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I'm fairly new to electronics, and I was recently looking at some LEDs listed for sale online. Some LEDs were mounted on "star pcb":

LEDs on star PCBs

From the picture, it looks like there are 6 connections. I'm pretty sure the LEDs just have 2 (an anode and a cathode).

Can someone explain the purpose of the other connections, or what the star shape pcb is used for? I tried googling, but all I could seem to find were more star pcb items for sale.

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2  
What is a "star PCB" - have you got a link or a picture? –  Andy aka Aug 11 at 18:52
    
@Andyaka the link provided by DoxyLover below is very similar. –  snapfractalpop Aug 11 at 19:28

3 Answers 3

up vote 9 down vote accepted

I assume you are looking at a unit such as this: Digikey link. The "circuit board" is actually a heat-sink heat-spreader to draw away the heat from the LED module in the middle. As you can see in the photograph, half of the pads are labeled + (anode) and the other half are labeled - (cathode). So yes, there are actually only two terminals.

[Edit: changed "heat-sink" to "heat-spreader", acknowledging Conner Wolf's correction.]

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As a heat sink, should that "pcb" be mounted with a thermal compound onto whatever it's attached to? Or can it simply be glued to something? –  snapfractalpop Aug 11 at 20:13
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@snapfractalpop Thermal compound will work much better. You'll severely limit the effectiveness of the heat sink if you put a layer of glue in between. The LEDs that are usually on these need to dissipate a lot of heat if you want them to last. –  horta Aug 11 at 20:18
    
I was looking into making a DIY grow light for my pepper plants (my window gets very little light). I guess I will have to carefully plan my design so that the heat doesn't mess with the plants. –  snapfractalpop Aug 11 at 20:21
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Technically, in this application, the PCB is acting as a heat spreader, not a heat sink. Basically, it provides a larger surface area for the thermal connection to the actual heat-sink, which makes the mechanical design for the overall system considerably easier. It does not actually dissipate any meaningful amount of power in the star PCB itself, so it can't really be called a heatsink. –  Connor Wolf Aug 12 at 11:23
    
@ConnorWolf - Thanks for the comment. I agree and have edited my answer. –  DoxyLover Aug 12 at 18:49

These are not usually ordinary PCBs, but rather metal-core PCBs. Rather than a fiberglass-epoxy composite material like FR-4 as the core of the PCB, the core is made of aluminium or sometimes copper. This has the advantage of far better heat dissipation which is a concern for high-power LEDs.

MCPCB cross-sectional geometry

Cree provides some detailed information in their application note: Optimizing PCB Thermal Performance for Cree ® XLamp ® LEDs. A typical high-power LED looks like this:

Cree XLamp XP LED package

Making good thermal contact to the thermal pad is tricky because it's so small. The star PCBs make it a lot easier.

As for why these star PCBs have as many as six pads when an LED has only an anode and cathode, I suspect it's an issue of cost. There are RGB LED modules which may have three LEDs on one die, and thus six connections are required. It is cheaper to manufacture just one PCB that can work for many kinds of LED package.

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I suspect the layout is also an attempt to style-in some product differentiation in a competitive market -- high power LEDs are probably more likely than than most components to need end-user appeal (look at all the cheap torches with CREE all over them) and a distinctive design is more recognisable. The notches also allow screws for attaching to a heatsink or other mounting. –  Chris H Aug 12 at 14:31

This star pcb board is also used for RGB LEDs so it gives the flexibility of breaking out all of the anodes and cathodes of each individual LED. Since one board will work for both RGB's and normal single color LEDs it's cheaper to just design one board that works for them all.

Here's an RGB LED with the star pcb.

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