an even number of layers for purposes of balance or symmetry.
Remember those lines? Traces, I mean. Those are made of special copper, selectively covered with soldermask (typically Liquid Photo-Imageable-LPI) with all the component interconnect pads coated with solder (HASL process) to prevent oxidation. (Unless it’s a lead-free board, but that’s a subject for another day.). PCB size and thickness are both significant cost drivers. The tighter those boards are designed or laid out, with high density fine traces and spaces, also drives up the cost. Even the number of drilled holes, and the sizes of those holes, can make a difference in the cost of the board.
Sometimes a customer will need all of the boards supplied as single pieces (1-up); you pick up one piece and that is one board. Sometimes they will prefer an “array” of boards; you pick up one piece, there may be two, or five, or twenty or more boards in your hand. Sometimes the arrays can be V-grooved (scored) & sometimes they are routed with tabs so that they can be snapped apart after assembly. Remember those “mouse bites” we alluded to? Those are little chunks of laminate that are removed from the tabs, so that the process of snapping boards apart is made easier. (No mice are harmed as a result of this procedure.)
Well, hopefully you haven’t crossed your eyes, gone to sleep, or worse, found an error in my explanations. Maybe this information will help some of our customers who are new to the game. We’ll cover flex boards and lead free process in later posts.
Thanks for coming along on the ride,