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In quite a lot of PCBs that I have seen, they often have these small metal bars going across from one point to another.

Here is an image of what I am talking about. In this example it is a charger: Charger with points

J2 and J3 are what I am talking about

For reference, here is the other side of the PCB:Other side of the PCB

So,

  1. What are these called?

  2. What is the point of these? Why not just use tracers in the board or a wire?

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4  
The PCB has copper on only one side of the board, so it is single sided. Not double sided. –  jippie Aug 25 at 20:43
    
Oh, I thought it was double sided if there were components on both sides! * Facepalm * - I'll edit the question now! –  George H Aug 25 at 20:45
    
You make me curious about multi-layer boards ;o) –  jippie Aug 25 at 20:47
    
Fascinating answers. Thank you. –  Phil Aug 26 at 11:55

3 Answers 3

up vote 34 down vote accepted

These are called "jumpers" or "jumper wires" and simply connect two parts of the PCB together. They are common for single-sided PCBs to make a connection that may not be routable. The alternative would be to have a double-sided PCB, but that would be more expensive.

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15  
... Which is what the "J" stands for. –  keshlam Aug 26 at 2:38

In addition to the direct answer already given by others, I want to point out that this is a different circuit board construction optimized for high volume. What you probably think of as normal PCBs are at least two layer, and the copper is etched and plated with the holes drilled and the outside shape of the board routed.

These boards are different, in that the whole board is punched with a custom die for the purpose. The single punching operation cuts the outside edges and any holes in the interior. The incremental cost per board is less than separately drilling and routing, but you have to pay for the custom die. That is why this technique is only used in high volumes where the cost of the die can be amortized over many boards, thereby being a small cost per board.

One telltale sign of such boards is the golden color of the material, which is phenolic instead of fiberglass because fiberglass doesn't punch well. Note also the lack of plating inside the holes. This process doesn't allow for plating the holes without extra steps that would negate the cost savings. Since everything can't be routed in a single plane, somehow connections need to be made out of the plane. Because the holes aren't plated, there can't be vias, and there is nothing on the other side for the vias to connect to if they did exist. The answer is to insert short wires on the top side that are jumpers. Note the designators start with "J", which stands for "jumper".

This is the cheapest way to make PC boards in high volume, but there are problems. The reliability is not so good, especially in a high vibration environment. Thru hole leads are only held by the solder meniscus on the other side of the board, not inside the hole and on both sides like a plated board. This means the joints are weak, especially when subjected to pushing forces from the top of a thru hole component.

Despite that, I've seen this type of board used in the dashboard of a car. A while ago I had a Dodge Neon, and the speedometer started acting flaky after about 100,000 miles. There were thru hole vertical mount connectors on the board, and over time the solder meniscus around the pins cracked so that contact was intermittent. You had to look with a jeweler's loupe or microscope to see it, but the effect was real. I took the board out, reflowed all the solder joints and added more solder, after which the dashboard worked again. I don't know how much Chrysler saved per board, but it couldn't have been all that much, probably less than a dollar. However, they were willing to give up reliability in return for that dollar.

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Wait... Small technicality... Do you mean Chrysler in the case of the Dodge Neon circuit board? Or was it some sort of generic component built by GM and then purchased by Chrysler? –  ParrotMac Aug 26 at 1:39
    
Thank you for this wonderful comment, Olin. I plan on outsourcing a PCB for a custom keyboard sometime soon, and I've learned more from this single post than from hours of random Googling the subject. –  dotancohen Aug 26 at 6:21
    
Thank you for the information! I too have learned very much from it! –  vlad b. Aug 26 at 10:56
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@Parrot: Oops, no, I should have said Chrysler. Fixed. –  Olin Lathrop Aug 26 at 11:33
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Solder fractures are a bitch - I had a similar one with a car stereo front panel, there were 4x large SMT resistors which had fractured their solder joints, basically these resistors were getting warm & expanding, then cooling off & shrinking when the power was off, and this cycling fractured the solder enough to kill the display backlight. To the naked eye it was almost invisible, but a quick dab with the iron fixed it. –  John U Aug 26 at 11:47

"Link" or "jumper (wire)". The PCB design engineer wasn't able to route the copper traces on the rear side to connect to all required places on the board so (s)he put in a couple "bridge wires".

Sometimes they double as an easily accessible test points or to set a specific circuit configuration (eg. a different voltage or current).

Why not "wire"? Well these are regular wires. Are you thinking about insulated wires? Do realize that insulated wires need to be prepared in exactly the right size, which is more effort and more expensive.

In this specific case they are implemented as a bare piece of wire, but sometimes 0 (zero) ohm resistors are used. These look like a regular "pin through hole" resistor, but have only one single black band. The advantage of 0 ohm resistors over wires is that they are easier to handle for pick-and-place machines.

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Another advantage of zero ohm resistors is that they don't look like jumpers. I've had a customer who included in the contract a ban of jumper wires. So our solution was zero ohm resistors :) –  slebetman Aug 27 at 5:37
    
The machines to auto-insert resistor-shaped components is different from that required to cut and auto-insert bulk wire, but I don't think they're called "pick-and-place", are they? I thought that term was used for machines that lifted things out of a package, as opposed to those which cut them from a reel. As for the customer's ban, it's entirely possible that the customer had access to equipment which was designed to handle reels of resistor-shaped components, but couldn't handle bare wires without something "blobby" to grab onto. –  supercat Aug 27 at 15:06

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