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PCBA Test

PCBA Test – The Key to

Guarantee PCB functional
by SMT Test

What is PCBA Test?

PCBA SMT processing is very complicated and includes multiple

important processes, such as PCB board manufacturing process,

component procurement and inspection, SMT assembly, DIP, and PCBA

testing. Among them, PCBA testing is the most critical quality

control step in the entire PCBA processing process. The

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testing determines the final performance of the product. So what are

the PCBA test formats?

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1. The ICT test mainly includes circuit continuity, voltage,current

values, fluctuation curves, amplitude, and noise.

2. FCT testing requires IC programburning, simulation testing of the

entire PCBA board function, discovery of hardware and software

problems, and equipped with

necessary SMT processing production fixtures and test racks.

3. Aging test is mainly to energize the PCBA board

and electronic products for an extended period. The test

continuously keeps the product in operation and observe any

failures. After the aging test, the electronic products can be sold

in batches.

4. The fatigue test mainly takes samples of the PCBA board and

performshigh-frequency and long-term operation to observe

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whether there are failures and determine the probability of

failure in the test. This is to provide feedback on the operational

performance of the PCBA board in the electronic product.

5. The test in harsh environments mainly exposesthe PCBA board

to extreme temperature, humidity, drop, splashing, and

vibration. It obtains the test results of random samples to infer

the reliability of the entire PCBA board batch product.

The PCBA process is complicated. Various problems may occur during

the production and processing due to improper equipment or

operation, and there is no guarantee that the products produced are

qualified. Therefore, PCB testing is required to ensure that each

product does not have quality problems.

How to test PCBA after SMT ?

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Several main common methods of PCBA testing:

1.Manual Test

Manual testing is to directly rely on visual inspection to confirm

the placement of components on the PCBA. This method is widely

used. However, there are many components in the PCBA, and

most of them are very small, which makes this method less

applicable. Some functional flaws are not easy to detect, and data

cannot be easily collected. Due to this, more professional testing

methods are needed.

2.Automatic Optical Inspection (AOI)

Automatic optical inspection, also known as automatic visual

inspection, is carried out by a special inspection instrument and is

used before and after reflow. It has a relatively good effect on the

polarity inspection of components and is a common method. But

this method is more difficult in identifying a short-circuited PCBA.

3.Flying Probe Test

The flying probe testing has been generally welcomed in the past

few years due to advancements in mechanical accuracy, speed,

and reliability. The test system’s current requirements include fast

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conversion, low-volume manufacturing, and no fixture capabilities

required for prototype manufacturing, making the flying

probe testing the best choice.

4.Function Test

This is a test method for a specific PCB or unit, which is done by

special equipment. Functional testing mainly includes the final

product test and the latest entity model (Hot Mock-up).

5.Manufacturing Defect Analyzer (MDA)

This test method’s main advantages are low initial cost, high output,

easy follow-up diagnosis, and fast and complete short circuit and open

circuit tests. The disadvantage is that the test cannot detect functional

problems. There is usually no test coverage indication, fixtures must be

used, and the test cost is high.

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PCB design and ICT test

PCB designs will be more complicated and difficult as electronic

products get lighter and thinner. In addition to the need to balance

functionality and safety, it also needs to be producible and testable.

When designing the PCB, it is necessary to consider setting up ICT

test points. The following are the precautions for ICT testing in PCB

design:

 Although there is a double-sided ICT fixture, it is best to

place the measured points on the same side.

 Priority stepsof tested points: A. Test-pad, Component

Lead, C. Through-hole (Via).

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 The center distance between the two measured points or

the measured point and the pre-drilled hole should not be

less than 0.050″ (1.27mm). It is better to be largerthan

0.100″ (2.54mm), followed by 0.075″ (1.905mm).

 The measured point should be at least 0.100″ away from the

nearby parts (on the same side). For parts higher than 3m/m,

the distance should be at least 0.120″.

 The measured points should be evenly distributed on

the PCB surface to avoid local high density.

 The diameter of the measured point should be no less than

0.035″ (0.9mm). If it is on the upper needle board, it should

be no less than 0.040″ (1.00mm). The shape is preferably

square,and the measurable area is increased by 21%

compared to the circle. The measured points less than

0.030″ need additional processing to correct the target.

 The pad and via of the tested points should not have solder

mask.

 The measured point should be at least 0.100″ away from

the edge of the board or folded edge.

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 The thickness of the PCB must be at least 0.062″ (1.35mm).

PCBs with a thickness less than this value can be bent

easilyand require special treatment.

 The diameter of the tooling hole is preferably 0.125″

(3.175mm). The tolerance should be “+0.002″/-0.001″and

the position should be at the corner of the PCB.

 The position tolerance between the measured point and the

positioning hole should be +/-0.002″.

 Avoid placing the measured point on the SMT part. Not only

is the measurable area too small and unreliable, but the part

will damage easily.

 The measured point should not be greater than 0.170″

(4.3mm),and the aperture should be less than 1.5mm, or

else it requires special treatment.

Information required for ICT fixture production:

 Layout CAD File: For example: PCADR–>* .pdf PADSR–> *.asc

 One blank PCB board

 Bill of material (BOM list)

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 PCB diagram

Precautions for ICT fixture and PCB Layout:

 Regardless of its shape, every copper foil needs at least one

testable point.

 The order of consideration of test point locations:

1. ACI DIPparts feet are prioritized as test points.

2. The exposed part of the copper foil (test PAD).

3. Vertical parts DIP feet.

4. Through-hole, but must not have mask.

 The diameter of test point:

1. Above 1m/m, the testing effect can be achieved with general

needle control.

2. Below 1m/m, a more precise probe must be used, but it will

increase the manufacturing cost.

3. PAD must have good contact.

 The shape of the test point can be round or square.

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 The distance between points must be greater than 2m/m

(center point to center point).

 Requirements for 2 layer PCB– Focus on the ability to make

single-sided testing:

1. The SMD surface trace must have at least one through-hole

to penetrate to the dip surface in order to serve as a test

point for testing from the dip surface.

2. If the through hole requires a mask, consider laying the test

pad beside the through hole.

3. If it cannot be made into single-sided, it is made by

double-sided fixture.

 If the empty foot is within the allowable range, testability

should be considered. If there is no test point, One must be

set.

 It is better to have a jumper for a back-up Battery, which can

effectively isolate the PCB during the ICT test.

 Positioning hole requirements:

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1. Each piece of PCB must have two positioning holes, and no

tin is allowed in the holes.

2. Choose the diagonal and the farthest two holes as

positioning holes.

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PCBA Test Equipment

PCBA test equipment includes:An In-Circuit tester, functional tester and

aging tester.

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These test equipment are common in the PCBA process. The PCBA test

in the processing link can ensure that the PCBA board meets the

customer’s design requirements and dramatically reduces the repair

rate.

1.In-Circuit tester

ICT is an automatic online tester with a wide range of applications

and simple operation. The ICT automatic online detector is mainly

for production process control and can measure resistance,

capacitance, inductance, and integrated circuits. It is particularly

effective for detecting open circuit, short circuits, and component

damages, etc., with accurate fault location and convenient

maintenance.

The most basic instrument used in electrical testing is the online

tester (ICT). The traditional online tester uses a special needle bed

to contact the components on the soldered circuit board and uses

hundreds of millivolts and 10 mA. It performs a discrete isolation

test with the internal current to accurately measure the missing,

wrong, and parameter value deviations of general and special

components.

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It measures components such as resistance, inductance,

capacitance, diode, triode, thyristor, field effect tube, integrated

block, welding joints, open circuit boards, short circuit faults. It

accurately tells the user which component is faulty or where the

open and short circuit is located.

The advantage of the needle-bed online tester is that the test

speed is fast. It is suitable for testing a single variety of civilian

home appliance circuit boards in mass production, and the

hosting price is relatively low. However, as circuit boards’

assembly density increases, there are some insurmountable

problems with the needle bed online tester.

This is because of the fine-pitch SMT assembly, new product

development, production cycles are getting shorter, and there are

more and more types of circuit boards. Also, needle bed fixture

production, long debugging cycle is expensive; for

some high-density SMT circuit boards, test accuracy problems can

not be tested, etc.

ICT has improved in recent years has overcome the limitations of

modern technologies. For example, when integrated

circuits became too large to provide detection targets for

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comparable circuit coverage, ASIC engineers developed

boundary-scan technology.

Boundary-scan provides an industry-standard method to confirm

component connections where probes are not allowed. The extra

circuit is designed for the inside of the IC, allowing components to

communicate with surrounding components and displaying test

results in an easy-to-check format.

Another vectorless technique applies alternating current (AC)

signals through a bed of needles to the test component. A sensor

board is pressed against the component’s surface under test and

forms a capacitor with the component lead frame to couple the

signal to the sensor board. Lack of coupling signal means that the

solder joint is open.

Manual generation of test programs for large and complex boards

is time-consuming, but the emergence of automated test program

generation (ATPG) software solves this problem. The software is

based on PCBA, CAD data, and component specification libraries

assembled on the board.

It automatically designs the required fixtures and test procedures.

Although these techniques help to shorten the generation time of

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simple programs, the demonstration of high-node test programs

is still time-consuming and technically challenging.

The flying probe tester is an improvement of the needle bed

online tester. It replaces the needle bed with probes. The XY

mechanism is equipped with 4 heads that can move at high

speed with 8 test probes, and the minimum test gap is 0.2mm.

The test probe moves to the test point according to the

pre-arranged coordinate position program. According to the test

program, each test probe performs open/short circuit or

component tests on the assembled components.

Compared with the needle-bed online tester, the test accuracy

and minimum test gap greatly improved, and no special

needle-bed fixture is needed. The test program can be directly

obtained by the CAD software of the circuit board. However, one

disadvantage is the slow test speed.

2.Functional tester

FCT function test provides simulation operating environment such

as excitation and load to the PCBA board, which can obtain

various state parameters of the board to detect whether the

board’s functional parameters meet the design requirements.

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The FCT functional test items mainly include voltage,

current, power, power factor, frequency, duty cycle, brightness

and color, character recognition, voice recognition, temperature

measurement, pressure measurement, motion control,

FLASH, and EEPROM burning, etc.

ICT can effectively find various defects and failures during the PCB

assembly process, but it cannot evaluate the system’s

performance composed of the entire circuit board at clock speed.

The functional test can test whether the entire system can achieve

the design goals. It takes the tested unit on the circuit board as a

functional body, provides input signals to it, and detects the

output signals according to the design requirements of the

functional body. This test is to ensure that the circuit board can

function normally according to the design requirements.

Therefore, the simplest method for functional testing is to

connect a dedicated circuit board on an assembled electronic

device to the appropriate circuit of the device and then apply

voltage. If the device passes the test, the circuit board is qualified.

This method is simple and requires less investment, but it cannot

automatically diagnose faults.

PCBA Test Equipment

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3.AOI (Automatic Optical Inspection)

As the assembly density of circuit boards increases, the difficulties

of electrical contact testing has also increased. The introduction of

AOI technology into the testing field of SMT production lines is

also a general trend. AOl not only inspects the quality of welding,

but also inspect the quality of light boards, solder paste printing,

and patch quality. The emergence of AOI in each process almost

completely replaces manual operation, and it has a lot to do in

improving product quality and production efficiency.

When using automatic detection (A01), AOI automatically scans

the PCB through the camera, collects images, and compares the

tested solder joints with the qualified parameters in the

database. After image processing, it checks out the PCB

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defects and uses the display or automatic marking to identify the

defects. In short, it marks it out for repair by maintenance

personnel.

The current AOI system uses an advanced vision system,

including a new light-giving method, increased magnification, and

a complex algorithm to obtain a high defect capture rate at a high

test speed. The AOI system can detect the following errors;

missing components, the wrong polarity of tantalum capacitors,

wrong or skewed solder pin positioning, bent or folded pins,

excessive or insufficient solder, solder joints bridging, or

false soldering, etc.

In addition to detecting defects that cannot be detected by visual

inspection, AOI can also collect and provide feedback on each

process’s work quality and the types of defects in the production

process for analysis and management by process control

personnel. However, the AOI system also has shortcomings, such

as detecting circuit errors and invisible solder joints.

4.AXI (Automatic X-ray Inspection)

AXI is a new type of testing technology that has only emerged in

recent years. When the assembled circuit board (PCBA) enters the

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inside of the machine along the guide rail, there is

an X-Ray emission tube above the circuit board. The X-ray emitted

passes through the circuit board and is received by the detector

(usually a camera) underneath. Because the solder joints contain

a large amount of lead that can absorb X-rays,

compared with other materials such as glass fiber, copper, silicon,

etc., the X-rays irradiated on the solder joints are absorbed and

appear as black spots. Thus, the analysis of solder joints is quite

intuitive, as image analysis algorithms can automatically and

reliably inspect solder joint defects. AXI technology has developed

from the 2D to the current 3D inspection method. The former is a

transmission X-ray inspection method, which can produce a clear

image of the component solder joints on a single panel. Still, for

the currently widely used double-sided mounting circuit boards,

the effect will be very poor, resulting in a visual image of both

sides’ solder joints. They overlap and are extremely difficult to

distinguish.

The 3D inspection method uses layered technology. The beam is

focused on any layer, and the corresponding image is projected to

a high-speed rotation to make the image at the focal point very

clear. In contrast, the images on other layers are eliminated. As a

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result, the 3D inspection method the image is eliminated. The 3D

inspection method can independently image the solder joints on

both sides of the circuit board.

In addition to inspecting double-sided mounting circuit boards, 3D

X-Ray technology can also perform multilayer image “slicing”

inspection on invisible solder joints such as BGA (Ball Grid Array,

solder ball display). The top, middle, and bottom of the BGA

solder joint are thoroughly inspected. At the same time, this

method can also be used to measure through-hole (PTH) solder

joints to check whether the solder in the through-hole is sufficient,

thereby significantly improving the quality of solder joint

connection.

Future SMT test technology outlook

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Predicting which test technology will succeed or phased out in the next

two decades is not a simple task. It requires a summary of the past

and a clear understanding of future applications. Judging from the

development trend in recent years, the use of multiple testing

technologies, especially the combination of AXI and ICT testing, will

soon become the primary choice in this field.

As the current circuit boards are becoming more complex, the

traditional circuit contact test is greatly restricted. It is difficult to

diagnose defects through ICT tests and functional tests. With the

increasing density of most complex circuit boards, traditional testing

methods can only increase the number of test contacts of the online

tester. However, as the number of contacts increases, the cost of

test programming and needle bed fixtures also rises exponentially.

It usually takes several weeks to develop test programs and fixtures,

and it may even take more than a month for more complex circuit

boards. In addition, increasing the number of ICT contacts will

increase the number of ICT test errors and retests. AXI technology is

not affected by the above factors, and its coverage of process defects is

very high, usually up to 97%. The process defects generally account for

80%-90% of the total defects, and invisible solder joints can also be

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inspected. Still, AXI technology cannot test the defects and faults in the

electrical performance of the circuit.

Combining AXI testing technology with traditional ICT online testing

methods can complement each other. The combined SMT PCBA testing

technology is nearly perfect because each technology compensates for

another technology’s shortcomings.

X-rays mainly focus on the quality of solder joints. It can also confirm

whether the component exists, but it cannot confirm whether the

components, the directions, and the values are correct. On the other

hand, ICT can determine the components’ direction and value but

cannot determine whether the solder joints are acceptable, especially

components with solder joints at the bottom of the package, such

as BGA, CSP, etc. Figure 2 is a complementary diagram of the

inspection scope of AXI and ICT test methods.

It needs to be pointed out that with the development of AXI technology,

the current AXI system and ICT system can communicate between

platforms. This technology, called “Aware Test”, can eliminate the

repeated testing between the two. By reducing the redundant test

coverage of ICT/AXI, the number of ICT contacts can be greatly reduced.

This simplified ICT test only needs 30% of the original number of test

contacts to maintain the current high test coverage, while reducing the

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number of ICT test contacts can shorten ICT test time, speed up ICT

programming, and reduce ICT fixture and programming costs.

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