Own Custom Build Computer With ECS Motherboard-No POST

[LENOVO-A880]

Re: Own Custom Build Computer With ECS Motherboard-No POST

...686-4954 should work as replacement.

Hi Agent24,

Do you have any idea about the correct soldering temperature for ST75185CTR or 686-4954. It should be an SOP-20 ic if not mistaken. The datasheet does not provide any reliable information on the reflow temperature for the ic. Can you please help me on this one? I'll still try to work on it this weekend...

Thank you

[Agent24]

Re: Own Custom Build Computer With ECS Motherboard-No POST

Most parts can follow a pretty standard reflow profile. If there is nothing specified in the datasheet just go with the usual temperature, taking care not to exceed any maximum temperatures listed.

[LENOVO-A880]

Re: Own Custom Build Computer With ECS Motherboard-No POST

....so lately I resoldered the ST75158CTR, an SOP-20 ic but unfortunately I lifted 2 tiny pads on the motherboard while using my soldering iron with temperature control. The reason is that it doesn't have any auto-reset on the temperature setting and i kept increasing the temperature till it was around 400°C. This is a big mistake and I had previously messed with its' setting so the temperature is all the way out of range..(refer PIC-A)

...you can actually notice that I rewired the lifted pads by using 2 pieces of tiny wires all the way to the ST75185CTR which has the lifted pads on the other side of the motherboard. I had used a bit of epoxy glue too, in order to tack those tiny wires down firmly on the motherboard before soldering them to the ic pins...(refer PIC-B)

...The IC itself might have failed but also there could be an issue with its supply rails going out of spec or some part on an output pin is shorted to ground, like one of the capacitors on a data line maybe.

...I think I eventually found the culprit to the no POST problem and it's the capacitor EC29 which actually is the output feeder to the CPU logic (V_FSB_VTT ). Its' (-) pin is shorted to the GND. It's as well coupling the DATA line signal to the ic ST75185CTR. The capacitor which as well couple to this ic failed to work in this case and causing over voltage to the ic where it's responsible to pull logic low to 5VSB then 12V can be drawn from the PSU. (refer PIC-C). Capacitor EC29 can be found on PG.04 of the SCHEMATIC or here (https://www.badcaps.net/forum/showpo...count=11). For your information I've ordered stock no.520-1438 from (http://my.rs-online.com/web/p/alumin...32303134333826) to replace the EC29 on the motherboard

[LENOVO-A880]

Re: Own Custom Build Computer With ECS Motherboard-No POST

Hi Agent24,

I had previously managed to desolder one of this array capacitor (flat pins) that ties to the ic ST75185CTR for pulling the logic level low as motherboard drive PS_ON# normally with this open collector driver that can pull the signal level down. However, unfortunately while trying hard to remove it from the motherboard, a few pins of the array capacitor have been destroyed by overheating them. Here, I would really like to know if you have any idea of what kind of tip should be used to rework this type of array chip. I've had a magnifier with me while I tried to resolder the chip but not that efficient cause the soldering tip of my iron is large conical tip...so do you have any recommendations on what type of tip should be small enough to be prepared and perform this job as I couldn't find any useful method anywhere else for this one..

THANK YOU.

[Agent24]

Re: Own Custom Build Computer With ECS Motherboard-No POST

A small chisel tip may help, but don't get some needle-point tip, they don't carry heat very well and actually work worse.

I find doing 0603 size or so components that a 1.2mm chisel is good, but even that doesn't work on one of those arrays very well.

Last time I soldered one of those arrays I used solder paste and hot air rework. You may be able to get away with a soldering iron if you use a 1.2mm chisel tip and LOTS of flux. Flux is one of the key things here.

[momaka]

Re: Own Custom Build Computer With ECS Motherboard-No POST

Here, I would really like to know if you have any idea of what kind of tip should be used to rework this type of array chip.

You can do this two ways without hot air:

1) By using two soldering irons...
Bridge all of the pins on the array component and heat the bridged pins on each side with each iron. Then slide the component off. Relatively easy method. That said, if your irons have pencil-shaped tips, you still might struggle a little because those tips have poor heat transfer. Get chisel- or bevel-shaped tips *OR* if the tips of your irons are made from copper, then sand the tips down to proper chisel shape.

2) Use a large spoon-shaped tip (large enough to cover the entire array component)...
Like method 1 above, bridge all of the pins of the array component first. Next, fill the spoon-shaped tip with solder. Finally, put some flux on the bridged pins of the array component and put the spoon-shaped tip on top of the array component. Let the solder melt and carefully slide the array component away from it's position or use tweezers to lift it while still heating with the iron.

I use both methods 1 and 2 above, depending on how big the component is. I have a 3.5 mm spoon-shaped (carved in) tip on my regular 30 Watt soldering iron. Actually, that tip used to have a conical shape, like yours. But over the years, it became worn and "pitted" on one side. I then sanded it down into a chisel tip. When it became worn again, it got that spoon-like shape, which now I find is excellent for doing SMD work. I can remove SOT-23 components in seconds. Not hot air needed at all.