Short circuit failure analysis of PCBA

- May 13, 2020-

Short circuit failure analysis of PCBA

1、 Foreword

As the carrier of various components and the hub of circuit signal transmission, PCB has become the most important and key part of electronic information products, widely used in all walks of life. In recent years, there are more and more cases of PCB failure and some of them are harmful. The plan for equipment manufacturing industry and standardization and quality improvement passed in April 2016 and made in China 2025 adhere to the basic policy of "innovation driven, quality first, green development, structural optimization, talent oriented", which shows that the country has higher and higher quality requirements for products and stricter control over product reliability. Therefore, the PCB quality and reliability water Flat determines the quality and reliability of the whole equipment.

In the past, only military products would put forward reliability requirements, but now many civil products enterprises would also put forward various reliability requirements. Different types of customers and different product application areas have different requirements for PCB reliability. For example, some customers' requirements for PCB reliability are 260 ° C and 10 hours baking, which can still meet PCB electrical performance requirements; some customers require that after 250 or even 1000 ist cycles, the product resistance change rate is less than 10%; some customers require PCBA products to meet 30 minutes resonance requirements under 25g acceleration, etc.

 

2、 PCB failure cases

As we all know, the failure of products will cause serious economic loss and quality impact. However, the failure modes of PCB are various, and the failure root causes are also different. For example, the corrosion failure of PTH hole copper, the open circuit failure caused by the crack at the bottom of HDI blind hole, the layered bursting plate failure, the hole ring crack of ENIG products and the short circuit ignition of PCB, etc. There are many reasons for its failure due to the complicated processing flow. Therefore, how to quickly locate the failure root cause of PCB and optimize the product performance has become one of the important issues in PCB industry. In recent years, the failure analysis cases related to big data calculation, navigation and medical field of the analysis and test center are listed as follows:

① PCB burn out failure

In 2015, a large data center reported that one or two equipment would burn on fire every month, so it is urgent to find out the cause of the fire quickly:

 

According to the observation of the failure site, the fire point of the burning plate is concentrated in the installation hole area of the hard disk connector, and the copper layer and positioning tin column of the hole wall in the installation hole area are completely burned. It is found that the metallization of npth hole wall will lead to CAF failure and short circuit fire in the process of power on.

Caf growth principle: when PCBA works with electricity under certain temperature and humidity conditions, CAF growing along the interface between resin and glass fiber may be generated between two insulated conductors, resulting in poor insulation or even short circuit failure. The principle of CAF generation is as follows:

There are three preconditions to produce CAF: ① certain temperature and humidity conditions; ② potential difference between two insulation conduction; ③ channels for copper ion migration.

Experiment recurrence: conduct 12V voltage stabilizing power on test for the insulation resistance failure module of the test board. The test board is short circuited and ignited at the moment when the power is turned on. If the power is continuously turned on, the sample will continue to ignite and carbonize (the test has video record);

Improvement plan: ① increase the distance between the inner and outer hole rings to the conductor to 300 μ m; ② replace the high Tg base material to ensure that it has good heat resistance and caf resistance.

② Crack failure of hole ring of electroless nickel plated gold plate

In 2017, a multinational SMT mounting customer reported that after reflow welding, the PTH holes of all ENIG boards purchased in the industry were corroded and blackened, the probe test failed, and the line was completely stopped, so failure analysis was needed.

In addition to affecting the contact performance of the probe, the crack defect of PTH hole will also lead to the corrosion of copper layer, reduce the corrosion resistance of PCB, and also bring greater failure risk to the solderability of the product.

 

Mechanism of crack generation: due to the principle of thermal expansion and cold shrinkage, PCB board is subject to high temperature expansion during reflow welding and wave soldering. Because the selection of PCB board does not match with the surface treatment process, the board will give an upward stress to the hole ring, jack up the hole ring, causing the deformation of the hole ring upwardly on both sides, leading to cracks in the hole ring.

Improvement plan: ① replace the plate with smaller CTE; ② replace the surface treatment process.

③ Electrochemical corrosion failure of PTH hole

In 2017, the 8-layer PCB of a certain model was used for 3 years in the sea and coastal areas, and the customer reported that the video signal of the whole product was blocked. There was a black foreign matter residue at the PTH hole of the abnormal signal network, so it was necessary to find the root cause and locate the failure point.

Through the analysis of PTH hole covered by foreign matters, it is found that there are obvious cracks between the resistance welding and the hole copper at the hole location of the plug hole, and the hole copper at the crack location has been corroded, and part of the hole copper is missing.

At the same time, compared with the normal PTH hole without foreign matter covering on the PCB, it is found that the resistance welding in the normal hole is well combined with the hole wall, there is no crack, the hole copper is not corroded, and the signal conduction is good.

According to the research, in the coastal environment with high salt and humidity, a thin water film will form on the surface of the object, and when the relative humidity is 65% - 80% in the air, the thickness of the water film on the object is zero point zero zero one ~ Zero point one μ m, when the PCB is working with electricity, the water film and the hole copper form the electrolytic cell together, and an electrochemical reaction takes place, resulting in black corrosion, as shown in the following figure:

Conclusion: when the solder plug hole is not full or there are cracks in the solder plug hole, the copper hole of PTH that is not covered by solder plug well will form an electrolytic cell, and electrochemical reaction will occur, and the copper hole will be corroded, which will bring serious risks to the reliability of PCB.

Improvement plan: improve the fullness of plug hole and improve the quality of plug hole.

Of course, there are various failure types and modes. The following are the pictures of other typical PCB level failure analysis cases accumulated in the laboratory:

From the above cases, it is not difficult to find that there are more and more PCB level failure modes, and the failure root causes are different. Therefore, it is necessary to summarize and refine the general failure analysis ideas and methods to form a set of methodology that can be popularized and applied. In the actual case analysis, the root cause can be quickly located with half the effort.

3、 PCB failure analysis method

This method is mainly divided into three parts, which can not only help us to solve the failure problem quickly and locate the root cause in the actual case analysis process, but also train the new engineers according to the framework established by us, so as to facilitate the borrowing and learning of all departments.

Next on the analysis of ideas and methods to explain, the first is the analysis of ideas;

Step one: "five steps" of failure analysis

The process of failure analysis is mainly divided into five steps: "① collect the information of defective board → ② confirm the failure phenomenon → ③ analyze the failure cause → ④ verify the failure root cause → ⑤ report the conclusion and improve the suggestions". The first step is to understand the failure content, process flow, structure design, production status, use status, storage status and other information of the defective PCB, so as to prepare for the subsequent analysis process; the second step is to determine the failure location and determine the failure mode according to the failure information; The third step is to analyze the failure mode, and check the root cause one by one according to the failure root cause fault tree. If the cause cannot be confirmed in the existing fault tree, it is necessary to study such failure problems through special project initiation and other ways, and add the research conclusions to the original fault tree, so that the fault tree will continue to enrich and improve, exhaust the root cause, and form an effective cycle of repeated iterative upgrading Mode; then through step ④ to carry out the repeatability experiment to verify the root cause; finally output the failure analysis report, and give the targeted improvement scheme for the failure root cause.

Step 2: establish failure root cause fault tree

Taking the bad solderability of the gold surface of the chemical nickel plated gold plate as an example, the method of establishing failure analysis fault tree is described

In view of the common board level failure phenomena of PCB / PCBA, we have established root cause fault trees of various failure modes, and continuously accumulated, refined and updated them in the actual combat. From the depth and breadth, we have increased iteratively, so as to form a relatively complete root cause fault tree analysis process of high-frequency failure modes such as delamination blistering, poor weldability, poor bonding, poor continuity and poor insulation, It can help you to follow the failure analysis process of fault tree, quickly locate the root cause of failure, solve problems and get twice the result with half the effort.

Step 3: establish standard library

By verifying the root causes of fault tree, a standard library file is formed. There are several sources of the root cause determination standard library: ① IPC, GJB, industry standards and other documents; ② comparison Library of normal products and abnormal products; ③ R & D project experience, production experience and other documents. At the same time, it summarizes and classifies the evaluation methods and standards involved in each failure root cause in the fault tree, summarizes and sorts out the common PCB standards and various abnormal data, and forms a PCB failure analysis standard library for reference in subsequent cases.

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