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PCBA Tech
The role of humidity in the PCBA manufacturing process
PCBA Tech
The role of humidity in the PCBA manufacturing process

The role of humidity in the PCBA manufacturing process

2021-11-06
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Author:Downs

Humidity plays a key role in the PCBA manufacturing process. Too low will result in dry things, increased ESD, higher dust levels, template openings are more likely to be blocked, 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.

The influence of humidity in the PCBA manufacturing process

Humidity has many effects on PCBA manufacturing. Generally speaking, humidity is invisible (except for weight increase), but the consequences are pores, voids, solder spatter, solder balls, and underfill voids.

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.

pcb board

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.

When the relative humidity is about 20% RH, there is a monolayer of hydrogen bonding of water molecules on the PCB substrate and the PCB 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.

The influence of humidity in PCBA manufacturing 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 will be 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 tight 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.

Solder paste

The influence of humidity in PCBA manufacturing. In fact, solder paste has a similar process to coating materials such as paint. As much flux as possible must be adhered to the surface of the substrate to effectively release the solder paste from the template openings. Solder paste close to the dew point of the surrounding environment will reduce the adhesive strength, resulting in poor solder paste release.

The air temperature of the ECU unit should follow the metal coating rules related to the dew point as much as possible, that is, for the metal coating, such as gold or tin, the substrate temperature should not exceed the dew point temperature of 4 ± 1 ℃. For porous/hydrophilic surfaces , Such as OSP, the minimum temperature we require should be ≥5 ℃.

DEK press settings

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

If we place a cold (19 ℃) substrate on other equipment, such as Fuji equipment, where the workshop humidity is greater than 60%RH, we will have a ΔT of 2 ℃, 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.

The influence of humidity in PCBA manufacturing 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 opening of the template.

The following is the critical temperature calculated according to the various temperature and humidity ranges of the workshop situation. Three substrate temperatures of 19 ℃, 20 ℃ and 21 ℃ are recorded. Figure 1 shows the safe workshop humidity and temperature range to avoid moisture absorption (the internal environment of the equipment needs to be measured).

The higher the substrate temperature, the lower the requirements for the workshop environment.

Dew point test (dyne value)

When the humidity increases (>50% RH), the surface temperature of the PCB substrate is within the range of 4 to 5 ℃ 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.