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68 DESIGN007 MAGAZINE I JANUARY 2019 Test Your Design Assumptions Keep in mind, PCB design is often as much an art form as it is a science. It's not as straightforward as plugging a bunch of numbers into an equation and receiving an answer. So, before you lean towards a thicker or thinner board design or accept your design tool's default thickness, consider as many relevant factors as possible: • Do you need to prioritize thermal transfer or insulation? • Are there weight restrictions? • How flexible or stiff does the board need to be? • Will it be well protected inside the assembly? • How much space do you have to work with? • Are component lengths long enough for thicker boards? • Is drill aspect ratio acceptable? • Can your design be manufactured, and does it fall within manufacturer tolerances for very thick or thin boards? Guidelines for Choosing PCB Thickness Determining the answers to those questions will go a long way toward establishing peace of mind during the transition from design to prototyping. At a high level, thicker boards make sense when: • There is a high layer count • The board needs to be stiff because of its size or lack of protection in the assembly • Durability is a priority Thinner boards are preferable or necessary when: • The board must be light • In rare instances, flexibility is required to facilitate installation or solve interconnection issues; in general, you don't want to bend your boards • The design calls for tiny vias; if the board is too thick when drilling through holes, drill bits will break, so consider keeping aspect ratios below 10:1 (preferably 8:1) • The assembly is smaller with support for a board sandwiched into a tight space • The device runs hot; a thinner board can help dissipate heat because thermal vias are shorter and pass through a smaller volume of insulating material We recognize cost can influence decision- making. Your fabricator will have a standard board thickness, probably 0.062". Optional thicknesses may be more expensive, but savings evaporate quickly if the PCBs aren't durable enough for their intended use. Thinner boards can be damaged and may warp during solder processes. Checklist for Determining Thickness As you consider thickness in your design, we encourage an easily replicated checklist approach. Here are some factors to consider as you design your board: • How big is your device? Thinner PCBs take up less space, so if you're building a device that will fit in your hand, go as thin as necessary to accommodate the space. • It ain't heavy, it's my board: PCB weight can be an issue more often than you might think. Is your board part of a cellphone, Bluetooth headset, or tablet device? Thinner boards are lighter and can help give your device that trim figure we all appreciate. • Know your connections: PCB edge or trace-based connectors, for example, require a board with thickness matched to fit in the mating portion of the connector. Some through-hole parts have pin lengths that limit board thickness. • What's your frequency? Layer-to-layer thickness will affect signal integrity, crosstalk, impedance calculation, and signal loss. For high-frequency