To define what a PCB board is. we have to look back at history that traces the evolution of printed circuit boards. back to the early 20th century. The first PCB patents for "printed wire" issued in the early 1900's but PCBs. that we would recognize first came into use after World War II. In 1925, Charles Ducas of the United States submitted a patent application for a method of. creating an electrical path on an insulated surface by printing. through a stencil with conductive inks. Hence the name "printed wiring" or "printed circuit." An Austrian scientist, Dr Paul Eisler. credited with making the first operational printed wiring board in 1943. It used as a replacement for bulky radio tube wiring.
pcb drill hole tolerance
Considering many potential issues before submission can impact your board’s turnaround time. quality and cost. An important issue that sometimes doesn’t get the attention it warrants is PCB drill holes. Most designers consider spacing and size as they relate to their performance objectives. but might not consider how they relate to the manufacturing process. Applying some simple PCB design for manufacturing (DFM) rules. and guidelines for drill holes can have a significant impact.
The most important DFM considerations for drill holes. are generally PCB drill hole tolerance. and PCB drill aspect ratio. When determining these, there is much more to consider than the actual drill hole size.
there are two types of drill holes: non-plated. through holes (NPTHs) and plated through holes (PTHs). NPTHs are holes that do not carry current, and thus do not need conductive plating. Mounting holes are the most common type of NPTH. PTHs, but, are usually signal carriers or ground returns. They carry current and need conductive plating. PTHs are usually vias between outer and inner layers. inner layers only or surface-to-surface.
There are some general rules that should applied for the NPTHs and PTHs of your design. Not employing the following PCB drill hole rules in your DFM. can impact the turnaround time for your board:
Minimum finished hole size = 0.006″
Minimum edge to edge clearance (from any other surface element) = 0.005″
Plated Through Hole (PTH)
Minimum finished hole size = 0.006″
Minimum annular ring size = 0.004”
Minimum edge to edge clearance (from any other surface element) = 0.009″
PCB drill board advantages
The backdrill process removes stubs from plated-through-holes (vias). Stubs are the unnecessary / unused portions of vias. which extend further than the last connected inner layer.
Stubs can lead to reflections, as well as disturbances of capacity. inductivity and impedance. This discontinuity errors become critical with increasing propagation speed.
Backplanes and thick Printed Circuit Boards in particular. can endure significant signal integrity disturbances through stubs. For High Frequency PCBs (e.g. with Impedance control), the application of backdrilling. as well as the application of blind and buried vias, can be part of the solution.
Backdrill can applied to any type of circuit board. where stubs cause signal integrity degradation, with minimal design and layout considerations. In contrast, when using blind vias, the aspect ratio has to kept in mind.
-Reduced deterministic jitter
-Lower bit error rate (BER)
-Less signal attenuation with improved impedance matching
-Increased channel bandwidth
-Increased data rates
-Reduced EMI radiation from the stubs
-Reduced excitation of resonance modes
-Reduced via-to-via crosstalk
-Aspect ratio can neglected (in contrast to blind vias)
The earliest PCB's (printed circuit boards). made from materials like Bakelite, Masonite, layered cardboard. and even thin wooden planks. Holes drilled into the material and then flat brass "wires" riveted or bolted onto the board. Connections to components were usually. made by pressing the end of the brass trace onto a hollow rivet. and the component's leads pressed into the open end of the rivet. small nuts and bolts used in place of the rivets. These types of PCBs used in early tube style radios and gramophones in the 1920's.
By the 50's and early 60's laminates using different types of resins . mixed with all sorts of different materials. were introduced but the pcbs were still single sided. The circuitry was on one side of the board and the components on the other. The advantages of the PCB over bulky wiring. and cables made it a prime choice for new products brought into the market place. But the largest influence on the evolution of the printed wiring board. came from the government agencies responsible for new weapons and communication equipment. Wire ended components were being used in some of the applications. In the beginning the leads of the components. held in place on the board by using small nickel plates welded to the lead after it placed through the hole.
processes developed that would plate copper onto the walls of the drilled holes. That allowed circuits on both sides of the board to connected . Copper had replaced brass. as the metal of choice because of its ability to carry electrical current, low cost. and ease of manufacturing. In 1956 the US Patent Office issued a patent. for the "Process of Assembling Electrical Circuits". that was sought by a small group of scientists represented by the US Army. The patented process involved using a base material like melamine to. which a layer of copper foil had laminated. A drawing made of the wiring pattern and then photographed onto a zinc plate. The plate used to create a printing plate for an offset printing press. An acid resistant ink printed onto the copper foil side of the board. that etched to remove the exposed copper leaving the "printed wire" behind.
Other methods like using stencils, screening, hand printing. and rubber stamping were also proposed to deposit the ink pattern. Holes were then punched in patterns using dies to. match the position of the component wire leads or terminals. The leads inserted through the non-plated holes in the laminate material. and then the card dipped or floated on a bath of molten solder. The solder would coat the traces as well as connecting the leads of the components to the traces.
They also used tinned eyelets, rivets. and washers to attach various types of components to the board. Their patent even has a drawing showing two single sided boards stacked on top of. each other with a standoff holding them apart. There are components on the top side of each board and one component shown with. its leads extending through the top board into holes on the bottom board. connecting them together, a rough attempt at making the first multi-layer.
Much has changed since then. With the advent of plating processes that allowed hole walls to plated. came the first double sided boards. Surface mount pad technology, something we associate . with the 1980's was actually explored twenty years earlier. in the 60's. Solder masks were being applied as early as 1950 to help reduce the corrosion. that was occurring to traces and components. Epoxy compounds spread over the surface of the assembled boards like. what we know now as conformal coating. the inks were being screen printed onto the panels before assembling the boards. Areas that meant to soldered blocked out on the screens.
It helped keep the boards clean, reduce corrosion and oxidation. but the tin/lead coating used to coat the traces would melt. during the soldering process causing the mask to flake off. Because of the wide spacing of the traces it seen more as a cosmetic problem than a functional issue. By the 1970's circuitry and spacing was becoming smaller and smaller and the tin/lead coating. that was still used to coat the traces on the boards began fusing traces together. during the soldering process.
Hot air soldering methods began in the late 70's allowing the tin/lead to stripped. after etching eliminating the problem. Solder mask could then applied over the bare copper circuits. and leave only the plated holes and pads free to coated with solder. As holes continued to get smaller. and trace work became more packed solder mask bleed. and registration issues brought on dry film masks. They used in the US. while the first photo-imageable masks were being developed in Europe and Japan. In Europe the solvent based "Probimer" ink applied by curtain coating the entire panel.
The Japanese centered on screen processes using various aqueous developed LPIs. All three of these mask types used standard UV exposure units. and photo tools to define the pattern on the panel. By the mid 1990's the aqueous developed liquid photo-imageable masks. were dominating the industry with specialized equipment designed for their application.
The increased complexities and densities. that were driving the evolution of solder mask were also forcing. the development of layers of copper traces laminated between layers of dielectric materials. 1961 marked the first use of multi-layer pcbs in the United States. The development of the transistor and the miniaturization of other components drew more. and more manufacturers into using printed circuit boards. for an increasing number of consumer products. Aerospace equipment. flight instrumentation, computers and telecommunication products. as well as defense systems and weapons all began to. take advantage of the space saving that a multi-layer circuit board provided. Surface mount devices were being designed that wers wilenth the size. and weight of the comparable through hole components.
Followed by the invention of integrated circuits the circuit board. has continued to shrink in almost every way. Rigid boards and cable applications have given way to flexible circuit boards. or combinations of rigid and flexible PCBs. These and other advancements will keep the manufacture of printed circuit boards. a dynamic field for many years.