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PCBD-Mar2016

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34 The PCB Design Magazine • March 2016 the unique hybrid design requirements that they are faced with. And yet many designers out there (and I used to be one of them) have no idea of what is meant when people start talking about hybrid design. It is therefore not uncommon for de- signers to avoid the subject directly while hop- ing to pick up little cues and pointers from oth- ers indirectly so that they are no longer in the dark. If that description sounds uncomfortably close to where you are today, then read on. My hope is that this three-part series will help you by serving as an introduction into the world of hybrid design. And before we jump in, I want to give a shout out to my colleague Bernd Pflueger. Bernd has been around the world of hybrid design for a long time now, and he has probably forgot- ten more about it then I'll ever know. I'm in- debted to him for his help and the depth of his knowledge and the valuable insight to hybrid design from the PCB designer's perspective that he brings. A hybrid design is an alternative to the stan- dard PCB. Components and conductors are at- tached to or fabricated onto a substrate which can then be completely encapsulated in a pro- tective coating. Hybrids are generally smaller and much more robust than PCBs, making them more adept for extreme environmental conditions. A hybrid design is best if the board is going to be subjected to moisture, excessive vibration, or high temperatures. In other words, if a board is going to be immersed in water, bur- ied in the ground, or shaken to death inside the hot confines of some kind of engine, then a hy- brid design would be the better choice over a PCB which may not able to survive under those kinds of conditions. Hybrid designs will generally have a higher reliability than traditional PCBs. They have few- er solder interconnections, while the other met- al interconnections in a hybrid design are more reliable than a solder joint. And with the ability to print ink resistors instead of using standard board-mounted packaged resistors, a hybrid de- sign can realize better precision in their resis- tors plus save on the cost of stocking and stor- ing these parts for manufacturing. But we are getting ahead of ourselves by talking about ink resistors this early in the game; more about that later. To start with, let's talk about the basic struc- ture of our hybrid design, which for the pur- poses of this series we will be referring mostly to an LTCC hybrid design (more on what LTCC means later). As the circuit board designer knows, a standard PCB is a made up of different layers of copper and dielectric material (usually FR-4). The PCB fabrication process etches cop- per away to form conductors (traces and fills) and all layers are eventually composited to- gether. Therefore the fabrication of a PCB can be considered a subtractive process due to the copper etching. An LTCC hybrid design is the complete opposite. It is built bottom-up from a substrate by printing conductive material for conductors (traces and fills) onto the substrate making its fabrication an additive process. LTCC stands for low-temperature co-fired ceramics. LTCC designs generally use a ceramic material for their substrate, although depend- ing on the needs of the design, different sub- strate materials can be used. Stainless steel and titanium are two alternatives for substrates, but these materials are more expensive then ce- ramic and add another level of complexity to the fabrication process. Each of these would re- quire the application of a thin layer of isolation material to the substrate before adding conduc- tive layers. Therefore, the majority of substrates are ceramic unless the high-temperature envi- ronments require the extra cost and weight of stainless steel. Titanium is reserved for those environments where heat flow, weight and sta- the principles of hyBrid design, part 1 " And yet many designers out there (and I used to be one of them) have no idea of what is meant when people start talking about hybrid design. "

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