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At work I've inherited a multilayer PCB design that I need to send out for quote and eventual fabrication. It contains two inner layers that are labeled "AIRGAP". What is the purpose of these air gap layers?

The board stackup is as follows:
 1. Top Silkscreen
 2. Top Soldermask
 3. Top Copper
 4. Ground Layer
 5. Ground Layer Airgap
 6. VCC Layer
 7. VCC Layer Airgap
 8. Bottom Copper
 9. Bottom Solder mask

The highest voltage on the board is about 40 volts, so I wouldn't think it's a high-voltage design.

Would this be considered a four-layer board, or more? Some of the board houses we've sent it to are confused as well.

Allen Moore
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  • I have not come across this air gap before .Study your circuit diagram and see where this airgap is .The air gap could be in some low leakage stuff like picoamps .or it could besomething that needs low capacitance . remember the dielectric constant of FR4 .lAlso it could be some low loss high Q thing .Remember the dissipation factor of FR4 .Maybe it could be some drift thing .The capacitance of FR4 does have a tempco that could be significant in your circuit .If you posted the circuit then the reason for the airgap could be ascertained – Autistic Mar 10 '17 at 23:33
  • Allen, what PCB design artifacts have you inherited? (The artifacts may include but are not limited to: Gerber files, design files in the native EDA software formats, physical samples of the PCB, original designers alive.) – Nick Alexeev Mar 10 '17 at 23:37
  • @NickAlexeev Artifacts include schematic, BOM, and Gerbers--but not the native design files. The two "air gap" layers are each separate Gerber files. – Allen Moore Mar 10 '17 at 23:40
  • @Autistic: Sorry, I can't post the [proprietary] circuit. As mentioned above, this "air gap" is an entire physical layer in the board stack up. The board itself is essentially all digital (up to 16 MHz), with some DC drivers in the one to five-amp range. – Allen Moore Mar 10 '17 at 23:43
  • @Allen, what is on the Gerbers for these mysterious "Airgap" layers? "Airgap" suggests that they should be [largely] blank. – Nick Alexeev Mar 11 '17 at 00:12
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    Do the airgap gerber layers have copper? What does it connect to? Are their vias? I have not come across this term either. – user57037 Mar 11 '17 at 00:12
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    I would expect the airgap layers to show areas that will be milled out of the board. Can you look at these layers, and known copper and silkscreen layers, to see if there are any tracks or components in areas marked by tracks on the airgap layers? Can you get a board that these Gerbers represent? – Peter Bennett Mar 11 '17 at 00:16
  • On a somewhat related [to @Peter 's post] note. Does this controller board have galvanic isolation barrier going across it? If so, what is the isolation rated for? Under what contamination conditions? – Nick Alexeev Mar 11 '17 at 00:28
  • @NickAlexeev: There are no galvanic isolation barriers. – Allen Moore Mar 11 '17 at 00:56
  • Airgap could be a misnomer for simply isolation layer. I'll bet 5cents its mostly blank bar the feed throughs. – Trevor_G Mar 11 '17 at 03:18
  • Are layers 5 and 7 actual physical sides of PCB, or are they features that end up on layers 4 and 6 respectively? For instance layers 1, 2 and 3 all end up on the top of the board. Could they be clearances for vias passing through the board, brought out to a separate design layer so they are easier to inspect and modify? – Neil_UK Mar 11 '17 at 06:02
  • How do you know that highest voltage on the board will be 40V? There could be some connections that could bring some spikes. And then some spark or air gaps make sense. – Chupacabras Mar 11 '17 at 06:41
  • since it only on the power/gnd planes, is there some other thin insulation coating thus used as a capacitance layer – Tony Stewart EE75 Mar 11 '17 at 07:10

4 Answers4

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As Peter Bennett said, the air gap layer is probably a Gerber containing areas to be milled out of the layers, possibly the top and bottom prepreg, leaving the core intact. Since there are only 4 copper layers, this would likely leave open cavities on the top and bottom with copper potentially exposed on the power/ground layers.

This could be used to recess components into the PCB.

In some cases, components are completely embedded into the PCB.

I believe this process typically would have the (in this case) core run through a pick and place machine, soldered, cleaned and then laminated and the holes plated through with the top and bottom prepreg.

Here is an example of a stackup with completely embedded components from Altium:

enter image description here

Spehro Pefhany
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An air gap is for creepage an clearance for high voltages to meet regulatory. I'll bet the designers have a different depth on the PCB for the milling trace and they use that distance in the stack up to achieve a custom depth. This is probably so the depth will show up in the 3D design or for manufacturing, and a milling track could be created with a custom depth in the PCB.

So if the design is for a power supply or something with creep-age and clearance then that's what it is. If it actually is an air gap layer I'd be shocked.

Edit: One other place I have seen air gaps (which this probably is) is in rigid flat flex PCB's which have kapton inner layers and FR4 outer layers. The air gap is to promote flexibility if you have more than 2 kapton inner layers as shown in the 8 layer stackup.

enter image description here

Voltage Spike
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An air gap is a physical less than conductive distance between two sections of a electronic circuit. It is intended to enforce a non conductive section between two points using non conductive (in normal circumstances) material. This air gap is chosen based on the typical working voltage of the circuit. A mains voltage air gap will be smaller than an air gap for 1k volt or higher circuits, for example. The spacing between two multi killivolt paths will be much larger than the spacing between two bare mains voltage paths.

The typical air gap is calculated based on the conductivity of atmosphere (a mix of various gases). Of atmosphere would conduct at that voltage at a given distance, the air gap is not enough.

Passerby
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As the question asked I looked into WikiPedia and found this statement on AIR GAP:

By insulating copper wires within a chip with vacuum holes, capacitance can be minimized enabling chips to work faster or draw less power. A vacuum is believed to be the ultimate insulator for what is known as wiring capacitance, which occurs when two adjacent wires on a chip draw electrical energy from one another, generating undesirable heat and slowing the speed at which data can move through a chip. IBM estimates that this technology alone can lead to 35% higher speeds in current flow or 15% lower power consumption.

Here also is the manufacturing tech from IBM on air gaps from WikiPedia:

IBM researchers have figured out a way to manufacture these "airgaps" on a massive scale, using the self-assembly properties of certain polymers, and then combine this with regular CMOS manufacturing techniques, saving enormous resources since they don't have to retool the entire process. When making the chips the entire wafer is prepared with a polymer material that when removed at a later stage leaves trillions of holes, just 20 nanometers in diameter, evenly spaced. Even though the name suggests that the holes are filled with air, they are in fact filled with nothing, vacuum. IBM has already proven this technique in their labs, and is already deployed in their manufacturing plant in East Fishkill, New York where they have made prototype POWER6 processors using this technology. Full scale deployment is scheduled for IBM's 45 nm node in 2009 after which this technology will also be available to IBM's customers.

As an after thought air gap could refer to spark gap for when circuit reverses current when hit buy a surge of energy like a lighting or over charging your device.

Ricardo
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William Laurence Clarkson
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