How to do the failure analysis of PCBA? Master analytical skills and methods

Date：2021-12-03 12:00:23 Views：2275

PCBA is a system composed of PCB and various electronic components. The main material of PCB is the composite of glass fiber and epoxy resin, which is divided into single panel, double-sided board and multilayer board. As we all know, product failure will cause serious economic losses and quality impact. However, PCB failure modes are diverse, and the root causes of failure are also different, such as corrosion failure of PTH hole copper, open circuit failure caused by crack at the bottom of HDI blind hole, layered plate failure, hole ring crack of ENIG product and short circuit fire of PCB, etc.

PCBA fault characteristics

1. Mechanical damage

Due to bending, twisting and other stresses, the components may be damaged. For SMD components, the solder joints of some components may crack or the components may be damaged. On a PCB with through holes, the package of the components may be broken, or the pins may fall off from the component body.

2. Thermal damage

-The voltage applied to the circuit board is too high, resulting in excessive current. For small lines or components, their power consumption capacity, resulting in over-current damage (EOS);

-Damage caused by external heat source of equipment;

-Thermal damage caused by component failure (epoxy resin will be carbonized and blackened due to long-term high temperature)

3. Pollution

-The flux is not cleaned completely;

-Leave fingerprints, dust or cleaning solution during treatment;

-Metal fragments or solder bridging from assembly;

-Encounter polluted atmosphere during storage, equipment installation or operation;

-Moisture or salt in the environment. Pollutants in the environment can lead to the erosion of copper lines and reduce the insulation. Contaminants

It can cause corrosion of copper lines. After a long time, pollutants may cause metal migration in two forms: whisker growth and dendritic growth.

When identifying pollutants, microscope, SEM, EDX, or FTIR, Sims, XPS and other technologies are most commonly used.

4. Thermal expansion mismatch

When materials with different coefficients of thermal expansion are physically connected together, their size changes with temperature, especially when the temperature changes greatly, which will cause mechanical failure.

5. Component interconnection failure on PCB

Interconnection failures often occur at solder joints, interfaces between multilayer boards and pad connections. Air holes between several layers and through holes and cracks in the line lead to connection failure. The pollutants in the solder can also cause the solder joint to be not firm, which can cause solder joint cracks. Thermal stress, mechanical stress and process problems can cause interconnection faults. In addition, the thermal expansion of VOC can cause pores or cracks, resulting in connection failure.

PCB failure analysis method

The method is mainly divided into three parts. The integration of the three parts can not only help us quickly solve the failure problem and locate the root cause in the process of actual case analysis; We can also train new engineers according to the framework established by us, which is convenient for all departments to borrow and study.

The analysis ideas and methods are explained below. The first is the analysis ideas;

Step 1: "five steps" of failure analysis

The process of failure analysis is mainly divided into five steps: "① collect bad board information → ② confirm failure phenomena → ③ analyze failure causes → ④ verify failure root causes → ⑤ report conclusions and suggestions for improvement". Among them, step ① is mainly to understand the failure content, process flow, structural design, production status, use status, storage status and other information of defective PCB, so as to prepare for the subsequent analysis process; Step ② determine the failure position and determine the failure mode according to the failure information; Step ③ is to analyze the failure modes and find out the root causes 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 approval and add the research conclusions to the original fault tree to continuously enrich and improve the fault tree and exhaust the root causes, Form an effective cycle mode of repeated iteration and upgrading; Then repeat the experiment in step ④ to verify the root cause; Finally, output the failure analysis report and give targeted improvement schemes for the root causes of failure.

Step 2: establish failure root cause fault tree

Taking the poor weldability of gold surface of chemically deposited nickel gold plate as an example, this paper expounds the method of establishing failure analysis fault tree:

Aiming at the common board level failure phenomena of PCB / PCBA, we establish root cause fault trees of various failure modes, continuously accumulate, refine and update them in practice, and iteratively increase in depth and breadth, so as to form a relatively perfect root cause fault tree analysis process of high-frequency failure modes such as layered blistering, poor weldability, poor bonding, poor conduction and poor insulation, It can help you to follow the failure analysis process of the 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 the fault tree, a standard library file is formed. The source of root cause judgment standard library mainly includes several aspects: ① IPC, GJB, industry standards and other documents; ② Comparison Library of normal products and abnormal products; ③ R & D project experience, production experience document library, etc. At the same time, the evaluation methods and criteria involved in each failure root cause in the fault tree are summarized and classified, and the common PCB standards and various abnormal data are summarized and sorted to form a PCB failure analysis standard library for reference for subsequent cases.

Analyze PCB failure cases and methods and DFM, standardize design standards for the majority of electronic engineers, improve design efficiency, and promote enterprises to shorten R & D cycle and reduce manufacturing cost. Huaqiu DFM is a kind-hearted PCB health examination doctor. It is a necessary desktop tool for PCB engineers, hardware engineers, PCB factories, SMT factories and PCB traders. At present, there are 200000 + engineers using practical manufacturability analysis software.

Core features:

1. Analysis and design hidden danger items 23+

2. Warn the items affecting the price, and give the optimization scheme for the hidden dangers and the items affecting the price

3. Supports one click resolution of Allegro, pads, Altium, PROTEL and Gerber file types

4. Multilayer board automatic matching laminated structure

5. Intelligent impedance tool, combined with production factors, calculates impedance data or inverse calculation.

6. Personalized panel, second kill rule board or special-shaped board, stamp holes can be added.

7. Open short circuit analysis (IPC network analysis)

8. One click output of production tools (Gerber, coordinate file, BOM list)

The above is the related content of "PCBA failure analysis" brought by the core creation test. I hope it will be helpful to you. We will bring more wonderful content in the later stage. Our testing services include: electronic component testing and verification, IC true and false identification, product design and material selection, failure analysis, function testing, factory incoming material inspection, tape braiding and other testing items. Warmly welcome to call, we will serve you wholeheartedly.