Since the early 1950s, printed circuit board (PCB) has been the basic construction 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 and reliability of the entire electronic packaging. With the miniaturization, lightweight and multi-function requirements of electronic products, as well as the promotion of lead-free and halogen-free processes, the requirements for PCB reliability will be higher and higher. Therefore, how to quickly locate PCB reliability problems and make corresponding reliability improvement has become one of the important issues for PCB enterprises.

Common PCB reliability problems and typical legends

Poor solderability

(No wetting)

Poor solderability (non-wetting)

Virtual welding

(Pillow effect)

Poor dependend

Layered plate blasting

Open circuit (through hole)

Open circuit

(Laser blind hole)

Open circuit

Open circuit (ICD)

Short circuit (CAF)

Short circuit (ECM)

Burning plate

However, in the failure analysis of practical reliability problems, the failure mechanism of the same failure mode may be complex and diverse. Therefore, just like investigating a case, correct analytical thinking, rigorous logical thinking and diversified analysis methods are needed to find the real failure cause. In this process, any link is slightly negligent, may cause “unjust, false and wrong case”.

General Analysis of reliability Problems Background Information collection

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

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

(1) If the single batch of mass production problems, or low failure rate, then the possibility of abnormal process control is greater;

(2) If there are problems in the first batch/multiple batches, or the failure rate is high, the influence of material and design factors cannot be excluded;

(2) Pre-failure treatment: whether PCB or PCBA has gone through a series of pre-treatment procedures before failure occurs. Common pretreatment include reflux before baking, / lead-free reflow soldering and/lead-free wave crest welding and manual welding, etc., when it is necessary to understand in detail all the materials used in the pretreatment process (such as solder paste, stencil, solder wire, etc.), equipment (soldering iron power, etc.) and parameters (flow curve and the parameters of the wave soldering, hand soldering temperature, etc.) information;

(3) Failure situation: the specific information when PCB or PCBA fails, some of which has failed in the pre-processing process such as welding and assembly, such as poor solderability, stratification, etc.; Some are in the subsequent aging, testing and even use of failure, such as CAF, ECM, burning plate, etc.; Detailed understanding of failure process and related parameters;

Failure PCB/PCBA analysis

Generally speaking, the number of failed products is limited, or even only one piece, so the analysis of failed products must follow the principle of layer by layer analysis from outside to inside, from non-destruction to destruction, by all means avoid premature destruction of the failure site:

(1) Appearance observation

Appearance observation is the first step of failure product analysis. 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 follow-up analysis accordingly. However, it should be noted that there are many ways to observe the appearance, including visual, hand-held magnifying glass, desktop magnifying glass, stereoscopic microscope and metallographic microscope. However, due to the differences in light source, imaging principle and observation depth of field, the morphology observed by the corresponding equipment needs to be comprehensively analyzed based on equipment factors. It is forbidden to make hasty judgment and form preconceived subjective guesswork, which leads to the wrong direction of failure analysis and wastes precious failed products and analysis time.

(2) In-depth non-destructive analysis

For some failures, the appearance observation alone cannot collect enough failure information, or even the failure point can not be found, such as delamination, virtual welding and internal opening, etc. At this time, other non-destructive analysis methods should be used to collect further information, including ultrasonic flaw detection, 3D X-ray, infrared thermal imaging, short-circuit location detection, etc.

In the stage of appearance observation and nondestructive analysis, it is necessary to pay attention to the common or different characteristics of different failure products, which can be used for reference for the subsequent failure judgment. After enough information is collected during the nondestructive analysis phase, targeted failure analysis can begin.

(3) Failure analysis

Failure analysis is indispensable, and is the most critical step, often determines the failure analysis success or failure. There are many methods for failure analysis, such as scanning electron microscopy & elemental analysis, horizontal/vertical section, FTIR, etc., which will not be described in this section. At this stage, although the failure analysis method is important, what is more important is the insight and judgment of the defect problem, and the correct and clear understanding of the failure mode and failure mechanism, so as to find the real cause of failure.

Bare board PCB analysis

When the failure rate is very high, the analysis of bare PCB is necessary as a supplement to failure cause analysis. When the failure reason obtained in the analysis stage is a defect of bare-board PCB that leads to further reliability failure, then if bare-board PCB has the same defect, the same failure mode as the failed product should be reflected after the same treatment process as the failed product. If the same failure mode is not reproduced, then the cause analysis of the failed product is wrong, or at least incomplete.