This version is now obsolete. The current version is located Here.

As an electronic hobbyist and inventor I often need to make printed circuit boards (PCBs) in single or small quantities. Usually these are relatively simple circuits: an MCU, some input conditioning circuitry, some output circuitry, and usually they are single sided or perhaps double sided, with just a few vias. And usually I want them right now!

Toner Transfer (TT) has become the method of choice for most hobbyists. A laser printer is used to print an image of the PCB on special “transfer paper” which is then placed on the bare copperclad board and either ironed or run through a modified laminator to transfer the image to the copper. When the PCB is etched, the toner acts as a resist, preserving the copper below it while the rest of the copper surface is etched away.

While TT works, it does have some problems. The process involves several steps, all of which are critical to success. Often there is pitting of large planes, and in many cases parts of the image don’t transfer, leaving gaps that have to be hand edited or completely redone. Trace widths under 10 mil, while achievable, are unreliable.

Since we know that laser printer toner is a great resist for making printed circuit boards, I have long wondered why we use Toner Transfer, rather than printing directly on PCBs with a laser printer. After all, if the boards produced by transferring toner first to paper and then to PCBs works well, then putting the toner directly on the PCB should be even better! And much faster!

The main arguments against the viability of the process, aside from the mechanics of getting a thick board through a paper-thin path, seem to be (a) the drum is too fragile and will quickly be damaged by the boards, and (b) the copper will dissipate the static electricity charge needed to transfer the toner to the board.

Over the last four years I have been experimenting with Direct Laser Printing (DLP), and it works! I have run hundreds of boards through my modified Lexmark E260 printer, and not only can I print on copperclad boards, I can print on aluminum sheets and other metals as well. Using Muriatic Acid/Hydrogen Peroxide etchant I can now make a PCB in under 15 minutes!

I am still experimenting with the process, but trace widths down to 4-5 mil (.004-.005) seem to be readily accomplished. Mechanical registration of the PCBs seems adequate to make simple double sided boards, though I have not made a serious effort to investigate this.

After trying several brands and models of laser printer, the printer I have settled on is the Lexmark E260. I use it because:

1 – It has an excellent Local Printer Utility that allows almost every printing parameter to be adjusted.

2 – It is readily and reasonably available on Ebay or Craig’s List. I paid $45 for each of the last two I bought on Ebay, including shipping.

3 – The drum is separate from the toner cartridge and is relatively inexpensive (<$30), so it can be replaced if it does become damaged.

4 – The printer has a manual feed slot in front so the paper path can be “flattened” with reasonable effort to pass PCBs.

5 – The charge on the drum seems to provide for essentially perfect transfer of toner to grounded metal sheets.

Step 1: Parts and Tools

The table above has a parts and tool list for this project. Most of the parts are available at McMaster-Carr, but many can also be found at local hardware stores.

Step 2: Testing the Printer

* Remove all tape and packing materials.

* Make sure there is a toner cartridge and drum present.

* Install the printer. Go to the Lexmark E260 site: Lexmark E260 Software Support. Select your operating system and your version. Click on the “Complete List of Software” link near the bottom of the page. Find and install the Universal Print Driver and the Local Printer Settings Utility (LPSU). Connect the printer and it should install and be ready for use. Note that when the printer is turned on it goes through a complex self-test. If there is an error, a series of error codes, most of which are detailed in the Service Manual, is displayed on the operating manual.

* Print something to be sure the printer is working correctly.

* Disconnect the printer, remove the power cord.

* Remove the Paper Tray.

* Remove the toner and drum. Since the drum is exposed and is light sensitive, it should be stored in a lightproof enclosure. A black plastic bag works well.

Warning: Wear safety glasses at all times! Some of these operations can result in metal or plastic pieces flying towards your eyes!

Step 3: Stripping Down the Printer

To successfully complete this project you will need to have and use the Service Manual (SM) for the printer. The Service Manual does not appear to be copyrighted, so a pdf of it is attached below. Steps that are thoroughly covered in the SM (Section 4: Repair Information, Removal) will not be detailed here. I will add (SM) to those steps covered in the Service Manual.

For the location of parts on the bottom of the Paper Platform see [Photos].

In this step you will be opening up the printer and exposing the Paper Platform for cutting. As you remove each part be sure to keep track of which screws were removed. You might want to tape the screws to the part or label them. Removal of some of the subsections below may require removal of other subsections or parts, so pay attention to the SM. To replace a subsection or part, reverse the removal steps. Using the SM as a guide do the following:

* Remove the Right Cover (SM). It is the side with the access door.

* Remove the Left Cover (SM).

* Remove the Lower Front Cover (SM).

* Open the Front Door.

* Remove the Paper Feed Guide with the 6 small rollers. It is held in place by 2 screws on each side, not by the 8 screws on top. Discard it.

* Remove the Manual Feed Paper Guide [Photo] and save it and its 2 screws. You will use it later.

* Remove the Transfer Roller (SM). Be sure to remove and save the small spring under the right bearing clip and save it. The spring on the left side is attached to the transfer roller cable and should not be removed. The left bearing can slide off the transfer roller, so remove it and put it safely aside with the spring. Note that the clips on the bearing are oriented to the outside of the shaft. Also note that the bronze bearing can slide out of the clip and get lost if you are not careful.

* Close the Front Door and remove and discard the pivoting beige Paper Holder.

* Remove the Rear Door and Cover (SM).

* Remove the Top Cover Assembly (SM).

* Remove the Rear Exit Guide Assembly (SM) – Note that the longer screws are at the top.

* Remove the Fuser (SM). The only part we need from the Fuser assembly is the connector and cable to the Exit Sensor. Cut the cable where it enters the Fuser, save the cable and discard the Fuser.

* Locate the Thermistor cable on the Controller Board at J-12. Cut the cable leaving about 1.5 inches of wire. Remove the connector and solder a 2.7K resistor across the two wires. This resistor makes the printer think it has a Fuser heated to 200 C still attached. Without this resistor the printer will error out when booting. The resistor can be covered with tape or heat shrink. Replace the connector with its resistor at J-12. [Photo]

* Remove the top rear 2 screws on the Rear Exit Guide assembly (save them) and remove the top section with the Narrow Media Sensor. Take the bottom section and remove the rollers and the reversal solenoid. Remove all the gears and small rollers. You will not be using the gears, rollers, or solenoid. Remove the Narrow Media Sensor and its flag from the top piece. Remove the piece of tape with its attached "bristles" and remove the 4 black plastic holders. You should now just be left with two bare green plastic pieces [Photo]. Replace the top section on the large lower section and screw them together with the two screws you saved [Photos].

* Replace the stripped down Rear Exit Guide assembly using only the 4 upper screws (SM). Be sure to put the longer ones in the top holes.

* Replace the top cover Assembly(SM).

* Turn the printer upside down.

* Remove the LVPS/HVPS (power supply) (SM). Remove and discard the thick 2 wire cable to the Fuser power supply. When removing the power supply note that one screw is different than the other 3 – that one screws into the plastic. Make special note of where the power supply tabs go under that screw. As you are removing the power supply, note which cable goes where and their orientation. Remove the Transfer Roller cable connector first using long nose pliers to pull it sideways out of its slot. Note its orientation. Press the connector tab to remove the two and three pin connectors. The multipin connector just pulls out.

* Remove the 3 screws holding the power supply shield and remove the shield.

* Remove the Duplex Assembly (SM). Discard it but save one of the large mounting screws to attach the new Exit Sensor.

* Remove the Main Motor Gear Drive Assembly (SM). Be careful detaching the motor cable. Since removing the door link (fuser link) will allow the door to "hang" open, disconnect the Operator Panel cable connector (J5) to avoid tension on the connector. You can also remove the door at this time to provide easier access to the Paper Platform.

* Remove the Media Feed Clutch (SM) and discard it.

* Remove the Media ACM ASM Feeder Mechanism (SM) [Photo]. All of these parts can be discarded, so it does not matter if you damage them in removal.

* Remove and discard the pivoting plastic piece and spring that sits just behind the paper drive shaft and rollers [Photo 02].

* Remove the Toner Patch Sensor [Photo]. It is held in place by 2 screws in the bottom of the paper platform, near the Media Feed Mechanism. Also remove the spring loaded sliding plastic window located just behind the Toner Patch Sensor These parts, too, can be discarded.

* Remove the screws holding the Paper In Sensor (PIS) and the Duplex Sensor in place [Photo 02]. Each is held by one screw. Remove the wires to these sensors from the wire guides. Cut the wires to the Duplex Sensor close to the connector (J27). Be sure you are cutting the correct wires (Red/Black/White). Save this sensor - we will use it as the new Exit Sensor.

* Remove the wires going to the Manual Clutch Solenoid from the same wire guide and weave it into the wire guide for the Main Motor cable [Photo].

* Remove the screw holding the Manual Feed Paper Sensor (MFPS) in place [Photo 02]. Save the screw and move the sensor temporarily out of the way so it won't be damaged when you cut the front of the paper platform around the rubber rollers.

E260 Service Manual.pdf
Download

Step 4: Cutting the Paper Platform

We are now going to cut a piece out of the Paper Platform. This is necessary to allow the carrier and pcb to easily pass through the printer without hitting the bottom of the toner cartridge. I used a Dremel [Photo] with a router bit, but you can also use a small saw. The most important precautions in cutting this platform are protecting the laser openings and removing all the plastic chips!

The openings to the laser mechanism must be covered. I used strips of two inch painter’s tape to cover all the openings. I also covered any areas where chips could lodge and cause trouble later on while printing [Photo].

We are going to remove an approximately 7.375 inch wide piece of the Paper Platform extending from just inside the Transfer Roller gutter to the edge of the Input Rollers. The right and left guides are the first and last "ridges" in the Paper Platform. The yellow lines in the photo outlines the area to be removed [Photos].

It is easiest to cut the left and right sides from the top, following the ridges in the platform. The front cut is also done from the top. Then turn the printer over and make the back cut, being sure the back cut includes the front edge of the Transfer Roller gutter [Photos].

In making the front cuts around the rollers, cut away the plastic between both sets of outer rollers so that these rollers can be moved close together. The central portion of the front should be cut so that any remaining plastic does not stick out past the rubber rollers. You can trim the edges back with a small side-cutter later, if necessary. About .75 inches of plastic past the outermost rollers on each side should also be removed to allow room for the springs that will hold the new top roller you will be adding [Photo]. Be careful while grinding around the rollers - you don't want to damage them!

Once the cut is done remove the section of plastic and smooth all the burrs. If you used the Dremel there will be pieces of melted plastic along the edges of the cut that can easily be removed with a screwdriver or your fingers. If necessary, use a file or sandpaper to smooth the edges.

Then clean up all the chips! Use a vacuum to suck up all the chips, then canned air to clean anything you missed. It is vital to prevent chips getting into the laser mechanism. It is also vital to keep chips from scratching the drum, so clean, blow and vacuum! Do not remove the protective tape from over the laser openings at this time.

Step 5: PCB Feed Rollers Assembly

While the original Manual Paper Feed Assembly in the E260 can successfully feed PCBs into the printer, it was designed to move paper, not PCBs and will eventually start slipping. In this step we will be converting the Manual Feed Input Rollers into a robust mechanism by adding a spring loaded set of passive aluminum rollers above the rubber drive rollers. See the photos and description below [Photos].

Since the carrier we will be using is 6 inches wide we need to first move the two outer rubber rollers on each side of the shaft till they are touching and located with the outer edges 6 inches from the other pair. The rubber rollers are held tightly to the shaft by friction, but they will move with finger pressure and some "wiggle". If you did not cut enough plastic to allow them to move, do so now. The correct position is between the second and third "ridges" from the outside.

To add the new rollers cut a 10.75 inch piece of 4mm stainless steel shaft. This shaft will fit into the groove located directly over the 6 mm input shaft. Slide on a 4 mm shaft collar, a 4mm x 10 mm diameter x 20 mm long aluminum spacer, followed by two more shaft collars, a second spacer and a fourth shaft collar. Position the rollers so they are NOT over the rubber rollers while the springs are added. You can tape the shaft in position temporarily.

Cut 2 pieces of extension spring, each about 1.25 inches long [Photo 16]. Use a wire cutter to gently separate the last two turns on each end from the body of the spring, creating “eyelets” to hook the ends of the spring together in a loop around the 2 shafts.

There are two ways to proceed. The simplest is to just loop each spring around both shafts just outside the rubber rollers, one on each side. Use a piece of 18 gauge solid wire to pull the two ends together and twist the wire to hold the spring in a loop. Position the spring with the joined ends at the bottom of the loop under the larger shaft. The problem with this method is that the springs will rub on the steel shaft as it turns. While this isn’t a problem in the short term it might eventually damage the shaft.

A better solution is to use a piece of brass tubing to protect the shaft. For each side cut a .5 inch piece of .253 ID brass tubing [Photo 13]. File the ends and deburr, then slit each piece longitudinally with a small tinsnip and remove a wide enough section so that the tubing will snap on the 6 mm input shaft [Photo]. Take a 2.5 inch piece of bare 18 gauge solid wire and solder it across the bottom of the tubing [Photo]. Slide one eyelet of the spring over one end of the wire, then bend that leg down. Bend the other leg of the wire up. Snap the tubing over the bottom of the shaft with the spring side towards the front of the printer. Pull the other spring eyelet over the upper shaft and onto the wire on the other side. Now pull this wire down, under the shaft while holding onto the wire that is already down. This pulls the two ends of the spring together at the bottom of the brass tubing. Turn the printer over and twist the wires together to hold the spring ends in place and cut off the excess wire. The photos show how to do this [Photos]. For clarity the printer shown has the entire center of the paper platform removed so you can see the rollers in their entirety.

Finally, position the aluminum rollers over the outer pairs of rubber ones with the inner edges lined up and tighten the shaft collars to hold them in place. Since the upper shaft does not rotate, the shaft collars should be loose enough to allow easy rotation of the rollers [Photo].

Step 6: Replacing the Sensors

The E260 has 3 sensors that are used to control the path through the printer. The first, the Manual Feed Paper Sensor (MFPS) detects the presence of an object at the Manual Feed Rollers. When it gets hit, the input rollers run for a second or so to pull the PCB into the rollers. After “Print” is clicked on the computer, the second sensor, the Paper In Sensor (PIS), goes low when the PCB reaches it. This sensor starts the actual print cycle. The final sensor, the Exit Sensor (ES), goes low when the PCB exits the printer. If these sensors are not hit at the correct times the printer will “jam” and flash an error light. In this step we will keep the original MFPS, remove and replace the PIS, and make and mount a new ES [Photo].

* If you removed the MFPS while cutting the paper Platform put it back in its original location using the original screw [Photo].

* The original Exit Sensor was located deep within the Fuser Assembly and is quite flimsy. We will be making a new one. Cut a 1"x2" piece of .016 aluminum sheet to use as the Exit Sensor mount [Photo]. We will use the original Duplex Sensor rather than the Exit sensor because it is much more robust and fits perfectly. Use 4-40 screws and nuts to mount the Duplex Sensor, using one nut as a spacer, making sure to mount it with the wires facing the large hole as shown [Photos].

* Cut the wires to the Duplex Sensor near the connector, then solder then to the wires from the Exit Sensor connector that you saved when you removed the Fuser, white to white, black to black, red to red. Use heat shrink tubing or electrical tape to insulate the connections.

* Mount the new Exit Sensor to the centermost (left) Duplex Assembly mounting boss located at the rear edge of the paper platform, using the screw you saved when you removed the Duplex Assembly [Photo]. Make sure it is oriented properly. Lace the cable into the cable channel with the Main Motor wires, then bring it through the hole near the corner of the blower. Plug the connector into the Exit Sensor receptacle (J-14) on the Controller Board.

* Replace the power supply shield. Each of the 3 screws has a pin near it to correctly position the shield. See the photo in the SM Removing the Duplex Assembly section.

* Replace the LVPS/HVPS (power supply) (SM). Replace the 3 internal connectors first, being careful to replace the cables in the correct connectors and in proper orientation. Replace the Transfer Roller connector using long nose pliers. See the photos and directions in the SM. Make sure the wires to the Paper In Sensor (PIS) do not get trapped under the power supply. The PIS should be hanging free by its cable. The power supply should slip easily into place - it should not need to be forced.

* Replace the Right Rear Foot by its 2 screws.

* Replace the Main Motor Gear Drive Assembly (SM). The large gear with the large helical spring can be removed and discarded. It was used to drive the fuser and is not needed. Be careful to attach the motor cable. The Fuser Link goes through the slot in the front of the printer. It will later be attached to the door. You may have to "wiggle" it a bit to get the gears to mesh, but the assembly should slip easily into place. Do not force it!

We will now make the new mounting plate and platform for the Paper In Sensor (PIS). See the drawings and photos for details [Photos]. Cut a 2 inch piece of 1x1x.062 aluminum angle. Drill two .125 inch holes along the bottom edge of one side, .125 inches from the edge and .750 inches apart. These will be used to mount the PIS [Photos].

Drill two .125 inch holes on the other leg of the angle, .5 inches from the edge and 1.5 inches apart. These will be used to mount this holder to the main support. Use a file to smooth the edges and the holes, then mount the PIS with the wires facing into the corner of the angle. Use 4-40 hardware with 2 nuts as spacers below the PIS.

Cut an 8.875 inch piece of 1x1x.062 angle. Drill two .125 inch holes centered on one leg of the angle, .375 inches from the edge and 1.5 inches apart. These holes mate to the mounting holes in the PIS holder. When you screw these two parts together with 4-40 hardware you should find that the PIS pivots smoothly, just clearing the top edge of the main support. You can open up the holes a bit if you need some adjustment room. Drill a .172 (11/64) hole as shown on the right side of this leg, 1.5 inches from the end and .5 inches from the edge. This will be used to hold a ground wire [Photos].

Now drill two more .172 (11/64) holes in the other leg of the main support angle, .3125 from each end and .5 inches from the edge. These will be used to mount the support and platform on the printer using 6-32 hardware.

Turn the printer upside down. Place the PIS assembly in position as shown in the photos. It should fit snugly against the bottom of the Paper Platform in the "channels" in the plastic. Make sure it is oriented properly with the PIS towards the rear of the printer. Be sure the laser openings are still covered by protective tape and drill two .172 holes through the Paper Platform using the PIS Assembly as a guide. Clean up the chips, remove the PIS Assembly and put it aside.

We will now make the new front platform for the PIS Assembly. Cut a 8.875" x 2" piece of the 6 inch wide x .016 aluminum. Following the drawing [Photo], lay out and cut as shown using tinsnips or a large pair of scissors. Drill two .172 inch holes where indicated [Photo]. When fully assembled, the PIS lever should move freely [Photo]. See the drawings and photos for details.

The front of the platform should be bent down at about a 45 degree angle to prevent the PCB carrier from sliding under the front edge of the platform when printing. Carefully orient the platform and bend the front down. This can easily be done by letting it hang over the edge of a block of wood and gently tapping it with a hammer or second block of wood.

Use 6-32 hardware to attach an 8" length of ground wire to the assembly. Mount the assembly to the printer using 6-32 hardware being sure to put the screw heads on the top of the platform to allow clearance when the Toner Cartridge is inserted. The Platform goes between the Main PIS Bracket and the plastic Paper Platform

Route the ground wire through the large opening on the right side of the printer as shown in the photo, and secure the ground wire to the metal printer case using a salvaged threaded screw in the hole as shown [Photo]. The screw will cut its own thread as you turn it.

The photos shows the completed sensor platform. [Photos]

Do a final cleanup of plastic chips, then remove the protective covering from over the laser openings.

Step 7: Replace Some Parts

* Replace the Transfer Roller.

* Replace the door. Be sure to reconnect the Fuser Link and the electrical connector (J-5)

* Replace the Manual Feed Paper Guide (2 screws).

* Replace the Front Door Cover.

* Replace the Left Side Door.

* Replace the Right Side Door.

* Replace the Paper Tray.

* Replace the Rear Door.

The Rear Door Cover is designed to swing open to guide media that is too stiff to bend well, out the back of the printer. For our purposes we will leave it closed, but we need to make a cutout to allow the PCB on its carrier to pass [Photos]. Using a Dremel or a saw, cut a rectangle from the bottom of the cover using the horizontal "shelf" [Photo] on the inside as a guide. Smooth the edges [Photo] but do not replace the Rear Door Cover until you have adjusted the Exit Sensor in the next Step.

Step 8: Adjusting the Exit Sensor

Before using the printer, the Exit Sensor has to be adjusted to the correct height. Make a 6x12 carrier by cutting a 12' piece of the 6 inch wide x .016 thick aluminum sheet, then beveling all 4 corners and filing all edges smooth. Place a piece of vinyl electrical tape over the leading edge of the carrier, folding it so half is on each side of the carrier. Cut the tape around the beveled corners. This protects the printer drum from getting gouged as the carrier enters [Photos].

Open a word processor to a blank page, select the correct printer, and using the directions in the next Step, run the bare carrier through the printer. Adjust the Exit Sensor by bending it up or down so that the carrier hits it about 2/3 of the way up the lever arm. Repeat as necessary until the carrier activates the Exit Sensor smoothly.

Replace the Rear Door Cover [Photo].

You have now successfully modified your Lexmark E260 for Direct Laser Printing of toner resist on PCBs. It's Time to make a print!

Step 9: Making a Print

Replace the Toner Cartridge and Drum. I suggest that you get a brand new Toner Cartridge and Drum for PCB printing. This will give you maximum toner density on your PCBs and should last a very long time. You might want to keep a used spare to experiment with and keep the new one only for prints that you actually want to etch.

You should have a carrier prepared from the previous Step. Using a 6" carrier the maximum PCB width is about 5 inches, but I usually use 4 inch wide PCBs. You can however, run 6 inch wide PCBs through the printer by not using a carrier.

It is important to note that the PCBs you run through the printer must be FLAT! Significant warping of the board, especially in thicker boards, will result in bare areas. Thinner boards are able to be flattened as they pass under the Drum, but .062 boards must be flat to work reliably. It is also a good idea to lightly sand the edges of the boards to eliminate sharp edges that might damage the Drum.

The surface of the PCB must be grounded for the process to work! The carrier is grounded via both the rollers and the metal paper platform. To ground the surface of the PCB use a small length of .25 inch wide copper foil tape [Photo]. Make sure that the tape you use has conductive adhesive!

Place the PCB centered left to right on the carrier and about 1 inch from the leading edge. Put a piece of the copper foil tape on the leading edge of the PCB, connecting the PCB to the carrier. Then place a length of .75 inch wide electrical tape over the leading edge of the PCB, covering the copper tape, and securing the pcb to the carrier. Lightly clean the surface of the pcb using a Scotch Brite or other pad, then wipe the PCB with acetone and allow it to dry [Photos]. We are now ready to print!

The Local Printer Settings Utility allows you to change most of the settings of the printer. I have not tried all the various combinations of settings, but I usually use Paper Source=Manual Paper, 1200 x 1200 dpi, Toner Density=10. After making changes you must click on Action and Apply Settings to make the changes permanent.

Set the paper guide to the width of the carrier. Insert the carrier and slide it towards the rollers. The rollers will “grab” the carrier and pull it in. If it doesn’t, simply remove the carrier and reinsert it. You can also gently push the carrier in as the rollers grab. Just be sure you don’t push the carrier in to the point where it hits the PIS as this will cause an error and you will have to restart the printer.

Once the carrier is in the printer, select the correct printer and click on “print”. The printer will pull the carrier in and print the image on the PCB. As the print emerges from the rear of the printer you can either slide it all the way out or leave it suspended in the exit opening. If you leave it, an error light on the Operator Panel of the printer will light. This occurs because the carrier is still hitting the Exit Sensor when the printing ends. Remove the print, then press first the Cancel button (X), and then the Continue button (>) to extinguish the error.

Above are some of the prints print I have made using this printer. You can see in the close up how dense the solid areas are and how solid the 10 mil traces are.

When it leaves the printer the toner is stuck tightly to the PCB by static electricity. You can tap it, blow on it and even drop it on the floor and it will stay put. However, all it takes is a light rub of your finger to wipe the toner off! To use toner as a resist in an etching bath, the toner needs to first be "fixed" to the copper.

Jim Hutchinson, a fellow DLP enthusiast, discovered that the easiest way to fix the toner is by immersing it in acetone vapor. Place a few tablespoons of acetone in a container. Suspend the PCB above the liquid (use some spacers), cover the container and let it sit for 2-3 minutes. Then remove the PCB and let it "air out" for a minute or so. The toner will now be hardened and bonded to the copper. You are now ready to etch and use your new PCB.

Step 10: Conclusion

DLP works, is reliable, and is very fast. Using Muriatic Acid/Hydrogen Peroxide high speed etchant I can go from design to ready-to-use PCB in less than 15 minutes!

Double sided PCBs are next. Using just the Manual Feed Paper Guide, the E260 can achieve registration that should allow non-critical double sided boards to be made. By using a more rigid alignment method and an improved, possibly optical, Paper In Sensor, it should be possible to make double sided PCBs of some complexity. Now that the basic DLP process is stable and repeatable, I will be continuing experimenting and hope to publish an Instructable on double sided printing in the near future.