PCB´s Design - How to proceed with the PCBs testing (part #3)
An electronic device which includes a PCB may not operate, so it needs to be somehow revived - tested.
At a PCB design it´s also necessary to consider possibilities of electrical testing. In such cases it is necessary to create some space where we can connect the sensor and measure the necessary parameters (voltage, current, etc.). This means that we need to test electrical functionality of the PCB. Proper sizing of conductive tracks width and the width of the isolating gaps between conductive connections has a significant impact on the current load capacity and voltage load capacity of the PCB conductors. Therefore it is necessary to know what is on that circuit current load, so there will not be overheating. It is also necessary to know what will be the largest tension between the two pads to avoid sparks and creating short circuits. These parameters need to also be applied to the design system.
For example - current load capacity is relatively high in comparison to wire leads, because a flat conductor features much higher cooling surface than a wire conductor. Copper wire with a 0,07 mm2 cross section will get melted at a current of 15 A, while a copper foil of the same cross section will get melted at a current of 60 A. This value approximately responses to 850 A/mm2 current density. However a continuous current load capability is lower, approximately 100 A/mm2. Maximum operating temperature of a PCB depends on a so called base material softening point what is for the most often used material FR4 approx. 125°C. From this reason, it´s necessary to size the track widths to meet this condition.
Value of a maximum allowable voltage among PCB tracks depends on many factors: isolation gap width, type of a used base material, usage of silkscreen and last but not least – operational and safety requirements for a PCB usage. Silkscreen helps maintaining basic PCB properties when exposed to harsh environment like dust and humidity. We differentiate breakthrough voltage and a maximum operation voltage.
At this parameter we should know where the board will be given to use and under what conditions. Values of these voltages and methods of their testing are subjects of regulations.
How to test the PCB?It´s worth to implement test pads (points), where testing jigs can be connected. If an automated test is possible, testing points should be placed in a matrix of a testing adapter. Placing components on a PCB should also suit needs of a visual inspection. For this purpose it´s necessary to maintain minimum components distances.
Recommended minimum distances between components on a PCB must be kept also because of a possibility of automated components placing by machines (minimum distance to a PCB border to grip the board in a machine) or a possibility of a manual placing.