Making double-sided PCBs
Making double-sided PCB’s is almost the same as making single-sided ones, with the obvious difference of course; that you need to transfer artwork and etch both sides.
For information and techniques on making single-sided PCB’s have a read here.
For double-sided PCB’s the hardest part is transferring the artwork and making sure that both sides align correctly, and stay that why during UV exposure, so that when you drill component holes through a pad on the top side, the hole is in the centre of the equivalent pad on the bottom side; incorrect artwork alignment is the most common reason for manufacturer failure of double-sided PCBs.
However, before you start etching your board, you need to follow some specific rules when designing your board
Commercial double-sided PCBs.
Commercial double-sided PCBs are manufactured with plated thru-holes. These are tiny copper tubes that are forced into each component hole and connect the pads and tracks on each side of the board. This is possible for the hobbyist to accomplish using special thru-hole rivet kits, but it’s very expensive and time consuming and it’s usually not worth the effort.
Let’s look at why thru-holes are important and how the lack of them affects boards made by hobbyists.
For information and techniques on making single-sided PCB’s have a read here.
For double-sided PCB’s the hardest part is transferring the artwork and making sure that both sides align correctly, and stay that why during UV exposure, so that when you drill component holes through a pad on the top side, the hole is in the centre of the equivalent pad on the bottom side; incorrect artwork alignment is the most common reason for manufacturer failure of double-sided PCBs.
However, before you start etching your board, you need to follow some specific rules when designing your board
Commercial double-sided PCBs.
Commercial double-sided PCBs are manufactured with plated thru-holes. These are tiny copper tubes that are forced into each component hole and connect the pads and tracks on each side of the board. This is possible for the hobbyist to accomplish using special thru-hole rivet kits, but it’s very expensive and time consuming and it’s usually not worth the effort.
Let’s look at why thru-holes are important and how the lack of them affects boards made by hobbyists.
Figure 1. Double-sided PCB with a plated thru-hole
Figure 1 shows a cut through section of a double-sided PCB with a thin copper layer on either side of the board. Where a component lead hole is required, a hole is drilled and a thru-hole rivet inserted, the component lead is then inserted and soldered, usually from the bottom side. As the solder melts and flows, it’s drawn into and along the inside of the rivet and flows out of the other end. This bonds the component lead to the copper layer on either side of the board. Electrical current can now flow from one copper layer to the other via the component lead.
However, our hobby boards don’t have these thru-platted holes so the component lead must be manually soldered on both sides of the board. This is fine for resistors and some capacitors, but for most IC sockets, switches and some other components, once the component is mounted into its holes, there is no access to the top-side copper to manually solder the connection.
Vias can also be a problem, especially if they are located under a component. A via is a way of allowing a signal from one side of the board, to be routed to the other side; the signal passes through the board and out the other side. Usually the via is just another thru-platted hole; often with a much smaller diameter that a hole designed for a component lead and this process can be duplicated on hobby boards by simply inserting a thin piece or wire (or an old component lead clipping) into the hole, and soldering both sides… assuming you can reach the pad on both sides that is, but being carful with the board assembly sequence can overcome a lot of these issues.
However, there are some situations that can’t easily be rectified and these need to be dealt with during the board design stage.
However, our hobby boards don’t have these thru-platted holes so the component lead must be manually soldered on both sides of the board. This is fine for resistors and some capacitors, but for most IC sockets, switches and some other components, once the component is mounted into its holes, there is no access to the top-side copper to manually solder the connection.
Vias can also be a problem, especially if they are located under a component. A via is a way of allowing a signal from one side of the board, to be routed to the other side; the signal passes through the board and out the other side. Usually the via is just another thru-platted hole; often with a much smaller diameter that a hole designed for a component lead and this process can be duplicated on hobby boards by simply inserting a thin piece or wire (or an old component lead clipping) into the hole, and soldering both sides… assuming you can reach the pad on both sides that is, but being carful with the board assembly sequence can overcome a lot of these issues.
However, there are some situations that can’t easily be rectified and these need to be dealt with during the board design stage.
Design requirements for hobby boards
Figure 2 - Pin to pin connections
Figure 2 shows a typical set of inter-connections between two IC’s using a double-sided PCB. The white tracks are the top-side, the dark grey tracks are on the underside of the PCB.
You can see that tracks are routed from pins on IC1 to IC2, on both sides of the PCB. The underside tracks are no problem to solder. The ICs (or IC sockets) are pushed through from the top side and are the pins are simply soldered on the underside. However, there are three topside tracks that also connect IC pins, and these cannot be reached once an IC socket has been mounted.
Actually this depends on the type of IC socket you use. The more expensive turned-pin IC sockets are actually much better suited to double-sided hobby boards as the tops of the pins are not encased in as much plastic so it’s simpler to solder them.
You can see that tracks are routed from pins on IC1 to IC2, on both sides of the PCB. The underside tracks are no problem to solder. The ICs (or IC sockets) are pushed through from the top side and are the pins are simply soldered on the underside. However, there are three topside tracks that also connect IC pins, and these cannot be reached once an IC socket has been mounted.
Actually this depends on the type of IC socket you use. The more expensive turned-pin IC sockets are actually much better suited to double-sided hobby boards as the tops of the pins are not encased in as much plastic so it’s simpler to solder them.
Figure 3 - Standard IC socket
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Figure 4 - Turned-pin IC socket
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Turned-pin IC sockets
Figure 5 - Modified PCB layout
When the turned-pin IC socket is seated on the board, the metal tops of the pins just below the plastic body are still visible. The problem is that if you solder these to the top tracks, the solder doesn’t always flow all the way around the pin and can compromise the quality and stability of the joint. Also, once soldered it’s almost impossible to get the socket off the board again intact.
So, it’s usually simpler and safer to cater for these limitations and restrictions during the board design stage.
Figure 5 shows an equivalent board layout to that in figure 2, but one more suitable to hobby construction. All tracks that are connected to IC pins are only on the underside of the PCB where they can be simply reached with a soldering iron. When the track needs to travel on the top-side of the board, a via is added away from the IC so both sides can be reached for soldering.
That’s basically it for the design considerations. Following these simple rules will mean that your board is actually constructible once you’ve etched and drilled it.
So, it’s usually simpler and safer to cater for these limitations and restrictions during the board design stage.
Figure 5 shows an equivalent board layout to that in figure 2, but one more suitable to hobby construction. All tracks that are connected to IC pins are only on the underside of the PCB where they can be simply reached with a soldering iron. When the track needs to travel on the top-side of the board, a via is added away from the IC so both sides can be reached for soldering.
That’s basically it for the design considerations. Following these simple rules will mean that your board is actually constructible once you’ve etched and drilled it.
Artwork
Figure 6 - Printed artwork
Once your board has been designed, the next step is to print the artwork out. I personally use DipTrace PCB design software for my PCB artwork, and when printing for double sided boards, you have to remember to print the artwork for the top-side of the PCB mirrored. Your software may do this automatically or you may have other steps you need to follow.
You can see from figure 6 that the art-work for the bottom layer (top right of the picture) appears darker than the art-work for the top layer (bottom left of the picture). This is because you should print your art-work so that the ink side will be positioned as close to the copper layer as possible.
You can see from figure 6 that the art-work for the bottom layer (top right of the picture) appears darker than the art-work for the top layer (bottom left of the picture). This is because you should print your art-work so that the ink side will be positioned as close to the copper layer as possible.
Figure 7 - Both artwork sides fixed together
Place the bottom sheet down, ink-side face up. Next, trim the top-side artwork so that it’s slightly smaller that the bottom sheet and carefully place it over the bottom sheet making sure it’s aligned correctly.
Figure 8 - Incorrectly aligned artwork
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Figure 9 - Correctly aligned artwork
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When designing your artwork, it’s a good idea to add alignment holes to enable alignment and you can see from the above images how these aid correct alignment.
Make sure you add at least three or four alignment holes (and in this case these are going to be the actual board mounting holes for mounting in the completed project).
Next, using a piece of masking tape, stick the top transparency to the bottom along one edge.
Once secure, slip in a piece of double sided laminate with the protecting plastic removed from both sides. Now tape along the three remaining sides.
Make sure you add at least three or four alignment holes (and in this case these are going to be the actual board mounting holes for mounting in the completed project).
Next, using a piece of masking tape, stick the top transparency to the bottom along one edge.
Once secure, slip in a piece of double sided laminate with the protecting plastic removed from both sides. Now tape along the three remaining sides.
Board ready for UV exposure
Figure 10 - Board and artwork ready for UV exposure.
You can see that along the bottom edge, the masking tape is also slightly stuck to the board laminate. This helps stop the board from moving around whilst your handling it.
That’s basically it. Assuming the quality of your artwork is ok, you can now expose both sides of the board to UV light, develop and then etch.
The proof of course is in the end-result.
Once the board shown in the above pictures was developed and then etched, I drilled a couple of random component holes at different parts of the boards to test the accuracy.
That’s basically it. Assuming the quality of your artwork is ok, you can now expose both sides of the board to UV light, develop and then etch.
The proof of course is in the end-result.
Once the board shown in the above pictures was developed and then etched, I drilled a couple of random component holes at different parts of the boards to test the accuracy.
Figure 11 - Topside of board with a hole drilled.
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Figure 12 - Bottom side of same board
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As you can see from the above two images, the hole alignment is almost perfect.
(The white specks you can see on the images are actually swarf that were picked up by the board after drilling).
(The white specks you can see on the images are actually swarf that were picked up by the board after drilling).
