Final Design

Components:

The fixture frame components are made from 6061-T6 Aluminum, machined using CNC milling. The dowel pins are made from stainless steel and are used in fixture contact points to prevent galling or seizing over many usage cycles.

The alignment combs are made from Kovar Nickel alloy, machined using super-high-precision Wire EDM. Kovar's coefficient of thermal expansion matches that of the ceramic substrate used to cut and mount the electrodes used in the TE device design; this prevents accidental shearing of TE pillars during cooling due to different expansion rates.

The purpose of the fixture and the combs is threefold:

1) Maintain accurate position, alignment, and perpendicularity for all TE pillars used in the design

2) Hold pillars in place during the soldering process

3) Allow for easy removal of the soldered assembly from all fixture components after soldering process completion

For more information regarding these components, see the Parts page.

Solder Reflow Oven

The purpose of the oven is to heat the fixture up to the solder reflow temperature.

This particular oven (Toshiba Convection Toaster Oven) was chosen for testing because it can reach a max temperature of 230°C in about 5 minutes and has a built in convection mode which makes it easier for the hot air to reach inside the fixture and melt the solder paste.

Additional testing using a K-type thermocouple determined that this particular oven is capable of holding a given target temperature with an error of +/- 2°C, rendering it more than effective for accurate process testing purposes.

For further development and production usage, an industrial solder reflow oven is suggested for optimal outcomes.

ChipQuik SMDLTLFP50T3 Solder Paste.

Solder paste is needed to solder the thermoelectric pillars onto the copper electrodes.

The solder paste is applied onto the copper electrodes using a stencil. This solder paste was chosen as it is lead-free (RoHS compliant) and has a low reflow temperature (165°C).

Aluminum Stencil

The stencil was custom made in order to accurately place the solder paste onto our alignment of thermoelectric pillars. Sourced from Stencils Unlimited, this reusable stencil was created using a high-precision UV laser, and is made from flexible yet durable aluminum. At 0.1mm (.004") thick, the stencil places just enough solder at each pillar-electrode junction to ensure a solid bond without any risk of solder drip or run.

Process Description:

The final design solution includes four main components: manufactured fixture frame/combs, solder paste/mask, solder stencil and toaster oven.

Step 1 - The solder paste is applied onto the copper electrodes using the stencil.

Step 2- The bottom ceramic plate with the copper electrodes are placed into the fixture frame, the combs are slid in, the thermoelectric pillars are placed in their positions and the top ceramic plate is placed on top.

Step 3 - The entire fixture is then placed in the toaster oven and baked at about 160 C for the solder paste to melt. The fixture is then removed from the oven and the combs are slid out to leave just the thermoelectric pillars successfully soldered onto the copper electrodes.

Step 1 - Solder Application

Applying Solder Paste.mp4

Step 2 - Fixture Preparation

Fixture and Pillar Placement.mp4

Step 3 - Reflow Process

Place fixture into heat oven until solder reaches a temperature above 165 C
Allow fixture to cool below 138 C to bond TE pillars with electrodes
Adhere to safety standards ASTM C1055 prior to handling hot material

Project Performance and Results

The final design was used to make a few prototype thermoelectric devices. These devices used hand-cut copper electrodes and copper wire mock pillars but proved viability of the final design solution. The pillars were successfully soldered to both the top and bottom electrodes and allowed for current flow through the device. Estimated resistance through the device was 200-300 Ohms which was confirmed to be acceptable by the sponsor, Professor Renkun Chen. Time to build one device was drastically reduced. Each stage in the process was tabulated below. The result was a reduction in the time taken to manufacture the device from 40 hours to under 2 hours, a 95% reduction. Additionally, labor time was reduced to 35 minutes with this design solution.

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