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PCB problems, packaging humidity-sensitive parts baking common problems sorting out
2021-10-09
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Author:Aure

PCB problems, packaging humidity-sensitive parts baking common problems sorting out




I have encountered many problems recently, one of which is why some SMD parts need to be baked? What is the Moisture Sensitivity Level (MSL)? Some frequently asked questions about this area and try to answer the following:

The purpose of the part rebake?

Which parts need to be rebaked?

How long does it take to bake?

Can reel packaging (reel) be baked in the oven?

Why do PCB manufacturers claim that their low-humidity drying oven can prevent moisture-sensitive parts from being affected by moisture, and what is its function?

Can the desiccant be reused?

Can the parts be repeatedly baked? Is it better to bake as long as possible? Are there any baking restrictions?



PCB problems, packaging humidity-sensitive parts baking common problems sorting out

 

1. What is the purpose of rebake parts?

First of all, we should first understand that moisture can cause damage to electronic parts? Moisture will oxidize the solder feet of the parts, resulting in poor solderability. In addition, if moisture enters the inside of the packaged parts, such as IC packaging parts, when these parts undergo a rapid heating process, such as Reflow, the internal Moisture water molecules will quickly expand their volume due to heating. At this time, if the moisture cannot effectively escape the inside of the packaged part, it will be stretched from the inside of the part due to the expansion of the water molecule volume, causing de-lamination (de-lamination). ), Even popped from the inside of the part to form popcorn... and other consequences.

 

If the solder leg of the part has been oxidized, the oxidation basically cannot be re-baked to return the oxidized solder leg to the state before oxidation, and it may be reprocessed by electroplating. Therefore, the main purpose of baking is only to remove moisture inside the packaged parts, so as to avoid the problem of delamination or popcorn when the parts flow through reflow.

 

2. Which parts need to be rebaked?

As mentioned above, the purpose of baking is to remove moisture inside the parts, so as to avoid the problem of delamination or popcorn when the parts are reflowed. Therefore, in principle, as long as the packaged parts (generally referred to as IC parts), especially the packaged parts that need to be reflowed, as long as there is doubt about moisture, they should be re-baked.

 

As for hand-soldered parts or packaged parts that have undergone wave soldering, do they need to be baked if they are damp? There is no clear stipulation in the J-STD-033B document. Although the temperature of hand soldering and wave soldering is much lower than that of reflow soldering, the temperature of the part body is much lower, but if time permits, I still suggest that you should compare J-STD -033B is more appropriate to bake the damp packaged parts. After all, the temperature during operation is still far above the boiling point of water, and there is still a chance of part delamination.

 

3 Can reel packaging (reel) be baked in the oven?

According to the regulations in sections 4.2.1 and 4.2.2 of J-STD-033B, the packaging of general moisture-sensitive parts must indicate whether the packaging can withstand high-temperature baking at 125°C or cannot withstand low-temperature packaging exceeding 40°C. . Generally, if there is no label, it means that it can withstand high-temperature baking at 125°C.

 

If the packaging material is low-temperature packaging, all packaging must be removed when baking, and then reinstalled in the original packaging after the baking is complete. Regardless of high temperature or low temperature packaging, the original paper and plastic containers (such as cardboard, bubble bag, plastic packaging, etc.) must be picked out before baking, and the rubber band and tray must be brought to high temperature. (125℃) It must also be picked out before baking.

 

4. Why do some circuit board manufacturers claim that their low-humidity drying oven can prevent moisture-sensitive parts from being affected by moisture, and what is its function?

According to the instructions in Section 4.2.1.1 and 4.2.1.2 of J-STD-033B, if the humidity sensitivity level (MSL) 2, 2a, 3 components are exposed to the workshop for less than 12 hours, and placed at ≦30°C/60% In the RH environment, as long as it is placed in a dry package or a drying cabinet below 10% RH, after 5 times the time of exposure to the atmosphere, the floor life of the parts can be recalculated without baking.
In addition, if the MSL 4, 5, and 5a components are exposed to the workshop for less than 8 hours and placed in an environment of ≦30°C/60%RH, just place them in a dry package or a drying cabinet below 5%RH , After 10 times the time of exposure to the atmosphere, the floor life of the parts can be recalculated without baking.

 

Therefore, some manufacturers claim that when moisture-sensitive parts are unused after being opened, they can be placed in an electronic drying cabinet below 5% RH to suspend or recalculate their workshop time. However, the storage space of the drying cabinet is limited after all. It is recommended to open the dry packaging of the parts before use to ensure that the parts are not damp, and to control the temperature and humidity of the factory workshop within 30°C/60%RH.

 

5. Can the desiccant be reused?

According to the description in section 4.1.2 of J-STD-033B, if the desiccant in the dry package is only exposed to the factory environment below 30°C/60%RH, and the time does not exceed 30 minutes, the original desiccant can be reused . But the premise is that the desiccant is not damp or damaged.

 

6. Can the parts be bake repeatedly? Is it better to bake as long as possible? Are there any baking restrictions?

According to the description in section 4.2.7.1 of J-STD-033B, excessive baking of parts may cause parts to oxidize or generate intermetallic, which will affect the quality of soldering and circuit board assembly. In order to ensure the solderability of parts, it is necessary to control The time and temperature of the baking of the part. Unless the supplier has special instructions, the cumulative baking time of the parts at 90℃~120℃ cannot exceed 96 hours. If the baking temperature is below 90°C, there is no restriction on the baking time. If the baking temperature needs to be higher than 125°C, you must contact the supplier, otherwise it is not allowed.

 

In addition, the compound used in the IC package colloid will cause the material to deteriorate and become brittle after repeated high temperature (>Tg (glass transition temperature)), and the high temperature environment will also make the IMC originally formed inside the IC speed up its electrons. The speed of migration, when the holes of the IMC increase, the original wire bond is easy to fall off, causing quality problems such as open circuits. Therefore, whether the baking time is as long as possible is quite debatable, at least in a high temperature environment. It is not. (Explained by circuit board manufacturer)