Assembling your DIY PCBs

Chris CorrellUncategorized

Finished silver Ink circuit drawing

In this post I am going to explore several options you have for assembling paper PCB’s. I will take you through the pros and cons of each option as well as some tips and some easy to implement design decisions that can greatly facilitate the assembly process. 

Lead Free Solder

The first option I am going to explore is lead free solder. The lead free solder I use is a tin, silver, and copper blend and it has served us well for both paper as well as traditional copper PCB assembly. I set our Weller soldering iron to around 450-500 degrees Fahrenheit. 

Tools/Materials

  • Soldering Iron
  • Lead Free Solder
  • Microscope (helpful when working with surface mount technology (SMT))
  • ESD safe tweezers

Pros

  • Readily available in most workshops/ maker spaces.
  • Cheap.
  • Easy to use if you are familiar with SMT soldering.

Cons

  • Components can break off if not secured appropriately.
  • Can dissolve traces if left on pads too long.
  • Pads must have enough silver ink for solder to adhere correctly.
  • Difficult for the beginners.

Tips

  • Try gluing larger components after soldering to reinforce mechanical attachment strength.
  • Make contact for at most 2-3 seconds between the silver ink and your part.
  • Deposit a layer of solder on your pad first and then wet it to slide the component in (check out this video for a good visual explanation of the technique).

Design for Success

  • Use the cricut footprint library. I have designed the footprints to lay down several layers of ink to make for easier solderability.

Solder Paste

Reflowing!

The next option I will explore is low temperature solder paste. I like to use a low temperature tin, bismuth, and silver blend. It is relatively inexpensive and reflows well with our heat gun hot plate set up. 

Tools/Materials

Pros

  • You can use a stencil for quick assembly (stay tuned for a tutorial on making a solder paste stencil with the Cricut!).
  • Low temperature solder paste does a good job of keeping traces and pads intact.

Cons

  • Hard to apply manually.
  • The heat gun can warp paper even if taped down.

Tips

  • If applying solder paste by hand be careful to add the right amount. If you look at the image above several pads do not have enough paste and needed more. The pad on the right with the most paste is what you should be shooting for.
  • Use a solder paste stencil for more uniform depositions.
  • Make sure your paste does not bridge any pads or traces.
  • If you do create a solder bridge I have found simply ripping it off the paper with an x-acto knife and reapplying more silver ink to the ripped pad is the easiest way to desolder.
  • Tape down your circuit with blue painters tape to keep the edges from curling when reflowing the solder paste.
  • Try gluing larger components after soldering to reinforce mechanical attachment strength.

Design for Success

  • Keep a little more clearance around pads to make applying the solder paste a little easier (especially if you are applying it by hand).

Silver Ink

The next option I will explore is silver ink in combination with super glue. I like to use this method to clean up any components that did not adhere well using the solder. This method is great to use if you do not have a soldering iron or are working with a large amount of students who would need to be supervised while soldering.

You dip your components in a small amount of super glue, place them on the paper using tweezers (use another pair of tweezers or an x-acto knife to release the components and position it before the glue dries), and then use a circuit scribe pen to draw over the terminals and create an electrical connection.

Tools/Materials

Pros

  • No warping or singeing of paper substrate due to heat.
  • No hot soldering iron needed.

Cons

  • Two part process is more involved then other methods.
  • Have to wait for the super glue to dry.
  • Can accidentally bridge connections when making electrical connections with the silver ink.

Tips

  • Apply as little glue as possible to the component you are gluing.
  • Deposit a small amount of super glue onto a piece of plastic and dip your components into it to uniformly coat the component in glue.
  • Work under a microscope for a greater chance of success.

Design for Success

  • Use larger components, they make glue application easier and allow for more surface area to electrically connect to.
  • Make sure your components have exposed contacts at the top as the bottom contacts will be insulated with super glue (or else you’ll have to make sure you do not cover the bottom contacts with super glue which is tedious).
  • The more clearance you add between components the more liberally you can apply glue as you will have more exposed silver to connect to with the ink.

Z-axis Tape

The final method of attachment I will explore is z-axis tape. Z-axis tape is an electrically conductive tape made by 3M. The tape conducts in the Z axis so components can be placed on top of a strip and make electrical connection with the pads but not short between the terminals. Z-axis tape is marginally easier to apply and work with then the other methods, however, I have found that it doesn’t work all that well, especially with smaller components. In order to maintain electrical contact some components must be pressed down into the tape otherwise contact is not always reliable.

Tools/Materials

  • Microscope
  • ESD safe tweezers (2 pairs is useful or you can use an x-acto knife)
  • 3M Z-axis tape

Pros

  • No soldering iron needed.
  • One step process: simply lay down the tape on your pads, peel back the paper and place your components on top.
  • No warping or singeing of paper.

Cons

  • Sticky and hard to remove once stuck.
  • Difficult to manage due to the stickiness.
  • Does not work super well with some components unless force is applied to maintain electrical contact.
  • Most expensive method.

Tips

  • Use tweezers to remove the paper backing from the tape.
  • Use different widths of tape for different components/areas.
  • Use the flat end of your tweezers to stick the tape well to the pads while the paper backing is still on.
  • Use a large portion of tape to attach multiple components as opposed to many small piece of tape.
  • Use a second pair of tweezers or an x-acto knife to help place and position components.

Design for Success

  • Place components in a line to make tape application easier/ use less tape.

Conclusion

As far as attachment goes the best way to assemble your paper PCB’s is the same way you would assemble a copper PCB, solder paste and a solder stencil. The low temperature solder paste makes reflowing manageable without burning the paper. Adding a layer of super glue over the soldered components helps keep components from breaking off. Please let us know if you have any ideas or questions about anything you read in the article. We are also very interested in seeing what paper PCB creations you come up with so send us those as well!