Why PCB Boards Must Be Baked Before SMT Assembly: Key Reasons Revealed!

Purpose of Baking PCB Boards Before Processing

The main reasons for baking PCB boards before SMT assembly are as follows:

1. Removing Moisture:During storage and transportation, PCB boards inevitably come into contact with air, absorbing moisture from the environment. If this moisture is not removed, during the soldering process of SMT assembly, when the PCB board enters a high-temperature environment above 200 degrees Celsius, the moisture will rapidly vaporize into steam. If the steam cannot be released from the board in time, it will create pressure inside the PCB, potentially causing the layers of the circuit board to delaminate, leading to broken vias, layer separation, and even phenomena such as bubbling, warping, or board explosion. Even if there are no visible signs on the surface, internal circuits may be damaged, resulting in unstable functionality of the electronic product and ultimately leading to product failure.

2. Preventing Soldering Bubbles:The moisture in the PCB board evaporates under high temperatures, and if not baked beforehand, it may form bubbles during the soldering process. These bubbles can weaken the solder joints, affecting the reliability of electrical connections. Pre-baking can significantly reduce the risk of soldering bubbles.

3. Improving Component Adhesion:Baking helps improve the adhesion of components to the surface of the PCB board. During the SMT assembly process, components need to adhere firmly to the PCB surface to ensure reliable connections, thereby guaranteeing the performance and stability of electronic products.

4. Avoiding Thermal Shock:Thermal shock caused by temperature changes can negatively impact electronic components. By controlling the temperature during pre-baking, the PCB board can gradually adapt to temperature changes, mitigating or avoiding thermal shock during subsequent processing, ensuring that electronic components remain undamaged throughout the manufacturing process.

5. Removing Volatile Organic Compounds:During the manufacturing of PCB boards, adhesives, solvents, and other materials may leave residues on the surface. If these volatile organic compounds are not thoroughly removed, they can vaporize at high temperatures during the assembly process, interacting with gases produced during soldering, leading to soldering bubbles, excessive soldering, and other issues. Baking can completely eliminate these volatile organic compounds, preventing various soldering problems during assembly.

6. Improving Surface Wetting:During the assembly process, solder paste needs to adequately wet the surface of the PCB board and the component leads to ensure good solder connections. However, moisture or volatile organic compounds on the PCB surface can affect the wetting properties of the solder paste, leading to poor soldering. Baking can remove these influencing factors, enhancing the wetting properties of the solder paste, thereby ensuring the reliability and quality of soldering.

7. Enhancing Dimensional Stability:During the assembly process, PCB boards may expand due to heat. If not baked beforehand, this can lead to poor fitting of electronic components and excessive lead spacing. Baking can minimize dimensional changes in the PCB during assembly, ensuring the fit and lead spacing stability of components, thus improving product consistency and reliability.

Necessity of Baking PCB Boards

Baking PCB boards is generally necessary in SMT assembly, especially for boards at high risk of moisture or those with strict soldering quality requirements. However, in some special cases, such as when a PCB board is taken directly from a sealed dry package and can be processed immediately, baking may not be required. When baking, the following aspects need to be considered:

1. Baking Parameter Settings

1. Temperature:Different materials and thicknesses of PCB boards require different suitable baking temperatures. Generally, for common FR-4 material PCB boards, the baking temperature is set between 100℃-120℃. If the temperature is too low, it will not effectively remove moisture and volatile substances; if too high, it may deform, discolor, or even damage the internal insulation layer and circuits of the PCB board.

2. Time:The baking time depends on the thickness of the PCB board, the degree of moisture, and the performance of the baking equipment. Typically, for standard PCB boards around 1.6mm thick, the baking time is 4-8 hours. Thicker or more severely moisture-laden PCB boards may require longer baking times, but generally should not exceed 24 hours to avoid adverse effects on the PCB board.

3. Heating Rate:To avoid thermal stress caused by rapid temperature changes, the heating rate should not be too fast, generally controlled at 5℃/min-10℃/min.

   2. Selection of Baking Equipment

4. Type of Oven:Common types include hot air circulation ovens and vacuum ovens. Hot air circulation ovens ensure uniform airflow within the oven, ensuring even heating of the PCB boards; vacuum ovens can more effectively remove moisture at lower temperatures, suitable for temperature-sensitive or severely moisture-laden PCB boards.

5. Oven Size:The size of the oven should be chosen based on the dimensions and quantity of the PCB boards, ensuring that there is enough space for the PCB boards in the oven, allowing hot air to circulate adequately and avoiding uneven heat distribution due to overcrowding.

   3. Placement and Protection of PCB Boards

6. Placement Method:PCB boards should be placed flat on trays or supports within the oven, avoiding stacking to ensure that hot air can fully contact all parts of the PCB board, achieving uniform baking results.

7. Protective Measures:To prevent contamination of PCB boards during baking, they can be placed in dedicated trays or fixtures, or covered with high-temperature resistant protective films.

   4. Post-Baking Treatment

8. Cooling Method:After baking, PCB boards should cool naturally in the oven or use a slow air-cooling method to avoid rapid cooling that could cause condensation on the surface or generate thermal stress.

9. Storage Environment:After cooling, PCB boards should be placed immediately in a dry environment, such as a drying cabinet or sealed bag, with desiccants to prevent them from reabsorbing moisture from the air. Additionally, SMT assembly should be performed as soon as possible to avoid prolonged exposure to air.

   Before performing baking operations, it is also important to refer to the technical specifications and recommendations provided by the PCB manufacturer, and to develop a reasonable baking process based on actual production conditions to ensure the quality of the PCB boards and the smooth progress of SMT assembly.

Consequences of Insufficiently Baked PCBs

If PCB boards are not adequately baked, various consequences may arise during SMT assembly and subsequent use:

Soldering Quality Issues

• Formation of Bubbles and Voids:Residual moisture in the PCB board rapidly vaporizes under high soldering temperatures, forming bubbles and voids within the solder joints. This not only makes the appearance of the solder joints uneven and unsmooth, affecting the aesthetic quality of the soldering, but more importantly, it reduces the mechanical strength and conductivity of the solder joints, making them prone to cracking and detachment during long-term use, leading to circuit interruptions.

• Cold Solder and False Solder:The presence of moisture and volatile substances can affect the wetting performance of the solder paste, preventing the solder from properly spreading and adhering to the PCB pads and component leads, resulting in cold solder and false solder phenomena. Cold solder and false solder joints may appear normal during initial testing, but during the use of electronic products, they can easily lead to poor contact due to vibrations, temperature changes, etc., resulting in intermittent failures that are difficult to diagnose and repair.

• Solder Splashing:Insufficiently baked PCB boards can cause internal moisture to suddenly evaporate during soldering, creating pressure that forces liquid solder to splash out from the solder joints, forming solder balls. Solder balls may fall on other areas of the PCB surface, causing short circuits and other issues, affecting the normal operation of the circuit, and may also attract dust and other impurities, further reducing the reliability of the electronic product.

PCB Performance Issues

• Unstable Electrical Performance:Residual moisture and impurities can reduce the insulation performance of the PCB board, leading to leakage and short circuits between adjacent circuits, causing interference and attenuation in signal transmission, affecting the stability of the electrical performance of electronic products. For example, in high-frequency circuits, reduced insulation performance may lead to signal distortion and increased noise, degrading the communication quality of the product.

• Deformation of PCB Boards:Uneven distribution of moisture within the PCB board can lead to different evaporation rates of moisture at different locations under high soldering temperatures, resulting in uneven thermal stress and causing the PCB board to deform. Deformed PCB boards may not fit well with other components, affecting the overall assembly accuracy of the electronic product, and in severe cases, may even cause the circuit traces on the PCB board to break, resulting in permanent damage.

Product Reliability Issues

• Early Failures:Due to soldering quality issues and unstable PCB performance, electronic products may experience various failures during the initial use phase, known as early failures. This not only increases the after-sales repair costs but also severely impacts the brand image and user satisfaction of the product.

• Shortened Lifespan:Insufficiently baked PCB boards may gradually deteriorate over long-term use due to issues such as bubbles, voids, and cold solder within the solder joints, under the effects of thermal cycling and mechanical stress. Additionally, the decline in insulation performance and deformation of the PCB board may worsen over time, ultimately shortening the lifespan of electronic products.

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