PCEA Chairman Stephen Chavez: How to Design Reliable Via
Stephen Chavez is a senior engineer at an aerospace company and chairman of the printed circuit Engineering Association (PCEA). Recently, he was interviewed by Andy Shaughnessy and happy holden to discuss how to design more reliable vias. During the interview, happy Holden and Stephen Chavez also discussed several aspects that need to be paid attention to to in order to improve reliability, and what steps designers should take to ensure the robustness of via, and introduced test resources and learning resources that PCB designers and design engineers should understand.
Andy Shaughnessy：Welcome. Almost every company will encounter via problems. Some OEMs send the same test board design with different sizes of through holes to different manufacturers, but they receive circuit boards produced in different ways. How can designers improve this situation? What's your opinion on reliability design of via?
Stephen Chavez：My first thought is to communicate with PCB manufacturers at the early stage of design, which will play a crucial role in the success of the whole PCB design. It is also a key step to discuss with PCB manufacturer when determining PCB stack. This provides an in-depth understanding of constraints, including the type of via technology used in the design, before placing components and wiring. By cooperating with PCB manufacturers and adopting the best method of PCB design, the probability of robust, highly reliable and manufacturable design will be very high.
PCB engineers are trying their best to cope with various challenges. If they want to design reliable PCB, there are three winning points: Design for layout solvability (DFS); design for performance (DFP) involving Si / EMC, power distribution and heat dissipation; design for manufacturability (DFM). The ultimate goal is a success. Through the maximum component placement density and wiring density, the best electrical performance, and efficient, defect free production, we can achieve high yield and low cost.
In response to the challenges of Multilayer PCB design, we should always keep these three winning points in mind, and the via strategy and the quality of via structure play an important role in them. No matter what kind of via technology is used in the design, whether it is standard PTH, HDI, micro via or the combination of these three technologies, PCB manufacturers are expected to obtain manufacturability design, while meeting the design requirements and DFM rules for downstream optimization. Otherwise, there will be many problems. I'm still surprised to find that there are so many OEMs who send designs directly to China PCB manufacturers instead of communicating with them.
You also need to know the materials you choose. Are appropriate materials selected for products and components? Is the original intention of the design for convenient production and high success rate, or do you want to design more complex? It is better to understand and skillfully use industry terms to communicate with PCB manufacturers to ensure consensus and avoid accidents.
Happy Holden： If the OEM wants to send the same PCB to multiple PCB manufacturers, they should not be surprised that the products sent back by each PCB manufacturer will be different, because each manufacturer optimizes the drilling, decontamination, metallization and electroplating processes to make them as reliable as possible, but their optimization methods are different. It is also because different PCB manufacturers will use different equipment and processes, so there may be more than 10000 kinds of arrangement and combination for each multilayer board. If OEMs really manage their supply chain in this way, the loss can only be inevitable. They should develop a statistically significant quality certification process, but if they don't, the manufacturer won't tell them it's unwise.
A lot of OEMs don't really understand reliability, so they will ask to spot check 15 circuit boards and check the cut plane. But this can not guarantee the reliability of the product at all, because this quantity is not statistically significant, unless only 15 circuit boards are purchased throughout the year. And it has been proved that the cut plane can not reliably show the real situation of interconnection, because you can only see the side through the hole on the cut plane, not 360 degrees.
I have provided a test plan for IPC PCQR2 to verify its reliability and quality. In order to pass the quality certification, the manufacturer must drill 250000 holes or laser drill 500000 blind holes. These circuit boards will be tested, and the reliability confidence level reaches 99.5%, that is to say, the probability of failure in the whole life cycle is less than 0.5%. Quality certification is required every year. If there are circuit boards with attached boards, they will use PCQR2 method to test the same circuit board for subsequent tracking.
Although this investment is very valuable, most companies do not want to carry out quality certification. I often cooperate with Asian PCB manufacturers who supply Intel chips. To pass the quality certification of Intel, we need to test 20000 circuit boards and modules randomly from a whole year's production batch. The test fee is paid by Intel. They need to know the situation at any time in the PCB production process, no matter which day of the week the PCB is produced, or whether the PCB is produced in summer or winter, because different seasons and different factors may lead to ups and downs of measurement results.
But some enterprises will invest a lot of money in reliability, such as Intel microprocessor packaging, they will invest money to let manufacturers do tests, so as to maintain a high degree of confidence in product quality. It is not feasible to randomly select 3 or 15 circuit boards for sectioning test to determine whether the manufacturer meets the requirements.
Do you think people really understand reliability requirements and statistical significance?
Chavez：I hope so, but there are still many people who do not know the details and are unwilling to pay attention to them. In many cases, the company will certify one or two specific PCB manufacturers. Once the model is put into production, they want to cut costs as much as possible, so they start looking for factories that can help them reduce costs. And this will make the previous plan fall short, because it can't ensure the quality.
Holden： But if you want to know more about vias, you can go to the articles published by Intel and some other aerospace companies around 2000, when they began to do quality certification for micro via manufacturers and adopted the pccr2 method of IPC, and they also provided batch reports. Each page is full of data, because the authentication process is to select the panel three times in a cycle of at least two weeks.
Chinese enterprises believe that after the production of 25 circuit boards, they only need to select 15 circuit boards with the best quality and send them out in three times within two weeks, five for each time, so that they can successfully pass the certification. But the test system is very sophisticated and complex. It will analyze each work order and detect when three purchase peaks are identified. One batch can only correspond to one peak. As a result, the enterprise failed to pass the quality certification for a whole year, so I asked about the working principle of the system and the reason for testing three different batches - in order to measure the difference between batches.
Maybe you think that only one batch can cheat customers, but you can't cheat these test equipment. PCQR2 is an excellent benchmark analysis tool and quality certification tool, and the cost is not high because of automatic testing. But most people have never heard of PCQR2, except those in the aerospace and military fields.
Shaughnessy：Have you ever had any problems with via?
Chavez：We are a large company with specialized technicians, experts and corresponding departments to verify the information. But do small companies have these resources? Will they learn from it and apply it to their own design? In some companies and in some cases, even if they have done a good job in benchmarking and said they have learned a lesson from it, this is not the case. In the one-stop departments of some companies, teams composed of few engineers are constantly omitting some steps. It is precisely because they have not learned from it, or they have not adopted the best method in the industry, that there are exceptions.
Holden：Strict quality certification process does not necessarily cost a lot, but can provide long-term help for the company. In the certification process, miniaturized attachment boards are also needed, so that an attachment board can be placed on each circuit board in the SMT assembly process, that is, there are sample through holes or nets on each assembled panel; then the attachment board can be retained for 3-4 months, and if there is a problem, the attachment board or net can be tested.
Each batch of goods may need a report to test whether the attachment board meets the basic requirements, so as to have confidence to say: "although you have passed our quality certification, we still need to ensure that the process is not only controllable now, but also controllable after six months." I learned from my PCB manufacturing experience in the past 50 years that no matter how good you are, if you don't pay attention to PCB technology, you may encounter problems unconsciously.
PCB manufacturing process is complex and complicated, so we must be able to control all the situations. This is why I always emphasize the importance of process control and automatic analysis.
Chavez：I agree. This feedback cycle must be continued, otherwise there may be problems. There are too many variables in PCB manufacturing process, which may be one of the most difficult manufacturing processes in the world. Look at our chemical process. We need to control the materials accurately and immerse and drain them for many times.
Chavez：I think every EE and designer in charge of PCB design should visit the PCB manufacturing plant in person to understand the manufacturing process and all the complicated details needed for a successful PCB production.
Shaughnessy：There are many designers who have not been to the factory once in 25 years, or have never been.
Chavez: what's worse is that many companies don't even allow the engineering team to communicate with PCB manufacturers. Instead, the supply chain staff go to the engineering team to communicate. I can't understand this all the time. The department structure of many companies is very complicated. Their engineering teams only know how to upload the design to PLM system, and then the supply chain receives the design from PLM system.
Holden：Taking a group of EDA software designers and programmers to visit the PCB manufacturing plant brought me too much fun. These designers and programmers have written all the programs of the design tools, but they have never understood the whole production process of PCB, and have never seen how PCB is made.
Chavez： It's very important to visit the factory. It's a good way to further education. Successful enterprises will ensure the continuous improvement of the engineering team. When working with current PCB manufacturers, whether providing training, re education courses on site or visiting factory facilities, continuing vocational training is the key to success. Another way to continue to learn professional knowledge is to participate in the activities of industry associations. I highly recommend that everyone join the PCEA and participate in its various activities, which is a good industry resource. The core purpose of PCEA is to promote cooperation within the industry, stimulate practitioners and provide them with re education services. I encourage all PCB related practitioners to participate in the activities of the local branch of PCEA.
Shaughnessy：Last year, I attended the IPC reliability seminar, which was almost all related to military PCB micro via failure. They did not realize that the failure would occur during reflow soldering and could not be detected at room temperature or during assembly. IPC team members have been investigating this problem, and the current recommendation is not to use stacked vias, especially complex stacked micro vias. Have you ever had a similar problem?
Chavez： Stacking and interleaving micro vias will be involved, but it depends on the product designed and the position of the product on the aircraft. For stacked micro vias, for the sake of reliability, it is better not to stack more than two layers. Because of our close cooperation with PCB manufacturers, reliability team, manufacturing team and production efficiency team, we are able to continue to work together to optimize our process and design. As a result, we are able to learn from the experience and adopt the best methods in the industry to produce end products to meet all the necessary requirements.
The potential failure problem you mentioned just now is a problem that can't be found at first. It will suddenly appear. The worst case is that the product fails in the use site, because this potential problem will bring a series of negative effects, including damaging the company's reputation, paying hundreds of millions of dollars, or causing permanent injury or death to users.