Toby's Automatic Ceramic Hotplate Is a First for DIY Reflow Soldering

While soldering breakout boards for parts in a TSSOP package, Toby Chui realized a hotplate would greatly speed up (and improve) the soldering process. After surveying the market options for low-cost hotplates, this software and hardware developer decided to design one instead. Toby's Automatic MCH Reflow Hotplate is open sourced, USB-C (PD)-powered, and a design you can build yourself.

Toby's Automatic metal ceramic heater (MCH) hotplate is a compact unit at 54 by 102 millimeters and standing 48 millimeters tall (assembled.) A 40 by 40 millimeter MCH rated for 50 watts dominates the surface area. This size makes it suitable for smaller boards, like breakouts for fine-pitched IC packages. Power comes from a USB-C Power Delivery (PD) supply. This device requires a supply capable of 20 volts and 3 amps operation. Toby says most supplies rated for 65 watts or higher should support 20-volt mode.

The automatic part of the name comes from the simple operation. The initial firmware has a reflow curve compatible with Tin-Lead solder alloys. There are two touch-sensitive buttons on the MCH hotplate. One starts the reflow process, and the other stops it!

USB-PD supplies default to 500 milliamps at 5 volts. There is a protocol and negotiation that occurs to enable higher power modes. The MCH hotplate design uses an IP2721 to handle PD communication and to move the supply to the 65 watts necessary for operation.

The microcontroller choice is interesting. The MCH hotplate uses a CH552G from WCH. This 8-bit microcontroller has an E8051 core. While its flash memory for code is 16 kilobytes, its RAM is somewhat limited. However, it does include a full-speed USB 2.0 PHY. You can program this chip with the Arduino IDE using a package called ch55xduino. Unfortunately, there is no free full-featured C++ compiler for the CH552. So, this package is not a 100% drop-in replacement for all Arduino libraries. Regardless, there is enough functionality for controlling the MCH hotplate.

Other DIY hotplate designs tend to use PCB or FR4 as the heating surface. Toby points out that the typical FR4 used for PCBs has a continuous operating temperature rating of about 110C. This temperature is far too low for reflow soldering, even with low-temp solder alloys.

The advantages of using a metal ceramic heating element are they use DC power, are relatively easy to use, and have long-term reliability. Toby says they are often used in (chemical) laboratories or medical devices. On the other hand, these elements are not easy to find because they are generally custom or made-to-order parts. This lack of generic part types means their price can be as high as 13 to 15 USD each.

Regarding temperature control, Toby looked at the sensors found in 3d printer hot-end designs. Those thermocouples seemed promising. However, their operational range is usually 200 to 250C, which is a bit low for reflow soldering. However, Toby found some 100K 1% thermisters rated for 300C, which have glass beads at their tip. These attach to the bottom of the ceramic heating element with some high-temperature thermal silicone.

You can build an MCH Hotplate using the PCB gerbers and Arduino code in Toby's MCH Hotplate GitHub repository. There are some schematics of the hardware available. However, they are not complete. For example, the schematic image for the CH552 lacks the power MOSFET and thermister voltage divider analog input. Detailed build instructions are on this project page. Another option is to join the waitlist on this Tindie product page. This reflow tool is so hot it sold out immediately!