PCBA Tech - Failure analysis ideas of pcb board and PCBA

PCB board manufacturers know very well that in the failure analysis of actual reliability problems of PCB, the failure mechanism of the same failure mode is complex and diverse. Therefore, just like the investigation, the smt technical engineer needs to have the correct analysis thinking., Meticulous logical thinking and diversified analysis methods can find the true cause of the failure of the PCB board; in the process, if any link is negligent, it is very likely to cause "unjust, false and wrong cases".

1. General analysis ideas for reliability issues

First, the collection of PCB board background information

The background information of the PCB board is the basis of the failure analysis of reliability problems, which directly affects the trend of all subsequent failure analysis, and has a decisive influence on the final mechanism judgment.

Therefore, before failure analysis, the information behind the failure of the PCB board should be collected as much as possible, usually including but not limited to:

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

1. If there is a problem in a single batch in the production of large quantities of PCB boards, or the failure rate is low, the possibility of abnormal process control will be greater;

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

(2) Treatment before the failure of the PCB board: before the failure occurs, whether the PCB or PCBA has gone through a series of pre-treatment processes; common pre-treatments include baking before reflow, whether there is lead reflow soldering, whether there is lead-free wave soldering and manual For process processing such as soldering, if necessary, you need to understand the materials (solder paste, steel mesh, solder wire, etc.), equipment (soldering iron power, etc.) and parameters (reflow curve, wave soldering parameters, hand soldering temperature, etc.) used in each pre-processing process in detail. 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.; smt technical engineers need to learn more about the failure process and related parameter data and other related information;

Second, PCB/pcba failure analysis method

Generally speaking, the number of failed PCBs is limited, or even only one. Therefore, the analysis of failed products must follow the analysis and processing from the outside to the inside, and the principle of layer-by-layer analysis from non-destructive to destructive, and special attention must be paid to the analysis. Do not destroy the failure site prematurely during the PCB board process:

(1) Appearance observation

Appearance observation is the first step of PCB failure 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 targeted follow-up analysis. But it should also 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 of the PCB board into the wrong direction and wasting valuable failed products. And analysis time.

(2) In-depth non-destructive analysis

Some PCB board failure products only use appearance to observe, can not collect enough PCB failure information, even the failure point can not be found, such as delamination, false welding and internal opening, etc. At this time, other non-destructive analysis methods are needed. Carry out further information collection, including ultrasonic flaw detection, 3D X-RAY, infrared thermal imaging, short-circuit location detection, etc.

At 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 judgment; after collecting enough information in the non-destructive analysis stage, targeted damage analysis can be started NS.

(3) Damage analysis

The destruction analysis of failed PCB boards is an indispensable and a particularly critical step, which often determines the success or failure of failure analysis; there are many methods of destruction analysis, such as scanning electron microscopy and elemental analysis, horizontal/vertical sectioning, FTIR, etc. . At this stage, although the failure analysis method is very important, what is more important is the insight and judgment of the smt technicians on the defects of the PCB board, and the correct and clear understanding of the failure mode and failure mechanism, in order to find the real PCB board Reason for failure.

Third, bare board PCB analysis method

When the failure rate of the PCB board is very high, the analysis of the bare board PCB board is also very necessary, which can be used as a supplement to the failure cause analysis. When the failure reason obtained in the analysis stage of the failure product of the PCB board is that a defect of the bare board PCB board leads to further reliability failure, then the bare board PCB board also has the same defect, after the same processing process as the failed product, It must reflect the same failure mode as the failed product. If the same failure mode does not appear, it can only mean that the analysis of the cause of the failure of the product is wrong, at least incomplete.

Recurrence test

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

Facing the increasing reliability of PCB boards today, failure analysis provides important first-hand information for design optimization, process improvement, and material selection, and it is also the starting point for reliability growth.