Thremal Relief problem on a PCB

[Ares6541]

Hi all,

I have the manufactured PCB in my lab which was designed by myself a week ago. During testing the PCB functionality i came across few problems, which are as follows:

1. The board starts sending in garbage values after operating for five minutes or more.
2. During diagnostics i accidentally put one finger on a part of the PCB and the garbage value was gone. (Sounds weird)

What i am guessing is my board is getting over heated or something and when i put my finger on it the board cools down a bit and shows the actual data to be received.
I have tested it by blowing hot air on that portion of the PCB and the garbage value starts coming and when i put my finger on it after a while i start receiving the actual data.
I am using three AVR contorllers, two on the top and one on the bottom, borad is double layered and the portion which is creating the problem has two capacitors and bunch of vias.

I am also attaching a snapshot of that portion which is creating problem. Its just left of the Two SMD capacitors with a bunch of vias above.



Need your help.

Thanks alot

 

[keith1200rs]

I am not sure why you think it is a thermal relief proble, or thermal (the two things are not the same). I would suspect a bad connection, floating inputs or bad clock oscillator operation. How hot do the chips get in normal use?

Keith

 

[Embedded_Geek]

Or is it a problem due to improper grounding???

 

[praveenlb]

It may be problem high current flow in that path..try to put more thermal via in copper plane area..

 

[Mattylad]

I too would be looking at poor grounding, poor sheilding of sensitive signals so your getting crosstalk, faulty via, even an incomplete net/bit of ground not actually connected to the rest of the gnd net.

Temporarily fixed by you adding yourself to ground that area perhaps?

 

[marce]

By you explanation I would suspect, Thermal problems, these are becoming more common as PCB density increases. Blow cold air over the devices and see if the signals improve (freezer spray is available to do this).
Knowing the exact devices would help, quite often devices are available in more thermaly efficient packages, this would also require thermal design considerations when doing the PCB layout. Initialy having a four layer board with 1oz power and ground layers would help both the quality of grounding and the thermal dissipation.
My first question on your design is why have you thermal relief on vias, it is not required and would imporove the ground copper pour. Also some of the vias look very close together with very little space. On these packages (quad flat packs) I would suggest using a decoupling capacitor that fits under the row of pins as this is dead area, plus you can reduce the decoupling loop.