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20 PCB007 MAGAZINE I JUNE 2019 a Class 1 or Class 2 part, if IPC netlists and any instructions expressing the need for a net com- pare are not provided, a separate netlist file is not required in the output package. Class 2 is for "dedicated service products" and requires a little more information to be provided, as continued performance and ex- tended life cycle are required. Uninterrupted service is not mandatory for Class 2 PCBs. It is important to note that while a Class 2 PCB does not require an IPC netlist be provided to do a design versus exported image data com- parison, it is highly recommended as part of the output package. Class 3 is specified for the highest reliability and continued high performance in segments where equipment failure simply cannot be tol- erated, such as the medical or aerospace in- dustries. For IPC 6012 Class 3 an IPC netlist comparison is required. Netlists typically fol- low IPC-D-356, IPC-D-356A, or a Mentor neu- tral file. Other older forms of IPC netlists can be used if a fabricator has the ability to use raw computer-aided design (CAD) data, but by far the most common formats for fabrication are the three previously mentioned. As a fabricator, we constantly see PCBs called out as IPC-6012 Class 3 that have not been designed to meet Class 3 requirements, such as minimum annular ring or clearance (Figure 1.1). If the PCB was not designed to meet Class 3 requirements, the fabricator does not have a chance to provide the customer with a Class 3 PCB. Designers should use the IPC- 2220 series of documents concerning board de- sign and fabrication to ensure a design-to-fab- rication match. Part sizes are shrinking while part densities are growing, which reduces space and makes it increasingly difficult to have enough sup- porting annular ring or clearance. One way to minimize delays in the fabrication quote or manufacturing process is to qualify this by say- ing the product is to be built to IPC-6012 Class 3 with an allowance for annular ring and clear- ance to Class 2. This may preclude a phone call from your fabricator to tell you the quote is on hold because the design does not meet Class 3 minimum annular ring requirements (Figure 1.1). Pre-quote Software Many PCB fabricators have some type of pre- quote analysis software that can quickly tell them if the PCB meets certain design rules, such as whether spaces on a given design do not support the copper callout specified on the drawing or stackup detail, or if the annu- lar ring is insufficient for the specified hole sizes. As a cautionary note, these are quoting tools the fabricator uses to catch obvious con- cerns. They are not intended to take the place of a full-blown computer-aided manufacturing (CAM) design rule check (DRC) tool. Pre-quote software is typically a quoting tool for managing and assessing the customer's incoming PCB data files. Depending on the Figure 1.1 Fun fact: The term "annular ring" comes from the reliable indicator of yearly tree growth.

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