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Overview
Overview
In this step, you'll be fabricating the 4 PCBs in the GelSlim 4.0 sensor. If you chose to purchase your PCBs pre-fabbed, you can skip this step. If not, you'll first solder the components onto the LED Driver board. Then, you'll solder the components onto each of the 3 LED Harnesses. Finally, you'll check that all of your PCBs are correctly soldered.
Step-by-Step Instructions
Step-by-Step Instructions
Acrylic Jig Laser Cutting
Acrylic Jig Laser Cutting
Load 1/16" clear acrylic onto laser cutter bed
Note: Leave the paper backing on both sides of the acrylic when you cut it for a cleaner cut.Load compatible led_driver_jig file into laser cutter software
Note: In the files provided, cut lines are red (R: 255, G: 0, B: 0) with weight 0.01pt. If your laser cutter software uses a different stroke color/stroke weight for cut lines, there are several ways to change the files provided to fit your setup. For example, you can load led_driver_jig.ai into Adobe Illustrator and change the stroke color/stroke weight of the cut lines to match the settings of your setup.In laser cutter software, set material to cast acrylic with 1/16" thickness
Note: Setting the material and material thickness in most laser cutter software should load the default laser power and speed for this material. It's likely that you will have to change these settings to achieve a clean cut. You can perform several test cuts with smaller parts until you're satisfied with the cut settings.In laser cutter software, align cut geometry with available space on the acrylic sheet
Execute the cut
Remove paper backing from both sides of the acrylic
LED Driver Soldering
LED Driver Soldering
Apply solder paste to LED Driver
Drop the LED Driver into the acrylic jig, as shown below
Cut a 1-inch piece of electrical tape and secure the acrylic jig to the soldering mat
Align the windows of the solder stencil with the copper pads on the LED Driver
Cut a 1-inch piece of electrical tape and secure the solder stencil to the acrylic jig, as shown below
Note: Take your time here! Hold down the aligned solder stencil to make sure it doesn't move while you tape down the stencil. If the copper is not well-aligned, you may cause shorts in the final circuit.Uncap the solder paste by twisting off the tip cover
Note: You don't need to use any solder paste dispensing needles or syringe tips for this step, a thick bead from the barrel is fine.Firmly depress the plunger and extrude an approximately 3cm-long and 2mm-thick bead of solder paste onto the solder stencil along the top side of the windows, as shown below
LED Driver seated in acrylic jig
Solder stencil aligned with LED Driver
Solder paste applied to aligned LED Driver solder stencil
Hold the solder paste spreader at an angle (about 45°) with your dominant hand while holding down the stencil and LED Driver with your non-dominant hand
Pull the solder paste spreader across the stencil while applying moderate downward pressure to spread the solder paste across the windows
Note: It is best if you are able to distribute the solder paste in one pass. If you take multiple passes, the solder paste might spread underneath the stencil and cause shorts in the final circuit.Check your solder paste application
Slip the tweezers underneath the solder stencil and gently lift it off the LED Driver
Flip the solder stencil all the way over the tape to expose the LED Driver
Use the diagram below to decide whether to continue with the instructions or redo your solder paste application
Note: Make sure to wipe both sides of the solder stencil with isopropyl alcohol thoroughly before trying again! Also make sure to clean out the interior of the windows on the stencil. If any solder paste is on the underside of the solder stencil or left in the windows when you re-align the LED Driver, it can spread underneath the solder stencil and cause shorts in the final circuit. Also wipe the LED Driver and the solder paste spreader before your next attempt.
Diagram showing solder paste application that is acceptable on the left and unacceptable on the right
Remove electrical tape from the acrylic jig and LED Driver
Lift acrylic jig off the LED Driver
Pre-arrange components on soldering mat
Remove 1 0Ω resistor, 2 45.3Ω resistors, 1 60.4Ω resistor, 3 FPC connectors, and 1 Micro USB connector from their packaging
Roughly arrange the components on the soldering mat in the layout they'll be applied, as shown below
Components pre-arranged how they'll be applied to the LED Driver. Top row, left to right: 45.3 resistorΩ, 60.4Ω resistor, 45.3Ω resistor, 0Ω resistor, Micro USB connectorBottom row: FPC connectors
Place components on the LED Driver using tweezers, as shown below
Components placed on the LED Driver
Heat solder
Bring ventilation system close to the hot plate and turn it on
Gently lift the LED Driver off the soldering mat and place it on the hot plate
Turn the hot plate up to 5-10C higher than the melting point of your solder paste
Note: If your hot plate has poor temperature control, you may find that you need to further increase the temperature.Wait until the solder flows and turns shiny on all the components, as shown below
Note: The time it takes for the solder to flow will vary depending on the thickness of your solder paste application and the time it takes for your hot plate to heat to the set temperature. Generally, it should take around 4-6 minutes for the hot plate to heat up and flow the solder.
Left: Solder paste before it flowsRight: Solder paste after if flows
Turn off the hot plate and wait at least 1 minute for the solder to cool and solidify before you remove the PCB
Note: If you remove the LED Driver from the hot plate before the solder solidifies, the components may slide out of place.Using tweezers, move the LED Driver to the soldering mat and allow it to cool
Note: This component will be hot! Only place it on heatproof surfaces, and exercise caution when handling it.
LED Harness Soldering
LED Harness Soldering
There are 3 colors of LED Harnesses, as shown below: blue, green, and red. Blue LEDs get soldered onto the Blue LED Harness, green LEDs get soldered onto the Green LED Harness, and red LEDs get soldered onto the Red LED Harness. These soldering operations also use surface-mount devices. The fabrication process is almost identical for each color of LED Harness except for one critical step: identifying the orientation of the LEDs. The blue and green LEDs have a chamfer on the anode side of the LED while the red LEDs have a chamfer on the cathode side. When you identify the orientation of the LEDs, make sure to keep this in mind. You'll repeat the steps below 3 times, once for each color of LED Harness.
Each color of LED Harness is shaped differently. From left to right, we show the Blue LED Harness, Green LED Harness, and Red LED Harness.
Apply solder paste to LED Harness
Align the copper pads on the LED Harness in the windows of the solder stencil, as shown below
Cut a 1-inch piece of electrical tape and secure the LED Harness to the soldering mat
Cut a 1-inch piece of electrical tape and secure the solder stencil to the soldering mat
Note: Take your time here! Hold down the aligned LED Harness and the solder stencil to make sure they don't move while you tape down the stencil. If the copper is not well-aligned, you may cause shorts in the final circuit.
Blue LED Harness aligned underneath the solder stencil
Green LED Harness aligned underneath the solder stencil
Red LED Harness aligned underneath the solder stencil
Uncap the solder paste by twisting off the tip cover
Note: You don't need to use any solder paste dispensing needles or syringe tips for this step, a thick bead from the barrel is fine.Firmly depress the plunger and extrude an approximately 1cm-long and 2mm-thick bead of solder paste onto the solder stencil near the windows, as shown below
Solder paste applied to solder stencil with Blue LED Harness
Solder paste applied to solder stencil with Green LED Harness
Solder paste applied to solder stencil with Red LED Harness
Hold the solder paste spreader at an angle (about 45°) with your dominant hand while holding down the stencil and LED Harness with your non-dominant hand
Pull the solder paste spreader across the stencil while applying moderate downward pressure to spread the solder paste across the windows
Note: It is best if you are able to distribute the solder paste in one pass. If you take multiple passes, the solder paste might spread underneath the stencil and cause shorts in the final circuit.
Check your solder paste application
Slip the tweezers underneath the solder stencil and gently lift it up while holding down the LED Harness
Flip the solder stencil all the way over the tape to expose the LED Harness
Use the tweezer to lift the LED Harness tape and move the LED Harness aside
Use the diagram below to decide whether to continue with the instructions or redo your solder paste application
Note: Make sure to wipe both sides of the solder stencil with isopropyl alcohol thoroughly before trying again! Also make sure to clean out the interior of the windows on the stencil. If any solder paste is on the underside of the solder stencil or left in the windows when you re-align the LED Harness, it can spread underneath the solder stencil and cause shorts in the final circuit. Also wipe the LED Harness and the solder paste spreader before your next attempt.
Samples of solder paste applications, where solder paste is the gray material. From left to right, the first two samples are fine to continue with. The middle sample would likely be fine, but since this step is quick and easy to repeat you should redo it. The last two samples should be redone.
Pre-arrange LEDs on soldering mat
Remove 6 LEDs from their packaging
Arrange the LEDs in a loose grid with the chamfers facing out, as shown below
Note: The chamfers can be difficult to see because they’re very small. You can use a magnifying glass and bright light source to see the chamfers better, or you can check the orientation of the LEDs using a multimeter. To do this, first set the multimeter to the "diode" setting (usually indicated by the standard diode symbol). Then, touch the tips of the probes on each terminal of the LED (one on each side). If the LED lights, the red probe is touching the anode and the black probe is touching the cathode. The blue and green LEDs have a chamfer on the anode side of the LED while the red LEDs have a chamfer on the cathode side. If it doesn't light, switch the probes and try again. If neither combination lights the LED, the probes may not be making proper contact with one or both LED terminals. Try using the tips of the probes to make contact. If the LED still doesn't light, it may be faulty.
Note: Another option to identify the chamfer on each LED is by using a magnifying device and a bright external light to enlarge your view of the LED.
For the blue LED, the chamfer is on the side the red probe is touching when the LED is lit. In this figure, the black probe is on the left side and the red probe is on the right (chamfer) side. The chamfer marks the anode side on the blue LED.
Blue LEDs pre-arranged in a loose grid with the chamfers facing out, highlighted in blue. The chamfers on the actual LEDs are not colored.
For the green LED, the chamfer is on the side the red probe is touching when the LED is lit. In this figure, the black probe is on the left side and the red probe is on the right (chamfer) side. The chamfer marks the anode side on the green LED.
Green LEDs pre-arranged in a loose grid with the chamfers facing out, highlighted in green. The chamfers on the actual LEDs are not colored.
For the red LED, the chamfer is on the side the black probe is touching when the LED is lit. In this figure, the black probe is on the left (chamfer) side and the red probe is on the right side. The chamfer marks the cathode side on the red LED.
Red LEDs pre-arranged in a loose grid with the chamfers facing out, highlighted in red. The chamfers on the actual LEDs are not colored.
Arrange LEDs on the LED Harness
Using tweezers, place the LEDs onto the LED Harness in a grid with the chamfers facing out, as shown below
Once all LEDs are placed, use tweezers to align them by gently bumping them into place
Note: The LED placement does not have to be perfect, but the LED array should be neat. Try to make sure that all of the LEDs are within the footprint of the LED Harness. When the solder paste is heated in the next step, the components will snap into place if they’re close enough.
Arrangement of blue LEDs on the LED Harness. In the top figure, chamfers are marked with blue triangles. The chamfers on the actual LEDs are not colored, as shown in the bottom figure. On the blue LEDs, the chamfer marks the cathode.
Arrangement of green LEDs on the LED Harness. In the top figure, chamfers are marked with green triangles. The chamfers on the actual LEDs are not colored, as shown in the bottom figure. On the green LEDs, the chamfer marks the cathode.
Arrangement of red LEDs on the LED Harness. In the top figure, chamfers are marked with red triangles. The chamfers on the actual LEDs are not colored, as shown in the bottom figure. On the red LEDs, the chamfer marks the cathode.
Heat solder
Bring ventilation system close to the hot plate and turn it on
Remove electrical tape from the LED Harness
Gently pull the LED Harness off the soldering mat and place it on the hot plate
Note: If you need to make adjustments to the LEDs, you need to make those adjustments before you turn on the hot plate. Do not use the tweezers to adjust the LEDs or otherwise move the LED Harness while the hot plate is on. It's very easy to accidentally bump the LEDs out of place when the solder is hot and flowing.Turn the hot plate up to 5-10C higher than the melting point of your solder paste
Note: If your hot plate has poor temperature control, you may find that you need to further increase the temperature.Wait until the solder flows and turns shiny on all the components, as shown below
Note: The tail of the LED Harness may curl up away from the hot plate as it heats. This is completely fine, you do not need to stop heating the PCB. The time it takes for the solder to flow will vary depending on the thickness of your solder paste application and the time it takes for your hot plate to heat to the set temperature. Generally, it should take around 1-3 minutes for the hot plate to heat up and flow the solder.
Left: Solder paste before it flowsRight: Solder paste after if flows
Solder paste flowing and snapping LEDs into place
Turn off the hot plate and wait at least 1 minute for the solder to cool and solidify before you remove the PCB
Note: If you remove the LED Harness from the hot plate before the solder solidifies, the LEDs may slide out of place.Using tweezers, move the LED Harness to the soldering mat and allow it to cool
Note: This component will be hot! Only place it on heatproof surfaces, and exercise caution when handling it.
Testing Soldered PCBs
Testing Soldered PCBs
Set the multimeter to the "diode" setting (usually indicated by the standard diode symbol)
Touch the tips of the probes to the corresponding locations shown in the diagram below
Note: Although there are 4 contacts on the LED Harnesses, they are paired. The first 2 contacts should be shorted together, and the second 2 contacts should also be shorted together. The Green LED Harness is flipped from the Blue and RED LED Harnesses because the Blue and Red LED Harnesses get physically flipped over in the final sensor assembly.
Blue LED Harness lit in the correct orientation. The black probe is touching the left pair of contacts and the red probe is touching the right pair of contacts.
Green LED Harness lit in the correct orientation. The red probe is touching the left pair of contacts and the black probe is touching the right pair of contacts.
Red LED Harness lit in the correct orientation. The black probe is touching the left pair of contacts and the red probe is touching the right pair of contacts.
If all the LEDs light when probed according to the above diagram, the LED Harnesses have been soldered correctly
If all the LEDs do not light when probed according to the above diagram, the PCB is incorrect and should be redone
If the LEDs light in the opposite probe position from the above diagram, the LEDs have their orientations flipped
If only some LEDs light in the probe position from the above diagram, the other LEDs may have their orientations flipped
If the multimeter detects a short in the circuit, the solder paste application is likely the culprit
Connect the tail of each LED Harness to the LED Driver, as shown below
Note: The connectors on the LED Driver are delicate. Do not push against them with a lot of force.Flip up the white latch on each FPC connector
Slide the tail of each LED Harness into its FPC connector, using tweezers to grip the tails
Flip down the white latch on each FPC connector to secure the connection
Connect the LED Driver to power via the Micro USB port
Note: Don't look directly at the LEDs. They're very bright and will hurt your eyes.If all 3 LED Harnesses light, the LED Driver has been soldered correctly
LED Harnesses connected to the unpowered LED Driver. From left to right: Red LED Harness, Green LED Harness, Blue LED Harness. Note that the Red and Blue LED Harnesses are flipped upside down.
LED Harnesses connected to the powered LED Driver. From left to right: Red LED Harness, Green LED Harness, Blue LED Harness. Note that the Red and Blue LED Harnesses are flipped upside down.
If all 3 LED Harnesses light, the LED Driver has been soldered correctly
If one or more of the LED Harnesses do not light, the LED Driver is incorrect and should be redone
Remove the LED Harnesses from the LED Driver
Using the multimeter, probe the individual FPC connector contacts to check for shorts
Using the multimeter, probe the Micro USB contacts to check for shorts
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