Impedance Tolerance: What to Specify What the Tolerance Applies To An impedance specification is meaningless until it says which impedance, measured where and over what frequency. A single line has a characteristic impedance that varies along its length because the dielectric thickness and the trace width vary, and a differential…

Laser Drilling Microvias: Parameters and Limits Why Laser Instead of Mechanical A mechanical drill below about 0.15 mm is impractical: the bit is fragile, the spindle speed needed to keep the surface speed reasonable becomes extreme, and the drill tends to wander. A laser removes material without contact, so the…

Silkscreen and Solder Mask: What Each Layer Does Two Different Layers The solder mask and the silkscreen are both printed onto the surface of a board and they are often drawn on the same screen by a designer, but they do completely different jobs. The solder mask is a permanent…

High Tg Laminates: When They Are Worth It What Tg Describes The glass transition temperature is the point at which a resin changes from a rigid, glassy state to a softer, more rubbery one. Below Tg the laminate behaves like a stiff solid; above it the resin softens, the coefficient…

PCB Copper Wrap and Edge Plating Tolerance What Copper Wrap Means Copper wrap is the connection of a surface copper feature around the edge of the board to copper on another layer, so that the two are joined by metal that passes over or through the edge rather than by…

X-Ray Inspection of BGA and QFN Joints Why X-Ray Is Needed A ball grid array hides every joint underneath the package, and a quad flat no-lead part hides its joints under the body edge. Optical inspection sees the package and nothing else, and functional test sees only the nets that…

Thermal Vias: Design and Copper Filling Where the Heat Goes A surface mount power device conducts most of its heat into the board through the thermal pad on its underside, not through the package body into the air. That pad sits on a copper land, and the land has to…

Stencil Aperture Design and Paste Volume The Deposit Decides the Joint Every soldered joint on a surface mount assembly begins as a printed deposit of paste, and the volume of that deposit is the single largest influence on whether the joint is sound. Too little paste and the fillet is…

Copper Filling Versus Conductive Paste Via Fill Why Vias Are Filled At All A plated via is a hole with a thin conductive wall and an open middle. That is fine for a signal via in an ordinary board, but it becomes a problem in three situations: when a via…

OSP Versus ENIG: Surface Finish Selection What the Finish Has To Do The copper on a pad has to remain solderable from the moment the board leaves the plating line until the last joint is made, which may be weeks later and several thermal cycles away. The surface finish is…