Flex-circuit Soldering & Assembly Tutorial and Notes

-William R. Provancher
Mechanical Engineering
University of Utah

The availability of inexpensive MEMS devices has made it possible to integrate arrays of sensors into otherwise ordinary structures.  It is therefore possible to inexpensively create meso-scale smart material structures without access to a clean room or being an expert in MEMS. Flex-circuits are a convenient way to create electrical connections to these small surface mount devices. Not only can they create the required electrical connections, but they can serve as a substrate for signal conditioning electronics (ref. shape sensor) or even create a scaffolding for placement of the sensor arrays.

There are many companies that make flex-circuits (Cirexx, Minco, Flextronics), however, unlike rigid PCBs which are an inexpensive commodity item that can cost less than $100 and take a few days to turn around prototypes, whereas flexicircuits start at $600 for a couple flex-boards. Making flex-circuits on your own may not happen flawlessly on the first try, but if one is persistent and methodical it is a skill that can be mastered quite rapidly (see flex circuit fabrication tutorial).

Once you have your flex-circuit and sensors, there is still the issue of soldering the sensor to the flex-circuit. There are a variety of methods for soldering these.  The primary tools include: (1) a standard soldering iron, (2) hot plate, (3) standard solder, (4) solder paste (no clean) (use either eutectic, Sn63Pb37 (e.g. Kester 276, 225 degree C melt) or low temp (200 degree C melt), Sn43Pb43Bi14 (e.g., Kester 286LT), you can get a free sample on the Kester website) (another option is to use low temp (135 degree C) "tix" solder I've never used it, but it's used commonly by watch makers, available at Bartlett & Co and Railroad Watch Tools), (5) flux pen (no clean), (6) hot air pencil (see Zephyrtronics), (7) hot air bath (see Zephyrtronics), tweezers, 3rd hand utility tool, tape


As an example for this tutorial, I have fabricated a simple 1-sided flex-circuit that supports a single 8-pin leadless LLC package accelerometer (Analog Devices), associated resistor and capacitors, and a crimp-on flex connector from FCI. In some ways, this tutorial is more difficult than it needs to be, because the flex circuit traces are not tinned and my flex board is made of mylar, meaning that you need to be pretty careful when soldering not to burn right through the mylar.  I'm also using an accelerometer with a leadless package (with 0.050 inch pitch contacts) and extremely small resistors and capacitors (0402 package, i.e., 0.040 x 0.020 inch footprint).  However, the same instructions generally apply regardless of the feature sized you're working with. The tutorial covers a few options for soldering the device and components, and makes recommendations for adding stiffeners to reduce stress concentrations that will naturally occur near solder joints and the connector.

Most labs have a decent soldering iron (including the Stanford BDML), so this is the primary tool in this tutorial. You'll also need a set of good tweezers (I prefer ones with a curved tip), some tape, a cutip and acetone or isopropyl alcohol, a flux pen, and either rosin-core solder (small diameter, 0.020") or solder paste. You'll also of course need an accelerometer (I've used an ADXL210E here, see datasheet) and 0402 SMT resistors and capacitors (link to Panasonic components in Digikey). If your flex circuit has a poly- imide substrate rather than the mylar (polyester) substrate I used for this example, a hot plate can also be a handy tool.

Here's the end product we're trying to make in this tutorial




Here are the basic steps to make this prototype (circuit layout & pinouts) :  
1. Solder the accelerometer in place
    (a) Tape the flex circuit down and the accelerometer to line up with the circuit's solder pads
    (b) Tack the accelerometer down at two points
    (c) Remove the tape from the accelerometer and solder the rest of the pads (and perhaps retouching the original tacked solder joints).
2. Solder the SMT resistors and capacitors in place
3. Check continuity and sensor outputs
4. Clean off flux/rosin residue
5. Place dielectric tape over traces (front side)
6. Laminate stiffeners onto the back side of the circuit
7. Crimp on flex-circuit connector
8. Double check sensor outputs
9. (Optional) Pot solder joints







1. Solder the accelerometer in place (Turn the solder iron on (~640 F if you have a temperature readout on you soldering iron)
    (a) Tape the flex circuit down and the accelerometer to line up with the circuit's solder pads. Taping the flex circuit down will keep the substrate from shifting around. I like using Kapton tape since its adhesive isn't gummy, can be restuck to reorient the circuit to get better access and won't leave a residue.

                    

    (b) Tack the integrated circuit (accelerometer) down at two points.  Use either conventional soldering techniques for this or use solder paste for this (see part (c) or step 2 for techniques..

             
    (c) After tacking into place, remove the tape from the accelerometer and solder the rest of the pads (and perhaps retouching the original tacked solder joints).
         Soldering these small contacts can be a bit difficult, but a little practice will make you a pro. There are two main techniques I've used with some success. Both use a standard soldering iron and either standard small diameter rosin-core solder or solder paste (mixture of minute spherical solder particles, activators, solvent, and a gelling or suspension agent). Here I'll cover a method that works with standard solder (see notes at the end of this step or step 2) for solder paste method). 

Soldering using rosin-core solder and soldering iron
In this method, a flux pen is your friend. Kester sells "no clean flux" pens (951 Flux-Pen) that can be purchased at any electronics store for about $5. The flux cleans and etches the surface oxides, prevents oxidation of the copper when it is heated, and helps the solder flow when heated by lowering the surface tension of the molten solder so that it spreads and penetrates more readily.

In all honesty, the above technique is pretty tough to master and would receive much criticism from my EE colleagues, so I strongly recommend getting some solder paste and a small tipped syringe with which to dispense it.

                 

             

Solder paste pointers
For pointers on how to solder the accelerometer using solder paste, see the Analog Devices technical library techsheet, or modify the above instructions, by dispensing a small amount of solder paste onto each solder pad and accelerometer contact.  Make sure to clean the solder iron tip, and quickly bring it into contact with both the substrate and accelerometer pad. The solder joint will form almost instantly.  If you're not convinced that the solder joint if formed, dispense a little more solder paste making sure that the solder paste bridges from the copper solder pad onto the pad on the side of the accelerometer.  Once again dab the joint with the tip of the solder iron.

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2. Solder the SMT resistors and capacitors in place.
Generally, solder paste is either sold in jars or large syringes, so buy yourself a small syringe with a xx gauge tip.




           
          

Another look (soldering an SMT resistor)

           
          

      

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3. Check continuity and sensor outputs
First check your solder joints for continuity with a multimeter.  Check contact from the connector side up to the accelerometer chip and check for shorts between solder pads.  Also check for continuity between the copper traces an adjacent contacts on the accelerometer.

It's useful to have a ZIF connector to insert the flex board into before you crimp on a connector.  Connectors are available from xxxxxxx.  Here I connect power and ground and use an oscilloscope to check the analog and the digital duty-cycle outputs of the accelerometers

   
       

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4. Clean off flux/rosin residue (Acetone or isopropyl alcohol and a cutip works well)
If you use "no-clean flux" this isn't strictly necessary, but you're better safe than sorry. Some fluxes can corrode your solder joints and lead to issues down the road.

       

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5. Place dielectric tape over traces (front side).  Shown here with Kapton tape.

   

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6. Laminate stiffeners onto the back side of the circuit (I've used carpet tape or double-back scotch tape and overhead transparency film)
This will reduce stress concentrations at the edge of the connector and accelerometer.  Such stress concentrations, can cause the copper traces to fatigue and fail.

       Back side
              

       Front side
      
       Placing a thicker stiffener near the accelerometer solder joints will prevent the solder joints from flexing.  Since there is a large stress concentration at each of the solder joints, it is judicious to use such a stiffener if the accelerometer will be exposed to active flexing.

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7. Crimp on flex-circuit connector
Here I use a connector that requires no special crimping tools and has a 0.1" pin spacing, so that it can be plugged into a standard solderless breadboard. This "Clincher" connector is made by FCI (part # 66226-006, also available with female sockets, part # 65801-006 ).

          
          
Once you're past the prototyping stage, I'd recommend going with a smaller connector (something like a Hirose Electronic Co. Ltd.  FH10A-6S-1SH with 1 mm conductor pitch)

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8. Double check sensor outputs (plug it in to your breadboard and check the outputs one last time.  This is especially important if you are going to do the next step)

      

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9. (Optional) Pot solder joints (be aware that this will cause a mechanical stress concentration at the edge of the potted area.
(dabbing 5-minute epoxy around the edges of the accelerometer works well.  Unfortunately the solder joints that create electrical connections, also create stress concentrations that are likely to lead to mechanical fatigue failures. Potting these connections prevents the solder joints from flexing.  However, stiffening this area underscores the importance of adding moderate stiffening in the adjacent area as shown in 6 above, otherwise, the copper traces will fail right at the edge of the potted area.)

      
      

      

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Notes:
Put in link to tutorial that Mark put together when he was in Pontedera.
Put in link to PCB tutorial (till I put together a tutorial for making flex circuits)