Editing Printed circuit board (section)

===Cordwood construction===
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[[File:Cordwoodcircuit.agr.jpg|thumb|upright=1.5|A cordwood module]]
[[File:MK53 fuze.jpg|thumb|Cordwood construction was used in [[proximity fuze]]s.]]

Cordwood construction can save significant space and was often used with [[Through-hole technology|wire-ended components]] in applications where space was at a premium (such as [[fuze]]s, missile guidance, and telemetry systems) and in high-speed [[computer]]s, where short traces were important. In cordwood construction, axial-leaded components were mounted between two parallel planes. The name comes from the way axial-lead components (capacitors, resistors, coils, and diodes) are stacked in parallel rows and columns, like a stack of firewood. The components were either soldered together with jumper wire or they were connected to other components by thin nickel ribbon welded at right angles onto the component leads.<ref>{{cite journal |last=Wagner |first=G. Donald |title=History of Electronic Packaging at APL: From the VT Fuze to the NEAR Spacecraft |journal=Johns Hopkins APL Technical Digest |volume=20 |issue=1 |date=1999 |url=http://www.jhuapl.edu/techdigest/TD/td2001/Wagner.pdf |archive-url=https://web.archive.org/web/20170510111809/http://www.jhuapl.edu/techdigest/TD/td2001/Wagner.pdf |archive-date=May 10, 2017 }}</ref> To avoid shorting together different interconnection layers, thin insulating cards were placed between them. Perforations or holes in the cards allowed component leads to project through to the next interconnection layer. One disadvantage of this system was that special [[nickel]]-leaded components had to be used to allow reliable interconnecting welds to be made. Differential thermal expansion of the component could put pressure on the leads of the components and the PCB traces and cause mechanical damage (as was seen in several modules on the Apollo program). Additionally, components located in the interior are difficult to replace. Some versions of cordwood construction used soldered single-sided PCBs as the interconnection method (as pictured), allowing the use of normal-leaded components at the cost of being difficult to remove the boards or replace any component that is not at the edge.

Before the advent of [[integrated circuit]]s, this method allowed the highest possible component packing density; because of this, it was used by a number of computer vendors including [[Control Data Corporation]].