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Precautions for PCB board technology MSD storage
PCB Blog
Precautions for PCB board technology MSD storage

Precautions for PCB board technology MSD storage

2022-04-27
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Author:pcb

For PCB board technology MSD storage considerations, usually, after the material is removed from the placement machine, it will be stored in a dry environment, such as a dry box, or repackaged with a desiccant until it is used again. Many assemblers believe that they can stop counting the exposure time of the device after the device has been stored in a dry environment. In fact, this can only be done if the device was previously dry. In fact, once the device is exposed for a long period of time (more than an hour), the absorbed moisture will stay in the package of the device and slowly penetrate into the interior of the device, which is likely to cause damage to the device.

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The findings clearly show that the time the device is in the dry environment is as important as the exposure time in the environment. Recently, Shook and Goodelle of Lucent Technologies published an incisive paper on this topic. There is an example of a PLCC device with a humidity level of 5 (normal unpacking life of 48 hours), after 70 hours of dry storage, in fact, only 16 hours of exposure will exceed its lethal humidity level. Studies have shown that after the SMD device is taken out of the MBB, its Floor Life has a certain functional relationship with the external environmental conditions. Conservatively speaking, it is safer to control the device strictly according to Table 1. However, the external environment often changes, and the actual environmental conditions cannot meet the requirements specified in Table 1. Table 2 lists the corresponding changes in the Floor Life of the device as the external or storage environment changes. If the MSD device has not been exposed to moisture before, and the exposure time after unpacking is very short (within 30 minutes), and the exposure environment humidity does not exceed 30°C/60%, then the device can be stored in a dry box or moisture-proof bag. If stored in a dry bag, the original desiccant can continue to be used as long as the exposure time does not exceed 30 minutes. For MSDs of Levels 2 to 4, as long as the exposure time does not exceed 12 hours, the holding time for the residual treatment is 5 times the exposure time. The drying medium can be enough desiccant, or a drying cabinet can be used to dry the device, and the internal humidity of the drying cabinet should be kept within 10%RH. In addition, for Levels 2, 2a or 3, if the exposure time does not exceed the specified Floor Life, the period of time the device is placed in a drying box of ≤10% RH, or the period of time it is placed in a dry bag shall not be counted. within the exposure time.

For MSDs of Levels 5 to 5a, as long as the exposure time does not exceed 8 hours, the holding time for the re-drying treatment is 10 times the exposure time. You can use enough desiccant to dry the device, or use a drying cabinet to dry the device, and the internal humidity of the drying cabinet should be kept within 5%RH. The exposure time of the device can be calculated from zero after the drying process. If the humidity of the drying cabinet is kept below 5% RH, it is equivalent to storing in an intact MBB, and its Shelf Life is not limited. MSD packaging Many companies will choose to repackage the unused MSD. According to the standard requirements, the basic material conditions for packaging include MBB, desiccant, HIC, etc. The packaging requirements of different grades of MSD are different. Level 2a to 5a devices must be dried (dehumidified) prior to sealing with MBB. The drying method is generally to use a dryer for baking. Since the trays containing the devices, such as Tray trays, Tubes, Reel tapes, etc., will affect the humidity level when placed in the MBB together with the devices, these trays are also dried as compensation.

The drying method generally used in the drying method of MSD is to perform constant temperature drying treatment on the device at a certain temperature for a certain period of time. It is also possible to use enough desiccant to dry and dehumidify the device. The drying process for different MSDs is also different depending on the humidity sensitivity level, size, and ambient humidity conditions of the device. After drying the device as required, MSD's Shelf Life and Floor Life can be calculated from scratch. When the MSD exposure time exceeds the Floor Life, or other conditions cause the temperature/humidity around the MSD to exceed the requirements, the drying method can refer to the IPC/JEDEC standard. If the device is to be sealed into the MBB, it must be dried before sealing. Level 6 MSDs must be re-dried before use and then reflow soldered for the specified time according to the instructions on the moisture-sensitive warning sign.

When baking MSD, pay attention to the following issues: Generally, devices installed in high-temperature trays (such as high-temperature Tray trays) can be baked at a temperature of 125 °C, unless the manufacturer specifies the temperature. The baking temperature is generally indicated on the tray. The baking temperature of devices installed in low temperature trays (such as low temperature Tray trays, tubes, and tapes) cannot be higher than 40 °C, otherwise the trays will be damaged by high temperature. Remove the paper/plastic bag/box before baking at 125°C. Pay attention to ESD (Electrostatic Sensitivity) protection when baking, especially after baking, the environment is very dry and it is easy to generate static electricity. Be sure to control the temperature and time when baking. If the temperature is too high or the time is too long, it is easy to oxidize the device, or generate intermetallic compounds at the internal connection of the device, thereby affecting the solderability of the device. During baking, be careful not to cause unidentified gas to be released from the tray, otherwise it will affect the solderability of the device. Be sure to make a baking record during baking so that you can control the baking time.

For MSD repair, if the device on the main board is to be removed, local heating is used, and the surface temperature of the device is controlled within 200°C to reduce damage caused by humidity. If the temperature of some devices exceeds 200°C and exceeds the specified Floor Life, the motherboard should be baked before rework. The baking method is described in the next paragraph; within the Floor Life, the temperature that the device can withstand and reflow soldering can withstand the same temperature. If the device is removed for defect analysis, be sure to follow the recommendations above, otherwise damage from humidity can mask the original cause of the defect. If the device is to be recycled after removal, follow the recommendations above. MSD is not a substitute for drying after several reflow soldering or rework. Some SMD devices and motherboards cannot withstand long-term high-temperature baking, such as some FR-4 materials, which cannot withstand 24-hour baking at 125°C; some batteries and electrolytic capacitors are also sensitive to temperature. Taking these factors into consideration, choose the appropriate baking method on PCB board.