Board Thickness Selection and Stack Up Tolerance Board thickness is usually quoted as a single number, but that number is the sum of several layers each with its own tolerance. Understanding where the figure comes from decides whether the specification can be met and at what price. How the Nominal…

Flex Rigid Board Design Considerations A flex rigid board is two constructions joined into one, and most of its difficulties come from the joint. The rigid sections behave like ordinary boards, the flexible sections behave like cables, and the transition between them carries the weaknesses of both. Designing one well…

Copper Balance and Thieving on PCB Panels Copper balance is the practice of arranging each layer so that the amount of metal is roughly the same across the panel and between the layers. Where it is uneven, the panel behaves unevenly during lamination, during plating and during reflow. The symptom…

Solder Mask Dam and Sliver Design Rules A solder mask dam is the strip of mask between two adjacent pads, and it exists to stop solder from bridging between them during reflow. When the dam is absent, the two pads behave as one wettable surface and the bridge is only…

Underfill and Corner Bond Selection for Area Array Packages Underfill and corner bond both reinforce a solder joint, and they are not interchangeable. One fills the gap beneath the package; the other reinforces the corners only, leaving the rest of the joint to carry its own load. What Each Material…

Component Sourcing and Approved Vendor List Control An approved vendor list is the document that decides which parts may be bought and from whom. It exists because two parts with the same function are not necessarily interchangeable, and because a supply chain that is not controlled will eventually deliver a…

Wire Bonding and SMT Hybrid Assembly A hybrid assembly carries wire bonded die and surface mount components on the same board. The two processes ask different things of the surface, and the sequence in which they are run decides whether both succeed. What Wire Bonding Requires A bond forms when…

Trace Fusing and Current Crowding in Power Nets A conductor fails when its temperature rises far enough that the copper no longer carries the load or the laminate degrades around it. The current at which that happens is set by the cross section at the narrowest point rather than by…

Shear Test and Pull Test Methods for Solder Joints Shear and pull tests are the two mechanical measurements that turn a visual opinion about a solder joint into a number. They are used to qualify a paste, a surface finish, a reflow profile and a component, and they are used…

Impedance Coupon Verification: Trace Geometry, Placement and Records An impedance coupon is a small test pattern on the panel that carries traces built to the same stack up as the product. It exists so that the impedance of the board can be measured without measuring the board itself. Why the…