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I've just ordered some sample LCD displays. The only model available for now is hot bar solder (instead of a regular connector).

Do you have any experience soldering regular thin wires to this type of connector? It seems like a 25 mil pitch.

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2 Answers

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This pdf has a description of hot bar soldering if you're unfamiliar with it. (It is the top Google result, but it's a popular intro.)

25 mil pitch on a connector isn't too bad; it's the characteristics of the flex cable that will bite you. If I misinterpreted, and you have the hot bar connector on the PCB they're sending you, then you shouldn't have too much trouble.

If, however, you're soldering off of the flex cable that attaches to the hot bar connector, you left out whether your flex cable has windowed, cantilevered, or unexposed leads. If you have a choice, cantilevered will give you your best chance of success because the leads will be thicker, and you won't damage the backing.

With any of the options, soldering leads onto the 25mm pitch should be alright. Use 28 gauge wire or finer. I did a 20mil pitch, 10 mil trace cable a few weeks ago, and that went OK. (I did have the use of a stereo microscope, you'll want a good magnifier at the very least.) I damaged one practice cable beyond repair - The backing burns like the insulation of a wire, and, and the traces are not copper leads, they're copper foil. Some form of strain relief will be absolutely mandatory to keep your wires from breaking off.

However, I would advise that it will be easier if you can solder directly to the connector leads instead. It's easy to damage the very thin traces on your flex cable, but the connector will be much sturdier.

Whatever you end up doing, make sure your wires are as short (and equal length) as possible, you might be dealing with some very high speed stuff! A 500MHz signal (for VGA resolution, using 2-wire differential signals) has a wavelength of just 5cm!

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I had seen that PDF as well, but I was looking more on the hands on experience from people around. You've just provided that. Thanks! I don't have the part in my hands yet, but I think I will be soldering leads directly on the connector. Thanks for the hint on the length, although it seems the controller is embedded in the package, so data is going through SPI, which I'd assume is less critical than raw VGA signals I'd assume. – Padu Jun 18 at 21:20
If you've got steady hands, a fine tip, and a magnifier (for inspection at the very least), you should be able to solder to the connector. – reemrevnivek Jun 18 at 21:59
What kind of resolution/frame rate/color are you working with? SPI seems like a strange way to send video data. 60(Hz refresh)*740(Horizontal pixels [1]) * 520(Vertical pixels [2]) *8 ((or 10 or 12) bit color) = 180MHz, way more than SPI (or anything that's not differential and/or DC balanced) can handle. If you're going over SPIa at a data rate that a microcontroller can handle, don't worry about length - Give yourself a comfortable amount of room. [1][2] Data sent while frame valid and line valid are off – reemrevnivek Jun 18 at 21:59
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I was worried about it myself, but decided to bite the bullet and try it out. It's relatively easy and straightforward.

I made the PCB pattern following the LCD data sheet for the cable terminal spacing and pitch. I made my pads a little longer so I could either solder on top of, or next to the cable if needed.

The PCB came with enough tinning that I simply lined it up, and pressed each terminal down with the tip of my soldering iron. It melted right into the solder already on the pads. I added a little more solder to a few terminals that didn't have a nice meniscus from the cable to the PCB. If the PCB isn't pre tinned, then you might consider adding a little solder to each pad, then swiping some flux across all the pads. If you add too much solder you'll find that the terminals will short as the solder squishes under the cable.

Also note that the terminals on the cable have small vias on the terminals themselves - if the solder draws up into them, it's a pretty easy and convenient way to know the connection is made.

Process

Tin the pads if the PCB doesn't have any solder on the pads already

Hot bar soldering without the hotbar

Apply flux to pads, then use the adhesive on the cable, if any, to secure the cable in the right spot.

Hot bar soldering without the hotbar

Press the soldering iron into the joint so it touches both the contact and the pad, if possible (some cables don't have exposed contacts like this one), then press the soldering iron down on the cable contact itself. It should pick up a litle of the solder on the board and make a connection.

Hot bar soldering without the hotbar

For this cable I also added more solder so the connections would be very secure.

Hot bar soldering without the hotbar

The images above link to a page where the full resolution is available. For reference, the board is 2.5" wide, and the LCD I was attaching is the 2.4" TFT LCD available from Newhaven Display International.

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Thanks for the pictures. I haven't received my samples yet, but I'll post the results of my experience here as well. – Padu Jun 21 at 17:13

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