What Are the Best Tips for Printed Circuit Board Repairs?

For the production of printed boards, because many designers do not understand the production process of circuit boards, the circuit diagrams they design are only the most basic circuit diagrams and cannot be directly used in production. Therefore, the line files need to be modified and edited before actual production, not only to produce a film that can fit the production process of our factory, but also to produce the corresponding punching data, mold opening data, and production useful data. Other data. It is directly related to the subsequent production projects.

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Engineering and technical personnel should understand the necessary production technology, and master the relevant software production, including common circuit design software such as: altium designer, Pads2000,

The level of PCB engineering production can reflect the design level of the designer, and it can also reflect the production process capability and technical level of the printed board manufacturer. At the same time, because PCB engineering production integrates computer-aided design and auxiliary manufacturing, it requires extremely high precision and accuracy, otherwise it will affect the electrical performance of the final on-board electronics, and may cause errors in serious cases, which will lead to the entire batch of printed boards The product was scrapped, delaying the contract delivery time of the manufacturer, and suffered economic losses. Therefore, as a PCB project maker, you must always bear in mind that you have a great responsibility, do not take it lightly, you must be careful, serious, careful, and serious. When processing PCB design files, you should carefully check: Does the received file comply with the rules set by the designer? Can it meet the requirements of PCB manufacturing process? Is there an anchor? Is the line layout reasonable? Wire to wire, wire to element

Printed circuit board description

Dense PCBs, higher bus speeds, and analog RF circuits all pose unprecedented challenges to testing. Functional testing in this environment requires careful design, well-thought-out test methods, and appropriate tools to provide credible Test results, adequate preparation and careful screening tools will achieve more results with less effort.

Meaning of printed circuit board

Functional testing is becoming more and more important. However, as with online testing, technological developments and PCB design will limit the scope of testing. Although great progress has been made in the programming software environment to help overcome some of these difficulties, there are still many issues to avoid and more thorough preparations to successfully implement functional tests in accordance with your testing strategy. This article introduces some factors that should be considered for successful implementation of functional testing and strategies for coping with them. Functional testing of electronic products has a history of prosperity. In the late 1960s, it was the first automated test method. With the advent of online testing technology in the late 1970s, functional testing seems destined to give way to faster and faster online testing. . Today, however, the tide has changed. One problem with online testing is getting worse: detection methods. According to the analysis of the United States NEMI (National Electronics Manufacturing Organization), the number of nodes that can be detected by the end of 2003 will be basically zero. If detection cannot be performed, then online testing is almost useless.

Functional testing is increasingly being used in post-production line processes and even for mid-process testing, but its system and implementation methods are almost completely different from previous tests. Today's test systems are faster and more compact in most cases. Functional testing is useful for verifying the overall functionality of a product, maintaining calibration information, providing data to ISO9000 procedures, and guaranteeing high-risk products such as the quality of medical equipment. Indispensable.

Test methods for printed circuit boards

The test implementation method is affected by factors such as budget, output, and the design of the product under test (UUT). It is the last item that has the greatest impact on what can be measured, and budget and output will limit the test item. In order to get the highest possible fault coverage in the test, you must pay attention to the selection of components and PCB layout during the design stage. Unfortunately, this is not always the case. Eager to enter the market and intensive development often disrupt you. Wishful thinking.

Here is a preliminary analysis of how to deal with these restrictions. Some concessions that have to be made for testing (especially in the early stages of design) may affect the design, but make testing easier and increase test failure coverage. Please note that the following problems and suggestions are not faced or need to be solved by every test engineer. Many of these problems affect each other, so each problem should be evaluated and applied flexibly when needed.

What are the test requirements for the product under test?

Before discussing design, test system, software, and test methods, we must first understand the "object"-the product under test, not only the PCB or the final assembly itself, but also how much will be produced, expected failures, etc. Including: product categories

Structure (single PCB / pre-made PCB / final product)

Test Specification

Plan test points

Expected output (per line / day / shift, etc.)

Expected failure type

Obviously, the "budget" is ignored above, but only after knowing the above items can we determine how much it costs to test a certain product, and then start to discuss the funding issue after clarifying what it takes to fully test the UUT. Know how to make compromises to get the job done. After the initial report is completed, the company may give you a budget and wish you "good luck"-figuring out what you can do. At this time, "good luck" is really needed, but there are other things, which are listed below. some.

High density problem

On the surface, component density does not seem to be a problem for functional testing. After all, the main consideration here is to "get an input and get the correct output." Granted, it's a bit too simple, but that's the case. Apply a given stimulus signal to the input of the UUT. After a certain time, the UUT will output a specific series of data. Connecting to the I / O connector should be the only access problem.

But component density also has a certain effect. Looking at the PCB sample (or your own design) in Figure 1, you must first answer the following questions;

Need to access the calibration circuit?

Is it important to diagnose specific UUT components or specific areas?

If the answer to the above question is positive, is the investigation done by humans or by some kind of robot?

Do you want to use an automated test setup?

Is the I / O connector used easily accessible or connected? If not, is the connector a through-hole mount that can be accessed through the needle bed?

Let's discuss these issues one by one.

Calibration circuit

Functional tests are often used for calibration or verification of analog circuits, including checking the interior of the UUT (such as the IF portion of an RF circuit) to verify its operation. To do so, test points or test pads may be required. A problem with high-frequency design is the relative impedance of the test points (path length, test pad size, etc.) plus the impedance of the probe will affect the performance of the circuit. You should keep this in mind when setting up the test area. And the needle bed fixture (discussed later in this article) only needs a smaller test area, which can alleviate this contradiction. This is mainly because compared with manual operation, the accuracy of the automatic machine can make the tester detect a smaller area. .

Troubleshooting

If you just use the functional test as a pass / fail screen and do not need to measure the calibration points, you can skip this section because the application may not require a probe at this time. In most cases, the function test is passed / failed because the function test is very slow in diagnosing the fault, especially in the case of multiple faults. But in some industries, functional testing is going deep into the manufacturing process, such as cell phone manufacturing. Some manufacturers need to make certain key measurements at the PCB level, that is, during the assembly process before final assembly. This is caused by The nature of mobile phones is easily eliminated. In other words, phones are designed to be assembled at a lower cost, and they are not easy to disassemble, so verifying functionality before final testing can save rework costs and reduce possible scrap (because the phone will be damaged when disassembled).

Therefore, there are sufficient test points to probe the PCB. For example, it is not very convenient to check the J-shaped leads of a surface-mount device with a 20mil pitch, and BGA is even more impossible. According to the recommendations of the American Surface Mount Technology Association (SMTA), the minimum test point interval is 0.040 inches, and the interval between the pads depends on the component height, probe size, etc. around the test area, but the 0.200 inch interval should be the minimum requirement. Especially manual exploration areas. Obviously, test fixtures and robotic probes are more accurate.

PCB test design

Needless to say, a test-friendly design is easier to handle in production than a casual design. However, engineers usually want to install more technology in the smallest volume at the lowest cost. This idea increases the limit of contact with the circuit board in online testing and functional testing.

The market has also responded to this kind of problem. Existing software tools can analyze the design, review according to the rules stipulated by the assembly and test equipment, and make recommendations to make PCBs easier to produce. If these tools are applicable to your product, it is recommended that you analyze each design, at least it can quickly indicate where test contact problems are found, and its ultimate purpose is to make the product easier to manufacture.

Structural configuration to meet high density requirements

High density can be a small PCB size, a large number of circuits on the UUT, or both. The title above indicates that the mechanical and electrical structure of the system must be considered to meet the requirements of the test. The mechanical issues to consider are:

How to support UUT

Test area

Multilayer board testing (Can the tester do parallel testing?)

I / O connector

On the electrical side, if it is a multilayer board, which one is more economical? Is it a multi-instrument method or a switch converter with a small number of instruments? Depending on the UUT structure or the type of instrument required, the answer may not be easy to come by.

Automatic test or manual test?

As the output and speed of each production line increase (a major way to achieve economies of scale is to increase the productivity of each test device), consideration should be given to automating the test process. Automated functional testing actually saves loading / unloading time, does not need to add other test systems, and generally does not take into account the increased cost of transportation equipment when considering the increase in output.

Disadvantages of test automation include an initial hardware investment, time to integrate with the production line, whether the test system can keep pace with the production line speed, and problems that can be caused to production if the equipment fails. The offline tester does not directly affect the assembly line. If the tester fails, you can take the product out of the production line and continue production, so that the production line is not affected, but processing time and labor are also issues.

Keep in mind that manual testing may often involve connecting the UUT with several cables and connectors. These cables generally have a shorter life than the probes on a needle bed fixture, so they should be included in a maintenance plan, which can reduce time Severe failure.

Fixture problem

Due to differences in production line production, workshop space, and labor rate, fixtures can range from simple plywood with pins and connecting cables to complex fully automated needle bed test fixtures that are connected to the assembly line by a conveyor belt. Obviously, these factors indicate that there is no fixed solution.

A manually loaded double-sided fixture, a ribbon cable connected to the main I / O connector, and a probe mounted on the top can access key test points on the UUT. This is the ideal design for a medium-sized plant, where the operator must connect the ribbon cable, close the roof, and start testing. There is no need for manual exploration for calibration and diagnosis, because the top plate can access all relevant areas. Ribbon cables and top probe connections should be designed to be easily replaced because these cables are often bent and subject to wear.

When dealing with fixture suppliers, it's important to keep these issues in mind while thinking about where the product will be manufactured. This is something that many test engineers overlook. For example, we assume that the test engineer is in California, but the product is manufactured in Thailand. Test engineers will think that products require expensive automated fixtures because of the high cost of a California plant, requiring as few testers as possible, and the use of automated fixtures to reduce the need to hire highly skilled and highly paid operators. But in Thailand, these two problems do not exist. It is cheaper to solve these problems manually, because the labor cost is very low and the land price is cheap. Large factories are not a problem. So sometimes top-notch equipment may not be welcome in some countries.

Operator skill level

In high-density UUTs, if calibration or diagnosis is needed, manual exploration is likely to be required, due to limited needle bed contact and faster testing (UUTs with probes can quickly collect data rather than feedback information to the edge Connector, etc.), so the operator is required to probe the test point on the UUT. Wherever possible, make sure that the test points are clearly marked.

Probe types and common operators should also pay attention to issues that need to be considered include:

Is the probe larger than the test point?

Is the probe at risk of shorting several test points and damaging the UUT?

Is there an electric shock hazard to the operator?

Can each operator quickly find and check the test points? Are the test points large and easy to identify?

How long does it take the operator to hold the probe on the test point to get an accurate reading? If the time is too long, there will be some trouble in the small test area, such as the operator's hand will slide due to the long test time, so it is recommended to expand the test area to avoid this problem.

After considering the above issues, the test engineer should re-evaluate the type of test probe, modify the test file to better identify the test point location, or even change the requirements for the operator.

Automatic detection of printed circuit boards

In some cases, the use of automatic probing is required. For example, when it is difficult to manually detect the PCB or the operator's technical level limits the test speed, the automated method should be considered.

Automatic probing eliminates human error, reduces the possibility of short circuits at several test points, and speeds up test operations. However, be aware that there may also be some limitations to automated detection, which vary depending on the design of the supplier, including:

UUT size

Number of simultaneous probes

How close are the two test points?

Test probe positioning accuracy

Can the system detect both sides of the UUT?

How fast does the probe move to the next test point?

What is the actual spacing required by the probe system? (Generally speaking, it is larger than the offline functional test system)

Automatic probing usually does not use needle bed fixtures to contact other test points, and it is generally slower than the production line. Therefore, two steps may need to be taken: If the detector is only used for diagnosis, you can consider using a traditional functional test system on the production line, The detector is placed next to the production line as a diagnostic system; if the purpose of the detector is UUT calibration, the only real solution is to use multiple systems, knowing that this is still much faster than manual operation.

How to integrate into the production line is also a key issue that must be studied. Is there still room on the production line? Can the system be connected to the conveyor? Fortunately, many new detection systems are compatible with SMEMA standards, so they can work in an online environment.

Printed circuit board boundary scan

This technology should be discussed as early as the product design stage because it requires specialized components to perform this task. In UUTs based on digital circuits, devices with IEEE 1194 (boundary scan) support can be purchased, which can solve most diagnostic problems with little or no probing. Boundary scan will reduce the overall functionality of the UUT, because it will increase the area of each compatible device (each chip adds 4 to 5 pins and some lines), so the principle of choosing this technology is the cost The diagnosis should be improved. It should be remembered that boundary scan can be used to program flash memory and PLD devices on the UUT, which further increases the reason for choosing this test method.

How to deal with a limited design?

If the UUT design is complete and finalized, options are limited at this point. Of course, you can also ask for changes in the next revision or new product, but the process improvement always takes some time, and you still have to deal with it.

The main guideline here is how many tests you can do. It may be sufficient according to the type of failure expected, but if it is not enough, it usually needs to strike a delicate balance between more expensive test systems. After weighing UUT's cost of goods sold (COGS) and marginal profit, choose a more accurate detection method. So, the answer is that there is no simple answer.

The best reference for future design is the completion of functional tests when they are limited. When facing these limitations, you should record the tests that can be completed within the time range specified by the production line speed and the number of testers on the production line. The time limit is critical, because it is impossible to make yields to you, so your job is to sacrifice test coverage for time, so you need to improve so that you can lift these limits in the future!