We have been developing a product to work outdoors.
During the winter months we have had no problems on our product and now in the summer the following problem came up:
Our product charges in a vehicle and we discovered that when the vehicle is outside and the product is charging the pcb resets. Once cooled down the problem is away.
Now all our microchips on the pcb is rated to work at 80 – 85 degrees celcius.
We took out the pcb and tested it again with a hairdryer. Heated up the product resets and the with more heat the pcb shuts down took it to under an aircon and while cooling down it started to work again.
Our PCB has the following circuits on Main Control Microchip IC which communicates with bluetooth chip. Also we have a MP3 decoder chip STA013 which communicates with an amp. Further more we have a lithium Ion charging circuit for the battery.
As all the components are rated for high ambient temperature we get resets of the pcb at around 50 – 60 degrees which is normal in a vehicle as our daily summer temperatures is around 30-40 degrees celcius.
With the scope we picked up that the MP3 and Microchip IC is normal. But on the MUte pin on the op amp we saw a drop once heated
I hope someone can help with this problem.
Best Answer
As jippie mentioned, reflow everything. Cold solder joints can wreck thermal efficiency.
Beyond that, lithium batteries are prone to fire hazards so they usually have thermal safety cut offs built in.
My assumption is that you are probably having a "thermal resistance" (Rth) issue. While from a distance, your board probably appears safe, up close, individual components are heating up on a much larger scale. Heatsinks and larger ground planes can improve this, as well as, thicker copper pours for the planes.
If you know the power draw of the failing components, you can easily calculate their temp by multiplying the power by the thermal resistance and adding it to the ambient temp. If you look at the attached datasheet, it gives a thermal resistance of 417 C/W. That means that with a 20 degree safety region between ambient and cutoff, you could only run it at 47mW before failure...
http://www.onsemi.com/pub_link/Collateral/MMBT3904LT1-D.PDF