PCBA Tech - The influence of humidity in PCBA manufacturing

The influence of humidity in PCBA manufacturing
Humidity plays a key role in the manufacturing process. Too low humidity will result in dry things, increased ESD, higher dust levels, template openings are more likely to be clogged, and template wear and tear. It has been proven that too low humidity directly affects and reduces production capacity. Too high will cause the material to damp and absorb water, causing delamination, popcorn effect, and solder balls. Moisture also reduces the Tg value of the material and increases dynamic warpage during reflow soldering.

Moisture absorbent layer on metal, etc.
Almost all solid surfaces (such as metals, glass, ceramics, silicon, etc.) have a moisture-absorbing layer (monolayer or multi-molecular layer), when the surface temperature is equal to the dew point temperature of the surrounding air (depending on temperature, humidity and air pressure), This moist water-absorbing layer becomes the visible layer. The friction force of metal to metal increases with the decrease of humidity. At a relative humidity of 20% RH and below, the friction force is 1.5 times higher than that at a relative humidity of 80% RH.

Moisture absorbent layer on organic plastic, etc.
Porous or moisture-absorbing surfaces (epoxy resin, plastic, flux, etc.) tend to absorb these water-absorbing layers. Even when the surface temperature is lower than the dew point (condensation), the water-absorbing layer containing moisture cannot be seen on the surface of the material. It is the water in the monomolecular water-absorbing layer on these surfaces that penetrates into the plastic encapsulated device (MSD). When the monomolecular water-absorbing layer is close to 20 layers in thickness, the water absorbed by these monomolecular water-absorbing layers will eventually lead to the failure during reflow soldering. Popcorn effect. According to IPC-STD-020, the exposure of plastic packaged devices in a humid environment should be controlled.

Influence of humidity in the manufacturing process
Humidity has a variety of effects on manufacturing. Generally speaking, humidity is invisible (except for weight gain), but the consequences are pores, voids, solder splash, solder balls, and underfill voids. For any process, the worst moisture condition is moisture condensation. It is necessary to ensure that the moisture on the substrate surface is controlled within the allowable range without adversely affecting the material or process.

The allowable range of control?
In almost all coating processes (spin coating, mask and metal coating in silicon semiconductor manufacturing), the accepted measure is to control the dew point corresponding to the substrate temperature. However, the substrate assembly manufacturing industry has never considered environmental issues. An issue worthy of attention (although we have published environmental control guidelines and various parameters that should be controlled in the global consumer team).

As the device manufacturing process moves toward finer functional features, smaller components and higher-density substrates make our process requirements close to the environmental requirements of the microelectronics and semiconductor industries. We already know the dust control problem and the problems it brings to the equipment and process. We now need to know that high humidity levels (IPC-STD-020) on components and substrates can cause material performance degradation, process and reliability issues. We have pushed some equipment manufacturers to control the environment in their equipment, and materials prepared by material suppliers can be used in harsher environments. So far we have found that humidity can cause problems with solder paste, stencils, underfill materials, etc.

Generally, coatings such as solder paste are formed by suspending solids in solvents, water or solvent mixtures. The main function of these liquids applied to metal substrates is to provide adhesion and bond to the metal surface. However, if the metal surface is close to the environmental dew point, water may be It will partially condense, and the moisture trapped under the solder paste will cause adhesion problems (bubbles under the coating, etc.).

In the metal coating industry, the dew point meter can be used to ensure the adhesion of the coating to the metal substrate. Fundamentally, this instrument accurately measures the humidity level on or around the metal substrate and calculates the dew point, compares this result with the substrate surface temperature of the measured component, and then calculates the ∆T between the substrate temperature and the dew point, if ∆T If the temperature is less than 3 ~ 5 degree Celsius, the parts cannot be coated, and voids will be caused due to poor adhesion.

The relationship between moisture absorption and relative humidity RH and dew point
When the relative humidity is about 20% RH, there is a monolayer of hydrogen-bonded water molecules on the substrate and the pad, which is bonded to the surface (not visible). Water molecules do not move. In this state, even in terms of electrical properties, water is harmless and benign. Some drying problems may occur, depending on the storage conditions of the substrate in the workshop. At this time, the moisture on the surface exchanges moisture and evaporates to maintain a constant monolayer. The further formation of the monolayer depends on the absorption of water on the surface of the substrate. Epoxy, flux and OSP all have high water absorption, but metal surfaces do not.

As the relative humidity RH level related to the dew point increases, the metal pad (copper) will absorb more moisture and even pass through the OSP to form a multi-molecular layer (multilayer). The key is that a large amount of water accumulates in the 20th layer and above of the monolayer, electrons can flow, and because of the presence of pollutants, dendrites or CAFs are formed. When it is close to the dew point temperature (dew point/condensation), the porous surface such as the substrate easily absorbs a large amount of water, and when it is lower than the dew point temperature, the hydrophilic surface will significantly absorb a large amount of water. For our electronic assembly process, when the moisture absorbed by the clinging surface reaches a critical amount, it will cause a decrease in flux efficiency, exhaust during underfill and reflow soldering, and poor solder paste release during stencil printing, etc. problem. Keeping the printer ECU close to room temperature will reduce possible solder paste release problems caused by dew point. Be aware that in a hot and humid workshop, the drying cabinet will produce cold low-temperature PCB components, and low humidity will increase the friction between metals and PCB produce The template is worn.