PCB Tech - PCBA technology: What is Moisture Sensitive Devices?

The following is an introduction to PCBA technology: What is Moisture Sensitive Devices:

The so-called Moisture Sensitive Devices (Moisture Sensitive Devices) basically refers to those electronic parts that are sensitive to [moisture].

"Moisture" is also generally referred to as "humidity", which refers to the "water vapor" content of the parts exposed to the ambient atmosphere. The environment here is usually referred to as the floor or the environment that the parts are exposed to after leaving the vacuum moisture-proof packaging. .

Please note: Water in liquid or solid state cannot be calculated into the humidity! The human body can actually feel humidity. In a high-humidity environment, you can feel the sticky feeling before it rains, and the whole body seems to be wet.

How does humidity affect electronic parts? This is because the molecular cluster of water vapor is smaller than that of liquid or solid water, and it is so small that there is a chance to get into the gaps of humidity-sensitive parts. At this time, if the parts containing humidity are taken through the reflow oven ( reflow) Direct heating, imagine what will happen? Everyone should have the experience of boiling water, or have seen the picture of boiling water in a gas stove on TV. Have you seen the scene where the water vapor can push the lid up after the water is boiled? This is water vapor The influence of heating, water vapor will expand rapidly after heating, and can produce a huge thrust, the previous steam trains relied on steam to drive the entire train, so you say whether the power of steam is very strong.

Back to the topic, if the inside of electronic parts contains water vapor and is quickly heated to exceed the boiling point of water, the general lead-free reflow temperature will reach up to about 250˚C, depending on the humidity inside the electronic parts, when the heated water vapor Before it can escape from the gap of the part, the worst result will be like a bomb burst from the inside of the electronic part. The lighter one may destroy the internal structure of the part, and the heavy one may also cause the part delamination., The phenomenon of cracking.

Since water vapor can cause such serious consequences to electronic parts, do you need to control the water vapor of these electronic parts? Of course, so the Industrial Standards (IPC) will have moisture sensitive parts (MSD, Moisture Sensitive Devices) control and moisture sensitive levels (Moisture Sensitive Levels) defined standards will appear.

Is it true that all electronic parts that need to go through Reflow have to worry about burst damage caused by water vapor? If you only answer the popcorn question, Shenzhen Grand Power will say that it is not entirely because the previous paragraph mentioned that there must be gaps in the electronic parts to allow water vapor to get in, and the gap should not be too large., Because the gap is big enough to allow the expanded water vapor to expand and escape smoothly, it’s okay, so only those parts with small gaps will have problems!

Which electronic parts have small gaps? It’s the packaged parts, so IC parts are the first to bear the brunt, because ICs are almost always packaged with upper and lower molds, or stacked layer by layer, and the joints of the upper and lower molds or between the layers are The place where the gap occurs, so generally we refer to the humidity-sensitive parts are referring to these IC parts, BGA parts may be even worse, because the chip has to be packaged on the PCB surface of the carrier, and there is a problem with the coefficient of expansion.

In addition, most of the IC parts use gold or copper wires to transmit the signals on the chip. These small gold or copper wires can not withstand the damage caused by the thrust generated by moisture. Shenzhen Grand Power has done it before. The engineers of the IC packaging factory know that most ICs will have holes due to the packaging process restrictions. If the moisture is not well controlled, the moisture will really easily appear in these holes. When the moisture expands rapidly, it will happen. Easy to cause problems.

At present, the definition of "moisture-sensitive parts" in the PCB industry is limited to the problems of de-lamination and popcorn on packaged parts, and there are two main industrial standards used to regulate humidity-sensitive parts and humidity-sensitive parts. Grade: Classification of humidity/reflow sensitivity of non-hermetic solid-phase surface mount components

This standard is used by PCB component manufacturers to judge and classify which moisture sensitivity level their parts meet. The document basically defines the test humidity of different moisture sensitivity levels and the conditions of exposure to the workshop, etc., and then used it for reflow. Welding furnace to confirm whether it meets the requirements. Humidity/reflow soldering surface with sensitive components attached to the operation, transportation, storage, and packaging standards

This standard defines how moisture-sensitive parts should be used, transported, stored, and packaged to prevent parts from getting damp and reducing the reliability of parts after reflow soldering. It also defines how to use baking conditions to restore the dryness of moisture-sensitive parts.

Speaking of this, you should have some understanding of "moisture-sensitive parts"! However, you may wonder why many parts other than IC are often required to perform vacuum drying packaging for humidity control, Why? This is because moisture not only causes the problem of delamination of parts due to volume expansion caused by heating, but also accelerates the oxidation of the metal coating of the part feet, causing subsequent welding problems such as solder rejection. In addition, the plastic material of some parts will absorb moisture. Problems, such as PA (nylon), and parts embrittlement, deformation, discoloration and other problems after flowing through high temperature.

Because these electronic parts do not have uniform humidity control conditions and cannot be fully regulated by a unified standard, they can only be defined according to the needs of individual parts to define their moisture-proof conditions. Looking at most of the current electronic parts, except for ICs In addition to the risk of explosion and delamination of parts and circuit boards (PCB), other parts may be Case by Case.