4-Wire Kelvin
4-Wire Kelvin testing has been around for many years but using this type of measurement on bare PCB's is a relatively new requirement. The requirement for PCB 4-Wire Kelvin testing was originally requested by digital commercial OEMs in the US with the aim to set out to improve the overall quality of the products. The first 4-Wire Kelvin test requirement for PCB's were on a limited hole criterion. Since that time automotive companies in Japan have also adopted such requirements. Medical applications are also joining in with their own 4-Wire Kelvin requirement.
What is 4-Wire Kelvin test?
If you have used an ohmmeter to make resistance measurements, you have probably heard of terms such as 2-wire and 4-wire measurements.
When you use an ohmmeter to measure the resistance of a trace touching one lead to the one end of the trace and the other end to the other end of the trace completes the circuit and allows for current to flow through the trace. Once current is flowing, a simple Ohm's Law calculation can be used to calculate the resistance as indicated in Figure 1.
Figure 1
Figure 1 shows a conventional ohmmeter that forces current through the wire, it measures the voltage that develops, calculates the resistance, and displays the result.
Figure 2
Ohm's Law: Resistance = Voltage/Current
The lead resistance is added into the total resistance. In this example (Figure 2) 1 ohm lead resistance has been added into the final resistance of 2 ohms.
In the case of a PCB flying probe tester the circuitry would be similar to that shown in Figure 3.
Figure 3
A flying probe machine contains a high-speed ohmmeter that allows the user to quickly gather resistance values for several nets on the PCB. A 4-wire measurement system will perform a similar action with greater accuracy.
Figure 4
On a 4-wire Kelvin measurement system a constant current is forced through the force lines negating the current flow through the voltmeter. This completely isolates the voltmeter to create a highly accurate resistance measurement.
In the case of a PCB flying probe tester the circuitry would be like that shown in Figure 5. Two probes are placed on the same pad allowing the lead resistance to be removed from the measurement. Lead resistance in a conventional flying probe tester is the wire that runs from the probe to the measurement system.
Figure 5
4-wire Kelvin testing is a methodology where high resolution measurements are taken to determine finite changes in resistance. These finite changes in resistance can then be used to locate plating defects or variations in plating thickness. The kelvin test is highly accurate because of a four terminal system that negates all current sources, lead and contact resistances. This allows for the finite measurements to only be measured on the PCB circuitry. Typically, these measurements are in the milliohm range.
What can 4-Wire Kelvin detect?
Figure 6 shows examples of what the 4-Wire Kelvin Test can uncover. These types of defects may pass the standard Electrical Test. Changes in copper thickness do not affect resistance enough to cause a fault at the 5 or 20 Ohm standard electrical test continuity threshold.
Figure 6
How the 4-wire Kelvin test detects hidden PCB defects
4-Wire Kelvin test will find the milliohm resistance changes from hole to hole.
This looks very easy now however the test in production is far from perfect currently. Most systems on the market are very good at measuring what there is but not so good at calculating what should be there and determining a pass or fail from that stage. This leads to a complex process of determining a pass from a failure. The typical process would be to measure a few boards or panels and determine a running average on the resistance measurements. Any resistance values that fall outside a certain threshold would be flagged as failures. When the test is specifically targeting barrels, variations in the plating thickness can be detected relatively easily. When the test is performed on a trace, depending on the length and width, the variations can have a more profound effect on the measurement. The current comparison test is good, but far from perfect in this case.
Once a barrel has been identified as a failure the only way to know for sure if it is defective would be to perform a destructive test on the hole by cross-section.
Figure 7: Examples of failures detected by the 4-wire Kelvin test.
Figure 8: Via Resistance versus Plating Thickness
Why Gardien's Solution?
Your Gardien Local Service Centre will offer different types of hipot services. Each hipot is specifically tooled/fixtured to test a part using a combination of UCAM and Fixgen Software (link to Ucamco). The Fixture production process is controlled to ensure an on-time fixture delivery in Gardien's proprietary job flow system called Ontrack (link to Ontrack). This creates a seamless and error free flow of information from the customer supplied data to the Service Floors at Gardien.
Gardien's team is trained and qualified on all internal process as documented Quality Management System. The internal process has specific inputs for incoming, certified, and not good boards as well as descriptive educational programs on various board types, and surface finishes.
Gardien certifies each order processed with a Certificate of Compliance with details about how the order was processed, what specifications were used to certify the product, equipment used with calibration expiry date, team member who processed the order, quantity, and failure analysis.
Gardien strongly recommends that the test sequence and parameters are clearly stated on the manufacturing drawing or at a minimum agreed upon during the quoting or contract realization phase. In addition, any PCB sent to us for processing should have a unique identifier on each peace for electronic traceability.