Exploring the Secret Layout of Copper PCBs and FPC Soft Cables in EC20

I have just bought an EC20 from ACEBEAM. It integrates RGB and white light in a compact size. It features a magnetic tail for easy attachment and is powered by a 3300mAh 18650 USB-C rechargeable battery.

In this article, we gonna dive deeper by disassembling it and having a closer view of its inner layout.

First, Remove the tail cap and take out the battery
Inside the lamp head is a circular circuit board, with the positive terminal spring for the battery soldered in the center and secured at both ends with screws. The circuit board is covered with a layer of rubber insulation.

Step 2, remove the rubber insulation layer
Once it is removed, surprisingly, we can see the two screws securing the circuit board were coated with transparent epoxy resin.

Step 3, remove the circuit board
Till now, we see that the mainboard and the light board are connected with an FPC (Flexible Printed Circuit) ribbon cable.

Finally, remove the FPC ribbon cable
Using a soldering iron, I removed the FPC ribbon cable and took out the mainboard. This is a typical layout for flashlights: a circular board and a square board are connected by corner welding, with the positive terminal spring of the battery soldered to the center of the other side of the circular board.

• On the side of the circular board where the positive terminal spring is soldered, there is an NMOS transistor (model DTQ2208) in the lower-left corner, which controls the on/off of the three main LEDs. Next to it, a 5 mΩ sense resistor is placed in series with the LED current circuit to measure the LED current.
• In the upper-right corner, there's an SOT23-5 package component, which is an operational amplifier. Its function is to collect the voltage from the LED current circuit, amplify it, and then feed it into the FB pin of the BOOST step-up chip, thereby enabling closed-loop constant current control for the LEDs.
• On the side of the circular board where it's soldered to the square board, the PMOS transistor on the left is a DTQ3205.
• On the right, there's an MPS MP3431 Boost converter chip, which boosts up the 3.7V from the Li-ion battery to the voltage required by the LED lights, and a built-in MOSFET, and it supports up to 21A of current, which is pretty impressive. Looking at the datasheet, sure enough, the built-in MOSFETs have RDS(on) values of 6.5mΩ and 10mΩ. So, in the current range of 100mA to 3A, with an input voltage of 4.2V, the efficiency can reach as high as 95%, and can even go up to 97% at its peak.
• One side of the square board features an FMD microcontroller, to the left of the microcontroller, there's an SOT23 component, which is used to step down the battery voltage and provide power to the microcontroller.
• The other side of the square board contains the button and a red-green dual-color status indicator light.

Now, let us remove the stainless steel strike bezel to have a look at the design of the light board.

The board features three SFT-25R HI white main light LEDs for illumination, along with three RGB LEDs. Notice the black part on the light board — that’s the FPC ribbon cable connecting the main control board to the light board. As expected, the light board uses a copper substrate with a thermally isolated design. 

The "thermally isolated copper substrate" has an astonishing thermal conductivity of 380W/mK — that's 380 times better than aluminum substrates! This technology is perfect for high-power lighting, and it seems there’s really no alternative that can match it.

Now, let's talk about the FPC ribbon cable. From the end, it appears to be a double-sided trace design. The thicker traces on both sides are used to power the white light main LEDs. The four thinner traces in the middle, on both the front and back, are for driving the three RGB LEDs.

Since the copper-based light board is a single-sided board, the connection between the FPC ribbon cable and the copper substrate is done as follows: six soldering holes are made at the FPC end and soldered to the copper substrate's pads.

The other end of the FPC is threaded through a slot in the center of the copper substrate, passes through the integrated heatsink structure of the lamp head, and is finally soldered to the pads on the main control board.