Electrical Test

What is Electrical Test?

The term Electrical Test (ET) is widely used across various industries, but what does it really mean?

Electrical Test is an evaluation of the parametric, functional, or timing performance of a component when electrical power is applied. Parametric tests typically involve a DC source and measures current or voltage. The current or voltage is measured to validate that the connection point has the expected voltage or current, hence creating the ‘TEST’.

When the switch is closed in figure 1 current will flow causing the light to turn on. When the switch is not closed the light will turn off.

When the switch is open in figure 2 there will be no current flow

When performing Electrical Test on unpopulated PCB’s the process uses requirements that are documented on the fabrication drawing (master build drawing). These requirements will vary from part to part and should be set by the design engineer.

Unpopulated (Bare) PCB Electrical Test: This is a test that is run after the circuit board has been fabricated to make sure that all nets in the board have electrical connectivity. The test can also be performed in the fabrication process before the final solder resist coat is applied (solder mask). The test could also be performed on a single PCB layer or on a sequential lamination fabrication panel.

Electrical Test follows the base principals of Ohms Law. Ohm’s Law is a formula used to calculate the relationship between voltage, current and resistance in an electrical circuit.

V – Voltage

I – Current

R – Resistance

V= I x R

When Electrical Test is performed typically resistance is used as the base measurement so Ohms Law would be

When voltage is applied to the net current flows allowing the resistance of the PCB trace to be measured.

R= V / I

What is a short circuit?

The term Electrical Test (ET) is widely used across various industries, but what does it really mean?
Electrical Test is an evaluation of the parametric, functional, or timing performance of a component when electrical power is applied. Parametric tests typically involve a DC source and measures current or voltage. The current or voltage is measured to validate that the connection point has the expected voltage or current, hence creating the ‘TEST’.

What is an open circuit?

An open circuit is when the current flow is expected to be on a point, but the current flow has been interrupted by a break in the conductor. The Electrical Test process is designed to detect opens because the netlist knows what points should be connected. When an open or opens occur on an unpopulated PCB the boards can be repaired or reworked, or the PCB will be scrapped. Whereas, if an open occurs on an assembled board a jumper wire can be installed to correct the current flow.

What is a netlist?

The word netlist can be used in several different contexts, but perhaps the most popular is in the field of electronic design. In this context, a “netlist” describes the connectivity of an electronic design. Netlists usually convey connectivity information and provide nothing more than instances, nets, and perhaps some attributes. Netlists can be either physical or logical; either instance-based or net-based; and flat or hierarchical. The latter can be either folded or unfolded.

The flat file called IPC-356 by IPC International Inc (IPC) coming from the schematic drawing can be used to verify the PCB fabrication process by comparing the IPC-356 generated from the schematic drawing to the IPC-356 file generated from the PCB manufacturing data. If there are any discrepancies found they should be reviewed before the fabrication process is initiated.

When using a third-party Electrical Test provider this process can be enhanced by the provider using the original design files (Gerber, ODB++, DPF, etc) and comparing these files to the fabricators IPC-356 file.

Testable Points on a PCB

Electrical Test points on an unpopulated PCB can defined by any open surface finish area on the PCB that can conduct electricity. The Electrical Test process may utilize all open areas or perform some test point optimization to reduce production costs. The best practice to ensure the electrical integrity is met for your design is to clearly define how the test points should be assigned. If adequate test point coverage is not provided for in the design, the manufacturer may resort to other less expensive options.

All these areas may end up passing the minimum testing rules that you have set up for your layout, but they may not be the most optimum conditions for the final product. By taking the time to look at these areas from a functionality perspective, the final assembly will have the in-circuit test points (ICT) that will ensure a seamless final product. Clearly define the Electrical Test point coverage on your manufacture’s fabrication drawing.

Testable test points on a PCB should have a suitable conductive area that is free of non-conductive material like solder mask.

The above example shows a shift in the conductive area and may not be able to be tested on some fixed mechanical test types.

Possible Test Points on a PCB

Plated Through Hole (PTH)
Points A & B could have test points on them

Non-Plated Through Hole (NPTH)
Points C & D are not testable because there is no conductive material on the outer surface

Back Drill / Counter Bore
Point E is not testable because there is no conductive material
Point F could have test points

Hole 1- Micro via, point G could be tested
Hole 2- buried via, point H is not testable because the outer surface is not electrically conductive
Hole 3- PTH, points I & J could be tested

Surface Mount Technology (SMT), points K & L could be tested

Plated Slot 4, could be tested
Via hole 5, could be tested
Point N cannot be tested because there is not conductive material
Point M cannot be tested because there is not conductive material

Interested?

We offer our services under 2 distinct frameworks: the OnDemand Solution and the Integrate Solution.
Depending on your needs, you can either outsource specific testing requirements or we can completely run your test area or test department on your behalf. Contact us now and we’ll find the best solution for your business.