PCB is one of the most functional inventions we’ve had in the field of electronics and manufacturing at large. It greatly helped in the area of cost-effectiveness and prototyping. But for us to come to terms with the various test carried out on a Printed Circuit Board (PCB), we need to understand what it really is in the first place.
What is a PCB?
In its most basic form, a printed circuit board is an electrical board that connects a series of points to each other. The board, in practice, permits signals and power to be transferred between devices that use electricity.
Every PCB has the following main components;
- Solder mask,
- The baseboard material made of Substrate (such as FR4)
Before the advent of the printed circuit boards, circuits were designed through a backbreaking process of point-to-point wiring. Those who have attempted this process in recent times can admit that it is prone to have failures at wire junctions, and most times will experience the sad phase of wear and tear.
With time, the necessity for circuits to be miniaturized and more affordable steered manufacturers to look for alternative approaches. This gave birth to the printed circuit boards.
Why Do We Test PCBs?
It is already a known fact that electrical and electronic products are prone to have flaws and errors in the system, even if the system is properly designed by the best of the best. When these failures occur, these products are usually recalled or discarded.
Now, it is easy to conduct a visual probing of PCB after construction has been completed, but this can be done on a two-layer PCB. For a multiple-layer PCB where connections are intricate and sophisticated, this is not the best course of action.
For a company to save hundreds of millions of dollars in damages, it is pertinent to run a series of tests on the printed circuit boards. This will result in early identification of flaws and ensure error-free productions.
We will be briefly looking at the various test that should be conducted on the PCBs before the product can be released for consumption.
Bare board tests are done on a free board to ensure that the circuit connection defined is properly noted on the circuit boards. This process usually involves using a multimeter able to carry out a continuity test, a highlighter pen, and a netlist printout. These tools are used to determine if the circuit is ‘short’ or ‘open’. With a minimum of $15 in purchasing these items, the test can be carried out successfully.
Assembly Level Test
This is a test done to check the functionality of the circuit after the assembly has been done. These tests are either Manual Visual Inspection (MVI) or with the use of Automated Test Equipment (ATE). This equipment is quite expensive with products ranging from $3,000 to $12,000. It includes a variety of testing devices such as Automated Optical Inspection AOI), Automated X-ray Inspection (AXI), etc.
In-circuit Test (ICT)
In-circuit test (ICT) is an electrical probe that not only checks for shorts or opens, but verifies the resistance, capacitance, and other component values which will determine if the PCB assembly was properly done.
This test is also used to note the functionality of the Integrated Circuits (ICs). It is performed with a bed of nails tester—a traditional test tool with numerous pins inserted at every net end. It could be a hole or a surface mount pad. The pins are usually 35mm in length. The test takes about 7 seconds, once everything is set up.
The cost of conducting this test can amount to $50.
JTAG Boundary Scan
This is a test that makes use of the Integrated Circuit’s test algorithm to check whether the product is properly soldered and inserted onto the board. It is also used to check the system level test, BIST access, memory testing, flash and CPLD programming and CPU emulation.
The cost for conducting this test ranges from $40 to $70.
This is the final test done on printed circuit boards. This test is aimed at ensuring that all the parts of the PCB are present and if the board is properly assembled, and to check if the circuit is function according to the design and specifications. After a functional test is completed, massive production can then take place.
These tests are not limited to:
- Peel test: This is done to ascertain the measure of strength required to peel the laminate from the board.
- Solder float test: This is done to determine the degree of thermal stress that the holes of a PCB can resist.
- X-ray Inspection: This is carried out to check the internal workings of the board, which includes all the layers. X-ray inspection is also done to check the authenticity of the product.
Common Terminologies in PCBs
In the course of carrying out tests on printed circuit boards, there are certain terminologies you might come across.
- Net. This is a connection of points from the first source point to the last target point. This includes lands and vias.
- Net List. This is a term used to describe a group of two, three or more points that common electrically.
- Mid Net Point. This is a point on the board that is neither the origin nor the target point of interconnected networks.
- Leakage. This is the flow of current between two conductors in the board even though they are not physically connected. The components should be isolated by a significant amount of resistance.
- Opens Test. This is done to make sure there is current flow from one node to the next, for every net on the board, by measuring the resistance of the conductor.
- Shorts Test. This is done to check sure there is no current flow between separate nets. This process is done by measuring the resistance between them.
- Node. This is a connection point between a network on the board and a component or a test probe.
- Isolated Land. This is a single point test on the board.
- Land. This is an area of conductive design used for connection or/and component attachments.
- 100% Net List Test. This is a check of every node on every net on the circuit board.
Even though you are confident that your device can perform optimally, you need to ensure that the functionality of your device meets your desired requirements. In essence, a PCB test is mandatory, not necessary.