Motherboard Construction

Hi,

I am slowly working my way through building a Mendel and am working on the electronics. It's a step into the dark for me as it's my first attempt at working with SMD components. I tried the reflow technique used in the wiki but have not been successful in creating a working board. I wonder if anyone on here can provide any assistance?

During the build process I ran into several problems. Firstly, it would appear I cranked the heat on the stove up a little too much, just as the solder started to melt a bubble appeared on the board, along with a burning smell. I killed the heat and the board returned to normal. Despite this incident the reflow appeared to work quit nicely with the solder forming nice joints. The only apparent problem from the bubble forming was that it had severed one of the bridges between the two PCB layers, but it was quite easy to repair with a piece of wire an a soldering iron. I double checked the rest of the board with a multimeter and there appeared to be no further problems.

Is it possible that I have cooked some of the components? I'm most concerned for the ATMEGA644P as the bubble formed within two centimetres of this microchip.

I then did a check for bridges and any other problems before hooking the board up to a USBTiny programmer. The power LED on the board lights, but when I try to load the boot loader it returns an error (rc=-1), which I understand to mean that the programmer can't communicate with the micro controller. I know the programmer is working as the red light flashes when is tries to communicate.

On further examination of the board I found a bridge across the 16Mhz crystal which I rectified. Would applying power to the board with this bridge have done any damage?

I tried to burn the boot loader again but still got the same error. I was loading the boot loader from Mac OS X, and for belt and braces tried loading using a PC running XP but get the same error, so at this stage I have ruled out the computer and programmer and suspect a problem with the board. I have triple checked all the component orientations and soldering joints.

Anybody got any ideas?

Regards,

Milly.

Milly Wrote:
-------------------------------------------------------
> Hi,
>
> I am slowly working my way through building a
> Mendel and am working on the electronics. It's a
> step into the dark for me as it's my first attempt
> at working with SMD components. I tried the
> reflow technique used in the wiki but have not
> been successful in creating a working board. I
> wonder if anyone on here can provide any
> assistance?
>
> During the build process I ran into several
> problems. Firstly, it would appear I cranked the
> heat on the stove up a little too much, just as
> the solder started to melt a bubble appeared on
> the board, along with a burning smell. I killed
> the heat and the board returned to normal.
> Despite this incident the reflow appeared to work
> quit nicely with the solder forming nice joints.
> The only apparent problem from the bubble forming
> was that it had severed one of the bridges between
> the two PCB layers, but it was quite easy to
> repair with a piece of wire an a soldering iron.
> I double checked the rest of the board with a
> multimeter and there appeared to be no further
> problems.
>
> Is it possible that I have cooked some of the
> components? I'm most concerned for the ATMEGA644P
> as the bubble formed within two centimetres of
> this microchip.

Yes if you got it hot enough to de-laminate the fibreglass then you may well have damaged some of the components. LEDs are the most vulnerable followed by any other semiconductors.

The recommended thermal profile for surface mount is quite strict. No only is there a peak re-flow temperature you have to achieve but not exceed more than about 20C (with unleaded solder), but also there are minimum and maximum rates of heating cooling of between about 1C and 3C per second. Some more info here: [hydraraptor.blogspot.com]

Hobbyists often advocate using cookware, but is very unlikely that the correct profile can be followed without a control system, so there is always a high risk of damaging components.

>
> I then did a check for bridges and any other
> problems before hooking the board up to a USBTiny
> programmer. The power LED on the board lights,
> but when I try to load the boot loader it returns
> an error (rc=-1), which I understand to mean that
> the programmer can't communicate with the micro
> controller. I know the programmer is working as
> the red light flashes when is tries to
> communicate.
>
> On further examination of the board I found a
> bridge across the 16Mhz crystal which I rectified.
> Would applying power to the board with this
> bridge have done any damage?

No it is unlikely that would cause any damage if it was just across the crystal.

>
> I tried to burn the boot loader again but still
> got the same error. I was loading the boot loader
> from Mac OS X, and for belt and braces tried
> loading using a PC running XP but get the same
> error, so at this stage I have ruled out the
> computer and programmer and suspect a problem with
> the board. I have triple checked all the
> component orientations and soldering joints.
>
> Anybody got any ideas?

If you are sure there are no shorts and opens and power is getting to the chip then I would suspect that it needs to be replaced.

>
> Regards,
>
> Milly.

Edited 1 time(s). Last edit at 11/28/2010 08:38AM by nophead.

---

[www.hydraraptor.blogspot.com]

Thanks for the reply, interestingly the SMD LED's were just as close to the area that bubbled and they are fine.

Is there a simple way to check the micro controller, using a multimeter or logic probe whilst it is still mounted on the board?

Milly.

Are you sure the solder joints are all good? I have never done work with 18+ pin SMDs, but on some SMD voltage regulators and caps I have seen bubbles of excessive flux cause bad joints. If the flux doesn't just burn away, it might look like a good solder joint but it isn't. In fact, this is what caused the power LED on my motherboard to not work. This is pretty rare though if there is enough solder on the pad when you started.

Thanks for the tip, I will go over the rest of the components with the soldering iron and see if it helps. I don't imagine the voltage regulator is being used for the boot loading, isn't the programmer supplying the 5v? I was able to check the voltages on the micro controller, all the pins that are meant to get 5v are recording it. I was able to watch the voltage of pin 5 drop to zero when the reset button was pressed. Starting to think the chip is fried!

Milly.

Yeah, Nophead is right. If it de-laminated the board, it probably got too hot, but you say the nearby LED didn't fry. On the off chance that the chip is still good, I was thinking maybe the communication pins didn't get soldered well enough.

I have never burned the boot loader. Is there some kind of acknowledgment message that would indicate something along the lines of "a chip was found, but communication failed"? If it at least knows a chip is there, that would be a good sign.

The rc=-1 error indicates the programmer can't find a chip. I have checked the resistance from the micro controller pins through to the header socket so I guess the solder joints are okay. Looks like I'm ordering a new 644p! The acknowledgement of a successful boot load should be on the screen, I'm not aware of the motherboard flashing any lights.

Thanks for the help!

Milly

Dazed,

In a last ditch attempt to salvage the board, and before I ordered the new microcontroller, I went round the board with my iron and remelted each joint. You were spot on, one of the joints must have been defective. It was a great feeling seeing the boot loader successfully load into the chip! I'm relieved as money is tight on this project, also this was my first stab at dealing with SMD's. Next time I will wind the temperature up a little more gently to try and avoid delaminating the board. Its amazing how much abuse these things will take and still work! Next step, load the reprap firmware!

Thanks again,

Milly.