Backlight LEDs - types, sources, tools, techniques

Please note that the links to the Aliexpress shop may be invalid at any time. This shop sometimes has the LEDs, other times it doesn't.

Direct backlight LED strips

LED strips are a common technique these days. The outstanding Philips 42PFL9803 once had a huge number of LEDs in a matrix. See this post. Its image brightness and homogeneity were second to none. Nobody does that anymore. It is too expensive.

Size
The LEDs used for direct backlight are 28x35mm up to 35x35mm, thus their size code can be 3528, 2835 or 3535
.
Voltage
The smaller LEDs (2835 or 3528) are usually 3 volts, 1 watt.
The bigger 3535 types can be 3V 1W up to 6V 2W.
The size is not a sufficient criteria for choosing LEDs.

Polarity
To my surprise, not all LEDs are equal. All have a small and a large contact pad. However, the cathode (minus) and anode (plus) can be either one.
In the following text, I use

• L- = cathode (minus) on large contact
• L+ = anode (plus) on the large contact

When a LED with opposite polarity is used, it will not sit in the original center and the plastic lens has to be offset from its intended place. This can cause difficulties with the homogeneous light emission and the repaired LED might get visible on the screen.

Contact shape
Some types can have S-shaped contacts. They will not fit on rectangular-shaped solder pads and the other way round!

In the following text I use

• S = S-shaped
• R = rectangular shaped

Top marking
Also the top marking does not reliably indicate the cathode, like it would be with ordinary diodes. Manufactures do as they please.

3V Types

Lextar 3030 3V 1W shape R, polarity L-

I found those in a Panasonic TX-P55DXX689. The image is that of a defect LED, I have not ordered any of them, yet. The dent at the right edge is normally not there, I did that.

Samsung 3535 3V 1W shape S, polarity L-

JUFEI 2835 3V 1W shape R, polarity L-

SEOUL 2835 3V 1W shape R, polarity L+

LG Innotek 2835 3V 1W, shape R, polarity L+
Identical parameters and light as the SEOUL. I think those are the original LEDs in the 50LN5406, for example.

UNI 3535 3V 1W shape R, polarity L+

LUMENS 3535 3V 1W shape S, polarity L-

6V Types

SEOUL 3535 6V 2W shape R, polarity L-

LG Innotek 3535 6V 2W shape R, polarity L+

SHARP 3535 6V 1.2W, shape R, polarity L-

How to know the most popular LED types

If you like to stock up on LEDs and don't know where to start: go with the masses. See what others have bought:

Most popular LEDs

Testing

The best tool I know is the LED backlight tester from China. It is available on Aliexpress, here for example: Latest (model GJ2 C) backlight tester. It is a real time saver.

There are older models (GJ2B) around, which are cheaper.

As of end of 2016 the number of different tester model has increased. I can only speak about the aforementioned model. It is fully automatic and adjusts the voltage by itself. It tests a single LED in one moment and a full panel right after that. Tester with a potentiometer where you have to adjust the voltage yourself are much too inviting for user errors.

This design is also available as a high voltage (>1500V) CCFL lamp tester: Lamp Tester

Soldering

For my first LED repair I used a hot air gun. That is ok, when you keep the temperature low enough to avoid melting the transparent plastic. However, swapping the LED would be much quicker if the solder on the strip were molten while pulling the old LED off and putting the new one on.

Shop Jimmy presented a LED rework station on YouTube once. It seems as if this very cool and cheap gadget is out of stock. They have blocked access from outside the US, so I cannot tell for sure. While looking for an alternative on AliExpress, I found the KADA 853B preheater:

It is basically a hairdryer in a box. It is a rough little machine. It vibrates like hell, because they did not decouple the noisy fan from the case. I guess I'll have to do that myself.

It is not powerful enough to heat a multi layer PCB with large ground planes quickly up to the melting point. I tried that on a small TCON board. They call it a BGA preheater, which is ridiculous. You will not want to solder BGAs with a device that has no temperature profile!

As it turned out in my first LG backlight repair (see this blog post), it was not possible to melt the solder in acceptable time with the device. Nevertheless it is useful as a preheater and with a little support from a hot air gun, soldering LEDs is very quick and not stressful for the LED.