Since the early 1950s, the printed circuit board (PCB) has always been the basic structural module of electronic packaging. As the carrier of various electronic components and the hub of circuit signal transmission, its quality and reliability determine the quality of the entire electronic packaging. And reliability. With the miniaturization, light weight and multi-function requirements of electronic products, and the promotion of lead-free and halogen-free processes, the requirements for PCB reliability will become higher and higher, so how to quickly locate PCB reliability problems and make corresponding measures The improvement of reliability has become one of the important issues for PCB companies.

Common PCB reliability problems and typical legends

Poor solderability

(Not wetting)

Poor solderability (non-wetting)

Welding

(Pillow effect)

Bad Bonding

Layered Explosion Board

Open circuit (through hole)

open circuit

(Laser blind hole)

Open circuit (line)

Open circuit (ICD)

Short circuit (CAF)

Short circuit (ECM)

Burned board

In the actual failure analysis of reliability problems, the failure mechanism of the same failure mode may be complex and diverse. Therefore, just like investigating a case, it requires correct analysis thinking, meticulous logical thinking and diversified analysis methods. Find the real cause of failure. In this process, any negligence in any link may cause “unjust, false, and wrongly-judged” cases.

General analysis of reliability problems background information collection

Background information is the basis of failure analysis for reliability problems, which directly affects the trend of all subsequent failure analyses, and has a decisive influence on the final mechanism determination. Therefore, before failure analysis, the information behind the failure should be collected as much as possible, usually including but not limited to:

(1) Failure scope: failure batch information and corresponding failure rate

① If there is a problem in a single batch in mass production, or the failure rate is low, the possibility of abnormal process control is greater;

②If the first batch/multiple batches have problems, or the failure rate is high, the influence of materials and design factors cannot be ruled out;

⑵Pre-treatment for failure: Whether the PCB or PCBA has gone through a series of pre-treatment processes before failure occurs. Common pre-treatments include pre-reflow baking, lead-free/lead-free reflow soldering, lead-free/lead-free wave soldering and manual soldering, etc. If necessary, you need to learn more about the materials used in each pre-treatment process (solder paste, steel mesh, solder wire, etc.) ), equipment (soldering iron power, etc.) and parameters (reflow curve, wave soldering parameters, hand soldering temperature, etc.) information;

(3) Failure scenarios: The specific information when the PCB or PCBA fails, some are in the pre-processing such as soldering and assembly process, such as poor solderability, delamination, etc.; some are in the follow-up aging, testing or even Failure during use, such as CAF, ECM, burn-in, etc.; need to understand the failure process and related parameters in detail;

Failure PCB/PCBA analysis

Generally speaking, the number of failed products is limited, or even only one. Therefore, the analysis of failed products must follow the principle of layer-by-layer analysis from outside to inside, from non-destructive to destructive, and avoid prematurely destroying the failure site:

(1) Appearance observation

Appearance observation is the first step in the analysis of failed products. Through the appearance of the failure site and combined with background information, experienced failure analysis engineers can basically determine several possible causes of failure and conduct targeted follow-up analysis. But it should be noted that there are many ways to observe the appearance, including visual inspection, handheld magnifying glass, desktop magnifying glass, stereo microscope and metallurgical microscope. However, due to the difference in light source, imaging principle, and observation depth, the appearance of the corresponding equipment needs to be comprehensively analyzed in conjunction with equipment factors. Avoid rushing judgments to form preconceived subjective guesses, making the failure analysis into the wrong direction and wasting valuable invalid products and analysis. time.

(2) In-depth non-destructive analysis

For some failures, only visual observations are used, and sufficient failure information cannot be collected, or even failure points cannot be found, such as delamination, false welding, and internal opening. At this time, other non-destructive analysis methods are required for further information collection, including Ultrasonic flaw detection, 3D X-RAY, infrared thermal imaging, short-circuit location detection, etc.

In the stage of appearance observation and non-destructive analysis, it is necessary to pay attention to the common or opposite characteristics between different failed products, which can be used as a reference for subsequent failure judgments. After collecting enough information in the non-destructive analysis stage, you can start targeted destruction analysis.

(3) Damage analysis

The destruction analysis of failed products is indispensable and the most critical step, which often determines the success or failure of failure analysis. There are many methods of destruction analysis, such as scanning electron microscopy & elemental analysis, horizontal/vertical sectioning, FTIR, etc., which are not described in this section. At this stage, the failure analysis method is certainly important, but more important is the insight and judgment of the defect problem, and a correct and clear understanding of the failure mode and failure mechanism, in order to find the real failure cause.

Bare board PCB analysis

When the failure rate is high, it is necessary to analyze the bare board PCB, which can be used as a supplement to the failure cause analysis. When the failure reason obtained in the failure product analysis stage is that a defect of the bare board PCB causes further reliability failure, then if the bare board PCB has the same defect, after the same processing process as the failed product, it should reflect the same The same failure mode as the failed product. If the same failure mode is not reproduced, it can only mean that the analysis of the cause of the failed product is wrong, or at least incomplete.

Recurrence test

When the failure rate is very low and no help can be obtained from the bare board PCB analysis, it is necessary to reproduce the PCB defects and further reproduce the failure mode of the failed product, so that the failure analysis forms a closed loop.

Facing an increasing number of PCB reliability failures today, failure analysis provides important first-hand information for design optimization, process improvement, and material selection, and is the starting point for reliability growth. Since its establishment, Xingsen Technology Central Laboratory has been committed to the research in the field of reliability failure analysis. Starting from this issue, we will gradually introduce our experience and typical cases in reliability failure analysis.