Considerations for MSD storage of PCB Technology

Normally, after the materials are removed from the PCB mounting machine, they will be stored in a dry environment until they are used again, such as a drying box or re-encapsulated with desiccant. Many assemblers believe that after the device is stored in a dry environment, the exposure time of the device can be stopped. In fact, this can be done only when the device is dry before. In fact, once the device is exposed for a long period of time (more than one hour), the moisture absorbed will stay in the encapsulation of the device and penetrate into the device, this may cause damage to the device.

Recent results clearly show that the exposure time of the device in the dry environment is equally important as that in the environment. Recently, shook and goodelle of longxun technology have published their papers on this topic. Some examples show that a PLCC device with a humidity rating of 5 (a normal sealing life of 48 hours) is exposed for only 16 hours after being dried for 70 hours, it exceeds its critical humidity level.

The results show that after the SMD device is removed from MBB, its floorlife has a certain functional relationship with the external environment. Conservatively, the safer approach is to strictly control the device according to table 1. However, the external environment changes frequently, and the actual environment conditions cannot meet the requirements set forth in table 1. Table 2 lists the changes in device floorlife as the external or storage environment changes.

If the MSD device has not been wet before and the exposure time is very short (less than 30 minutes) after unpacking, the exposure environment humidity does not exceed 30 ℃/600, you can use a drying oven or a moisture-protected bag to store the device. If the drying bag is used for storage, as long as the exposure time does not exceed 30 minutes, the original desiccant can continue to be used.

For levels2 ~ MSD of 4, as long as the exposure time does not exceed 12 hours, the retention time of its re-drying processing is 5 times the exposure time. The drying medium can be enough desiccant, or the drying cabinet can be used to dry the device. The internal humidity of the drying cabinet should be kept within RH.

In addition, for levels 2, 2a, or 3, if the exposure time does not exceed the prescribed floorlife, the time when the device is placed in the drying oven of ≤ rh, or in the drying bag, it should not be calculated within the exposure time.

For levels5 ~ MSD of 5a, as long as the exposure time does not exceed 8 hours, the retention time of its re-drying processing is 10 times the exposure time. You can use enough desiccant to dry the device, or use a drying Cabinet to dry the device. The internal humidity of the drying cabinet should be kept within 58rh. After drying, the exposure time of the computer can be calculated from scratch.

If the humidity of the drying cabinet is kept below 5% RH, it is equivalent to storing it in intact MBB, and its shelf life is unrestricted.

MSD packaging many companies will choose to re-package the unused MSD. According to the standard requirements, the basic packaging conditions include MBB, desiccant, hic, etc, the packaging requirements for MSDS of different levels are different. See table 3.

Level 2a ~ 5A devices must be dried (dehumidification. The drying method is generally to use the dryer for baking.

Because the material disks of the device, such as tray, tube, and reel tape, which are placed together with the device into MBB, the humidity level is affected. Therefore, it is used as compensation, these pallets must also be dried.

MSD drying method generally adopts a constant temperature drying process for the device at a certain temperature. You can also use enough desiccant to dry and dehumidify the device.

The drying process varies depending on the Humidity Sensitivity Level, size, and ambient humidity of the device. After drying the device as required, MSD's shelflife and floor life can be calculated from scratch.

When MSD exposure time exceeds floorlife, or the temperature/humidity around MSD exceeds the requirement in other cases, the drying method can refer to the latest IPC/JEDEC Standard. If the device is to be sealed to the MBB, it must be dried before sealing. MSD of level6 must be re-dried before use, and then reflow welding is performed according to the description on the humidity-sensitive alarm mark.

Pay attention to the following issues when baking MSD: devices installed in High-Temperature Material disks (such as high-temperature tray disks) can be baked at 125 ℃, unless the manufacturer explicitly specifies the temperature. The top of the tray disk is usually filled with the highest baking temperature.

The baking temperature of a device in a low-temperature tray (such as a low-temperature tray, tube cartridge, or tape) cannot exceed 40 ℃. Otherwise, the tray will be damaged by high temperature.

Remove the paper, plastic bag, or box before baking at 125 ℃.

WHEN baking, pay attention to ESD (electrostatic sensitive) protection, especially after baking, the environment is particularly dry, the most likely to produce static electricity.

Make sure to control the temperature and time during baking. If the temperature is too high or the time is too long, it is easy to make the device oxidized, or the generation of intermetal compounds in the device, thus affecting the device welding.

During baking, note that it is not possible to release unknown gases from the material tray; otherwise, the welding of the device will be affected.

Make a baking record during baking to control the baking time.

MSD repair if you want to remove the device on the motherboard, it is best to use local heating, the surface temperature of the device is controlled within 200 ℃, to reduce the damage caused by humidity. If the temperature of some devices exceeds 200 ℃ and exceeds the prescribed floorlife, bake the motherboard before rework. For the baking method, see the following section. Within floorlife, the temperature that the device can withstand is the same as that of reflow soldering.

If the device is to be removed for Defect Analysis, follow the above suggestions. Otherwise, damage caused by humidity will mask the original causes of defects.

If the device needs to be recycled after removal, follow the above suggestions. MSD cannot be used for drying after several reflux welding or rework.

Some SMD devices and boards cannot withstand high temperature baking for a long time, such as some FR-4 materials, cannot withstand 24 hours 125 ℃ baking; some battery and electrolytic capacitors are also very temperature sensitive. Consider these factors and select an appropriate baking method.

This article is from the "9392039" blog, please be sure to keep this source http://9402039.blog.51cto.com/9392039/1554462

Considerations for MSD storage of PCB Technology