PCB Tech - The influence of humidity during PCBA processing

Humidity often plays a key role in the PCBA manufacturing process. This article mainly introduces the effect of temperature on PCBA processing. Too low will cause things to dry, ESD will increase, dust levels will be higher, and template openings will be more likely to be blocked and template wear. Increase, it has been proved that the humidity is too low to directly affect and reduce 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.

Introduction to Surface Wetness

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. Porous or moisture-absorbing surfaces (epoxy resins, plastics, flux, etc.) on organic plastics, such as moisture-absorbing layers, tend to absorb these layers. Even when the surface temperature is lower than the dew point (condensation), no moisture-containing surfaces can be seen on the surface of the material. Absorbent layer. 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 moisture 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. Humidity affects during the manufacturing process. Humidity has a variety of effects on manufacturing. Generally speaking, humidity is invisible (except for weight gain), but it brings The consequences are pores, voids, solder spatter, solder balls and underfill voids, etc. 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 bonds of 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.

DEK press settings

In the workshop, DEK ECU actually set a temperature of 26 degree Celsius. The relative humidity of the internal environment is 45% RH, and the dew point temperature of the substrate calculated under the internal environment is 15 degree Celsius. The coldest substrate temperature recorded before entering the screen printer is 19 degree Celsius, ΔT (the difference between substrate temperature and dew point) is (19 degree Celsius-15 degree Celsius) 4 degree Celsius, which only meets the metal safety coating ASTM and ISO coating specifications (Minimum 4±1 degree Celsius) low limit, but on-site production operations may fail. The porous surface coating specification requires the substrate temperature to be higher than 5°C, so we can assume that the substrate will absorb moisture.

If we place a cold (19 degree Celsius) substrate on other equipment, such as Fuji equipment, where the workshop humidity is greater than 60%RH, we will have a ΔT of 2 degree Celsius, which will not meet the requirements of ASTM/ISO coating specifications at all. Because the substrate is too wet. A good setting for optimization should be ≥5°C above the dew point.

Workshop measurement

The moisture absorbed by the substrate surface depends on the surface temperature, ambient air temperature and relative humidity (dew point). When the substrate temperature is close to the dew point, due to the formation of a thick multi-molecular layer of water, the pad is wet, which will cause the adhesion of the solder paste, etc. (Viscosity) is low, resulting in poor release of solder paste in the template opening.

Dew point test (dyne value)

When the humidity increases (>50% RH), the surface temperature of the PCBA substrate is within the range of 4 to 5 degree Celsius close to the dew point temperature, and all substrate surfaces have poor wetting. We designed a test with an indoor relative humidity level of 43% RH, which is basically far lower than the worst case (60% to 65% RH) of the actual workshop measured. The influence of humidity on the process is very common. We conducted a test and put a clean substrate in the refrigerator in the workshop for half an hour until it was cooled to the dew point temperature required by the low-humidity workshop. When tested with a dyne pen, the dyne value had dropped from> 40 dyne to 37 dyne. Because this is enough to explain the influence of humidity on the process, the influence will be greater under high humidity and room temperature, and the dyne value will definitely drop more sharply.