PCB News - Assessment and analysis of circuit board MSL​

Assessment and analysis of circuit boardMSL

1. The Huai Principles of Interpretation of Results
(1) A detailed description of the rules for failure study and judgment
If one or more of the samples that have been tested has the following shortcomings and fails and fails, then such packaged components can be judged as failing. The research and judgment adopted are:
1. The appearance of cracks can be seen in the 40 times magnification of the light lens.
2. Fail the electrical test.
3. The internal crack has penetrated the key positions such as the line, the first hitting point, or the second hitting flat point.
4. Internal cracks extend from any wire-bonded gold finger pads to other internal metal points.
5. There are cracks inside the body, so that any metal site inside reaches the outer surface of the package, and 2/3 of the distance has been affected by the cracks.
6. When the flatness of the package is warped, swollen, or swollen that is visible to the naked eye, but the component can still meet the requirements of coplanarity and elevation, it should still be judged as a pass.

(1). When the internal crack is found by the C-SAM ultrasonic scanning microscope, it can be judged as a failure, or continue the microsection verification at the designated point.
(2) When the package body is very sensitive to vertical cracks, the micro-section verification should be carried out on the approximate cracks of the molded body or the appearance of the package body.
(3) If a failed SMD seal is found, another new set of samples must be tested against the original moisture-sensitive water level of the seal and the first-level condition is adopted.
(4) When the component in the examination has passed the above 6 requirements, and no delamination or cracking is found after inspection by C-SAM or its methods, the component shall be deemed to have passed the examination for a certain MSL.

(2) Rules for passing the second examination
In order to understand how much delamination and cracking will have on the reliability of the seal, the packaging industry must take three further second-level exams to determine the extent of their impact. The content of the three exams are:
A. According to the following detailed procedures, find out what kind of degradation occurs in the crack layer between the two examinations after "pre-absorption" to "complete reflow".
B. Perform reliability evaluation according to JESD22-A113 and JESD 47 specifications.
C. Perform in-depth assessment or re-examination according to the original method of the semiconductor industry manufacturer.
As for the content of the reliability assessment, it must include stress test, and historical general data analysis. The appendix of the original 020C is the logical thinking diagram for implementing this kind of passing rules.

In addition, when the SMD package product has passed the subsequent electrical test, cracks are found in the heat sink and the bottom surface of the chip in the crystal area. However, if cracks and delamination are not found in other areas, and the standards can still be met, the SMD can be regarded as a pass in the MSL level re-examination.

All failures must be further analyzed and confirmed that the reason for the failure is directly related to humidity sensitivity. When the failure of a certain moisture-sensitive water is not found after reflow, the packaged components that should be tested can obtain the MSL certification.

Second, graded packaging
When the seal can pass the "Level 1" two-stage examination, it means that the seal has nothing to do with moisture sensitivity, and there is no need to deliberately perform dry packaging. However, if the seal has not passed the "Level 1" top-level exam, but still obtains a higher MSL level, it should still be classified as a moisture-sensitive product, and it must be dry-packed in accordance with J-STD-033 . If the seal can only pass the lowest level of Level 6, it should be classified as an extremely moisture sensitive level, and even dry packaging cannot guarantee its safety. When these products are delivered, the customer must be informed of their moisture-sensitive properties, and a warning label must be attached, and it must be baked and dehumidified according to the instructions of the label before re-soldering, or not directly welded. On the PCB surface, another plug-in card type indirect assembly is adopted. As for the minimum pre-bake temperature and time, it depends on the results of the dehumidification study of the components to be tested.

3. Analysis of on-demand weight increase and decrease
The weight gain analysis after moisture absorption is extremely valuable for the "on-site time limit" temporarily stored in the factory. This term refers to the time from the opening of the dry package until it has experienced a certain "period" of moisture absorption that is sufficient to cause damage to the packaged product in the reflow process. This period of time on site is called Floor Life. In addition, the weight loss analysis of dehumidification is extremely helpful in determining the baking time required to drive off moisture. The implementation of these two analyses can be carried out by selecting 10 seals from the sample, and using their average readings as reference data. The calculation method is as follows:
.Final weight gain = (wet weight-dry weight) / dry weight
.Final weight loss = (wet weight-dry weight) / wet weight
.Mid-term weight gain = (current weight-dry weight) / dry weight
.Mid-term weight loss = (wet weight-reappearing weight)/wet weight
Here, "Wet" is a relative concept, which means that when the packaged component has been placed in a specific temperature and humidity environment for a certain period of time, it has absorbed moisture. As for "Dry", it is a kind of saying, that is, when it is kept in a high temperature of 125 degree Celsius, and it has been tested that it can no longer remove more water.

(1) Moisture absorption curve
The horizontal axis (X axis) of this kind of graph is the passage of moisture absorption time. The initial period can be set within 24 hours, and the latter can be extended to 10 days until asymptomatic. The vertical axis 1. Fine weighing of dry weight
The sample must be placed in an oven at 125°C for more than 48 hours, and the dry weight must be weighed on a balance with an accuracy of 1 μg within 1 hour after being taken out and cooled. As for the smaller components, the dry weight should be accurately weighed within 30 minutes.
2. Fine scale of saturated wet weight
After the dry weight is weighed, the seal can be placed in a clean and dry small plate, and sent to the desired temperature and humidity environment for moisture absorption. Take out the moisture-absorbing seal and let it cool for more than 15 minutes but not more than 1 hour at room temperature. Be sure to weigh the wet weight during this stable period. However, for small items with a height of less than 1.5mm, it cannot exceed 30 minutes. After weighing the wet weight for the first time, the seal must be returned to the temperature and humidity box to continue to absorb moisture (the time outside the box should not exceed 2 hours). Weigh the wet weight repeatedly until the number is stable.

(2) Dehumidification curve
The X time axis of this XY curve is divided into 12 hours, and the weight change of the V axis can be from 0 to the above saturation weight loss. The method is to take out the moisture-saturated seal from the temperature and humidity box, and then put it into the oven within a period of 15 minutes to 1 hour that is stable at room temperature, and bake it at a predetermined temperature and time course to remove moisture. Then take it out to cool, and complete the preliminary weighing within 1 hour. After that, it is sent back to the oven to continue the dehumidification and weighing action, until the light humidity reaches a constant weight, so that the dehumidification curve can be obtained. ipcb is a high-precision, high-quality PCB manufacturer, such as: isola 370hr PCB, high-frequency PCB, high-speed PCB, ic substrate, ic test board, impedance PCB, HDI PCB, Rigid-Flex PCB, buried blind PCB, advanced PCB, microwave PCB, telfon PCB and other ipcb are good at PCB manufacturing.