Capacitance Testing in PCB Electrical Performance Analysis
23 May 2025 17:46:07 GMT
PCBASAIL
Table of Contents
In PCB electrical performance testing, there are various testing methods, and the
capacitance testing method is a very important means. This article starts with the basic
principles of capacitance and introduces the application of the capacitance testing
method in PCB electrical performance testing. By comparing it with the ordinary
open-circuit and short-circuit testing method (resistance testing method), the
advantages of the capacitance testing method are identified, and practical cases are
analyzed to demonstrate the important role of the capacitance testing method in PCB
electrical performance testing.
1. Introduction
With the rapid development of industries such as electronic products, industrial
control, and aerospace, the electrical performance requirements for PCB products are
getting higher and higher, and correspondingly, the difficulty of electrical testing is
also increasing.
For PCB manufacturers, when facing large quantities of products, fixture needle bed
testing is mainly selected. Although there is a cost for manufacturing fixtures, it can
greatly save testing time. For samples, small-batch products, and HDI products with
dense patterns, flying probe testing is mainly used. On the one hand, it can save the
cost of fixture manufacturing. On the other hand, in BGA or MBGA with fine pitches,
fixture testing cannot guarantee the testing accuracy, so flying probe testing is an
indispensable testing means for an enterprise.
The common flying probe testing method is the open-circuit and short-circuit testing
method, also known as the resistance testing method. This method can meet the testing of
precision circuits, but the testing time is relatively long. It needs to carry out two
tests of open circuit and short circuit, and it often takes more than 10 minutes to test
one product, with very low testing efficiency. Therefore, we introduce the capacitance
testing method to change this situation.
2. Introduction to the Capacitance Testing Method
2.1 Basic Principles of Capacitance
The capacitance testing method carries out open-circuit and short-circuit testing based
on the basic principles of capacitance. The capacitance calculation formula is:C=4πdεS
where C is the capacitance; ε is the dielectric constant, which is determined by the material of the insulating medium and is a constant when the medium remains unchanged; S is the vertical area of the relative overlapping part between the two plates; d is the distance between the two plates.
where C is the capacitance; ε is the dielectric constant, which is determined by the material of the insulating medium and is a constant when the medium remains unchanged; S is the vertical area of the relative overlapping part between the two plates; d is the distance between the two plates.
2.2 Basic Principles of the Capacitance Testing Method
As shown in Figure 1, in the capacitance testing method, according to the capacitance
calculation formula, the insulating medium ε
is the dielectric constant of the board material. One pole of the two poles is the
entire large testing table, and the other pole is the copper surface of the testing
network. Therefore, the value of S
depends on the size of the copper area of the testing network; when the product is of
the same model, the board thickness d
is also a constant value. It can be seen that in the capacitance testing method, the
size of the capacitance value ultimately depends on the size of the copper area of the
testing network and is proportional to it. Therefore, this method is also called the
electrostatic capacitance comparison method.
Schematic Diagram of Capacitance Testing for PCB Products.jpg
2.3 Judgment Methods for Open and Short Circuits in the Capacitance Testing Method
2.3.1 Judgment Method for Open Circuits in the Capacitance Testing Method
If there is a break in the conductive pattern, the area of the conductive pattern will
decrease, and the electrostatic capacitance value will also become smaller. For example,
as shown in Figure 2, the electrostatic capacitance value measured between endpoint 1
and endpoint 2 in a qualified product is 100, while the electrostatic capacitance values
measured between endpoint 1 and endpoint 2 in a broken product are 70 and 30
respectively. By the change in the area of the conductive pattern, the position of the
break in the pattern can be accurately detected.
Schematic Diagram of Capacitance Testing for Open Circuit.jpg
2.3.2 Judgment Method for Short Circuits in the Capacitance Testing Method
If there is a short circuit in the conductive pattern, the area of the conductive
pattern will increase, and the electrostatic capacitance value will also become larger.
For example, as shown in Figure 3, the electrostatic capacitance value measured between
endpoint 1 and endpoint 2 in a qualified product is 100, and the electrostatic
capacitance value measured between endpoint 3 and endpoint 4 is 60. In a short-circuited
product, the electrostatic capacitance values measured at endpoints 1, 2, 3, and 4 are
all 160. By the change in the area of the conductive pattern, the position of the short
circuit in the pattern can be accurately detected.
2.3.3 On-Site Simulation of Open and Short Circuits in the Capacitance Testing Method
As shown in Figure 4, it is the simulation diagram of the flying probe capacitance
testing in our company. Our company adopts horizontal flying probe capacitance testing,
which can play a good adsorption role on thin substrates. Between the conductive
patterns on the substrate and the electrodes (Sensor Board) used for electrical
performance inspection, there is an electrostatic capacitance proportional to the
pattern area. Suppose that the electrostatic capacitances measured for the circuit
endpoints A, B, C, D, E, F, G, H, I, J, K, L, and M are: A = C = H = K = L = 100, B =
10, D = G = M = 90, E = F = 30, I = J = 30.
Capacitor Test Simulation Diagram.jpg
When a break occurs in the pattern line, as shown in Figure 5, the measured
electrostatic capacitances are: A = C = H = 70, B = 10, D = 20, E = F = 30, G = M = 70,
I = J = 30, K = L = 30. It can be found that the capacitance values of A, C, H, K, L, D,
G, and M become smaller, and it can be determined that the networks where these
endpoints are located have open circuits.
Capacity measurement.jpg
When a short circuit occurs in the pattern line, as shown in Figure 6, the measured
electrostatic capacitances are: A = C = H = K = L = 100, B = 10, D = E = F = G = M =
120, I = J = 30. It can be found that the capacitance values of D, E, F, G, and M become
larger, and it can be determined that the networks where these endpoints are located
have short circuits.
Capacitor Test Short-Circuit Simulation Diagram.jpg
3. Comparison between the Capacitance Testing Method and the Ordinary Resistance Testing Method
The resistance testing method measures the impedance between two points and needs to
confirm the conduction between endpoints in the same network and the non-conduction
between different endpoints. Because it is necessary to confirm the non-conduction of
all combinations between different lines, the number of inspections is very large. The
capacitance testing method, by measuring the electrostatic capacitance (area) of all
circuit endpoints at one time, can confirm short circuits and open circuits at the same
time. Therefore, the number of inspections is greatly reduced compared with the general
detection method, sometimes by more than 90%.
For example, when the total number of lines is 100 and the total number of endpoints is
500, we compare the number of tests between the resistance testing method and the
capacitance testing method. As shown in Table 1, the total number of tests by the
resistance method is 5350, while the total number of tests by the capacitance method is
only 500. From this example, we can see that the capacitance testing method has a great
advantage in testing time.
Table 1: Comparison of the Number of Tests between the Resistance Method and the
Capacitance Method
| Resistance Testing Method | Capacitance Testing Method |
|---|---|
| Number of Conductive Tests: All endpoints within the same network: n=500−100=400 times | Conduct capacity tests on all endpoints and simultaneously detect short-circuits and open-circuits based on area changes. n=500 times |
| Number of Non-conductive Tests: nCr=100×(100−1)/2=4950 times | n=500 times |
| Total Number of Tests: 5350 times | Total Number of Tests: 500 times |
As shown in Table 2, it is the detection time of a certain product by the capacitance
testing method in our company. This product has a total of 364 lines, 1207 endpoints,
and 124 independent pads. A single device has two probe heads and can test two products
at the same time.
Table 2: Detection Time Data of Capacitance Testing
| Substrate Type | Detection Status | Number of Work Points (steps) | Detection Time | Unit Detection Time |
|---|---|---|---|---|
| FR-4 Four-layer Board | Detection of Two Boards Simultaneously | 1207 | 119s | 0.049s/step |
From the data, it can be seen that the time required for the capacitance testing method
to test two products at the same time is 119s, which is equivalent to 1 minute per
product. For ordinary open-circuit and short-circuit testing, only the non-conduction
test requires 364×(364−1)/2=66066
steps. Plus the conduction test time, it takes at least 20 minutes to test one product,
which is more than 20 times the testing time of the capacitance method.
4. Special Role of the Capacitance Testing Method in PCB Electrical Performance Testing
It is well known that the detection effects of the capacitance testing method and the
resistance testing method are the same, both of which test the continuity of each
network in the product. However, the detection principles of these two testing methods
are different. So, can we make use of the special detection principle of the capacitance
testing method?
Usually, after electrical performance testing, we will analyze the defective products.
In general, the defects can be divided into two categories: short circuits and open
circuits. Among these two defects, we focus more on the analysis of open circuits
because the location of an open circuit may occur on the line or in the hole. When the
open circuit is in the hole, it is necessary to confirm the cause of the hole break.
When the cause of the hole break is a batch factor, the entire batch of products has a
hidden danger of failure, especially for HDI products, where the networks with blind
vias account for a large proportion. Once the blind vias are abnormal, the entire batch
of products will face scrapping. As shown in Figure 7, it is the three main sources of
open-circuit defects in the electrical performance testing of HDI products.
| Origin of Open-Circuit Defects in HDI Product Electrical Performance Testing | Remarks |
|---|---|
| Abnormal pattern transfer leading to open-circuit and AOI oversight | Isolated issue, normal products can be released |
| Scratches during product turnover after AOI inspection, resulting in open-circuit | Isolated issue, normal products can be released |
| Open-circuit in vias, generally blind via open-circuit | For blind via open-circuit, the cause needs specific analysis |
For a batch of HDI products with open circuits, the cause of the open circuit may be one
of the above or all three. If one of the causes of the open circuit is a blind via open
circuit, then this batch of products needs to be specifically analyzed, and the cause of
the blind via open circuit needs to be found to determine whether there is a risk of
failure during welding of the entire batch of products. However, how to determine the
blind via open circuit is a complex process. The ordinary resistance testing method can
only confirm that one or some networks have open circuits. To confirm the blind vias, it
is necessary to slice all the blind via positions in the network. If there are many
open-circuit products, it is necessary to slice all the open-circuit products one by one
for analysis, which will take a long time.
At this time, we can use the capacitance testing method to carry out electrical
performance testing on HDI products. Usually, the starting endpoints of HDI product
networks are blind vias. When a blind via has an open circuit, the capacitance value at
this point is very small, no more than 1fF. In a normally conducting network, the
capacitance value of the entire network is in the range of hundreds to thousands of fF.
Usually, the judgment criteria for open and short circuits are: when the tested
capacitance value is greater than (1+20%) of the standard capacitance
value, it is judged as a short circuit; when the tested capacitance value is less than
(1−20%) of the standard capacitance
value, it is judged as an open circuit. When a line has an open circuit, the measured
capacitance value is at least tens to hundreds of fF. Therefore, we only need to slice
the blind via positions with a capacitance value less than 1fF, and this position must
be a blind via open circuit. This method can greatly reduce our analysis time, and at
the same time, we can make a preliminary judgment on the number of blind via open
circuits and their proportion before slicing.
A company once produced a certain HDI product. The entire batch of products was tested
by the capacitance method, and finally, 5 open-circuit products were detected. Among
them, two had an endpoint capacitance value less than 1fF. The blind vias at the
endpoints of these two products were sliced, and it was confirmed to be a blind via open
circuit. The cause of the defect was analyzed as dust blocking the hole, which was an
individual problem, and the remaining qualified products could be released. From testing
to analysis to judgment, this product only took a short time, greatly reducing the
production cycle of the product and improving production efficiency.
Another HDI product produced by a company was also tested by the capacitance method, and
finally, 20 open-circuit products were detected. Among them, 8 had an endpoint
capacitance value less than 1fF. The blind vias at the endpoints were sliced, and it was
confirmed to be a blind via open circuit at the position. The cause of the defect was
that the buried hole was not blocked firmly, resulting in copper plating depression at
the position of the plate-in hole, so that the laser-drilled blind via did not hit the
bottom copper. This kind of problem has a great hidden danger to the electrical
performance of the product and is very easy to cause micro-connection between the blind
via and the buried hole. Therefore, this batch of electrically tested products, whether
qualified or not, were all scrapped. It is precisely because of the high efficiency and
accuracy of the capacitance testing method that we can quickly and accurately analyze
this problem, and also win precious time for re-manufacturing this batch of products.
Through the above two cases, we can find that using the capacitance testing method to
test HDI products can quickly detect the position of blind via open circuits, reduce the
analysis time of products, and thus improve production efficiency. On the other hand,
the capacitance testing method can accurately locate the position of defective blind
vias, find the problem directly through slicing, and then formulate improvement measures
in time according to the cause of the problem to avoid the occurrence of such problems.
5. Conclusion
(1) The testing of the electrostatic capacitance of the conductive pattern by the
capacitance testing method mainly depends on the area of the conductive pattern. If
there is an open circuit in the conductive pattern, the area of the conductive pattern
will decrease, and the electrostatic capacitance value will also become smaller. If
there is a short circuit in the conductive pattern, the area of the conductive pattern
will increase, and the electrostatic capacitance value will also become larger.
(2) The capacitance testing method tests the area of the conductive pattern, can carry
out open-circuit and short-circuit testing at the same time, and the two probes of the
capacitance testing machine can test two products at the same time, which is many times
faster than the ordinary resistance testing method in testing time.
(3) The capacitance testing method plays a special role in the electrical performance
testing of HDI products. If the starting blind via endpoint has an open circuit, the
capacitance value at this point is extremely small, and the open-circuit position in the
network can be directly determined as this blind via. The resistance testing method can
only detect that a network has an open circuit and cannot directly determine whether it
is a line break or a hole break, and a large number of slices are needed for analysis.
Therefore, the capacitance testing method can be widely used in the electrical testing
of HDI products.