About a month ago, my trusty W490 started to give me a blank white screen whenever I flipped open the phone. It was originally intermittent, but grew more frequent as time passed. Eventually, it became unbearable. Searching online, I discovered that many people have had the same problem. The solution seems to be that because the flex cable (the cable that connects the two halves of the phone) has worn out, the connection becomes flaky. After almost 2 years of flipping this phone open and closed, that explanation made sense. So, I set out to replace the flex cable (I bought the replacement off ebay). Therefore, there are two reasons, I can think of, that may cause you to want to disassemble the W490: either you want to replace a worn out flex cable, or you want to replace the shell. This guide will then help you with that. You will need a TORX T5 screwdriver and/or a small screwdriver.
First, slide off the back cover, by pushing down with your thumb on the back side of the phone, and sliding your thumb up towards the hinge while maintaining pressure. There should be a click. Take the cover off by gently lifting it away.
Remove the battery by sticking one of your fingers in the little gap between the battery and phone close to the hinge. Lift up the battery.
Next, remove the chin piece. There should be an elegant way of doing this, but for me, I ended up ripping it off. Perhaps, if you slide something under the piece, it might pop right off.
You can see a picture of the underside. Hopefully this will help you so you won't have to brute force it like what I did.
With the chin piece removed, two screws should be exposed. Remove them. Though it seems that a TORX T5 is needed, I made do with a small flathead screwdriver. It came out ok. I just had to be careful not to strip (wear out the teeth) the screw.
With the six screws removed, the back of the phone should come off pretty easily. Just lift it off, and set it somewhere safe.
Now, the circuitry should be exposed. Be careful not to inadvertantly scrape the sensitive electronic components. Detach the flex cable connector. I did it by wedging my thumb under the connector, and popping it off.
Now, the whole circuit board should come out pretty easily. I took a metal piece on the bottom of the phone, and lifted the board up by that. That piece of metal moves, so care should be taken so that it isn't bent out of shape, which could prevent you from putting the board back when you reassemble it.
Notice that there is a shiny piece of metal on the top left of the phone. Remove that. It is used to keep the hinge in place. Using a small flathead screwdriver, widen the gap between the case and the piece of metal going into the hinge. Then, sticking the screwdriver under the flat part of the metal should make it come out easily enough. Make sure to not open and close the phone (flipping it open and close) anymore after this piece is removed, because it could send the spring-loaded hinge shooting out and getting lost somewhere. After taking the board out, the numpad comes out very easily too. Also take that out so that there aren't any loose pieces falling everywhere.
Now, we remove the hinge. For the right hinge, using a small screwdriver, gently stick it into the little hole that the flex cable comes out of, and push against the side, of the phone. The hinge piece should start coming out. When enough of it comes out, take it out. Be careful that you don't destroy the flex cable in the process.
For the left hinge, the easiest way is just to open and close the phone repeatedly, with the hinge pointed towards a safe spot, like towards the table, and the hinge will shoot out eventually.
Notice the way the hinge is shaped. The exposed metal should match up exactly with the metal piece we removed a couple steps earlier. Keep that in mind when you put the phone back together.
This hinge is spring-loaded. Note the shape; the hinge piece is also has the same shape. Make sure to align the two pieces when you put the phone back together.
To separate the two halves of the phone, slide the two halves apart after using something (like a small screwdriver) to push the left (spring-loaded) hinge down.
Having removed the screws, pry open the cover by wedging your finger inside. This step does require brute force, as a strong piece of tape is holding the cover down. So, use enough force to where the tape lets go. Note also that the clear part is also attached to the black part only by the tape. For me, I did not keep the tape in one piece. Your skill may be superior (or inferior).
The rest is not important if you just want to replace the flex cable. If you want to replace the shell, then read on. Detach the camera cable. The camera can't be taken out yet. It is being held in place by the metal housing.
The metal housing is being held on the two sides. To remove it, I first used a small flathead screwdriver to separate one side, and lift that side of the housing up. Then, the other side comes off more easily. Be careful not to bend the housing out of shape. The first time I took this phone apart, it got bent out of shape, and the phone became very buggy when I put it back together, prompting me to take it apart again, and make this guide while I was at it.
Next, the whole screen also comes out very easily, except that the speaker is glued or taped on. Find away to take the speaker out, and you're done.
Personally, this solved my problem, with my phone which was the first phone that I actually paid money for ($50) about 2 years ago. It's been the best phone I've owned, the best looking, and most functional as well. The GUI was my only complaint. It was a bit slow, but that got mysteriously fixed after I flashed the phone. I'll post those instructions up later. Overall, I'm pretty happy that I extended the life of my phone.