Musings, deliberations, and end results.
Felix Jen – 19 March 2023 – 7 min read
The manufacturing of printed circuit boards (PCBs) is a complex and multi-step process that involves several stages. One critical stage in PCB assembly is the reflow soldering process. This process is used to attach electronic components to the board by melting solder paste, which then solidifies and forms a strong bond between the components and the board.
In the reflow process, the PCB is first coated with solder paste using a stencil or a dispensing machine. The components are then placed on the board, with their leads or pads aligned with the solder paste deposits. The board is then heated in a reflow oven, which typically consists of multiple heating zones, each with a specific temperature profile. The temperature is gradually increased to reach the solder’s melting point, allowing it to flow and form an electrical and mechanical connection between the components and the board. The temperature is then lowered, allowing the solder to solidify and create a strong bond.
Glass transition temperature (Tg) is a critical material property that plays a pivotal role in determining the performance and reliability of printed circuit boards (PCBs). Before we dive into the technical details of Tg, it is essential to understand the structure and composition of the base material used in PCB manufacturing.
The substrate, commonly employed in PCB manufacturing, is a composite material consisting of a woven glass fiber cloth impregnated with a thermosetting resin such as epoxy or polyimide (also known as fiberglass). The glass fibers provide mechanical strength, rigidity, and dimensional stability to the composite material, while the thermosetting resin acts as a binder that holds the glass fibers together and provides electrical insulation. While there are a variety of types of fiberglass, the one most commonly found in PCB manufacturing is FR-4 (otherwise known as Flame Retardant 4).
The glass transition is a unique physical phenomenon that occurs in amorphous or partially amorphous materials, such as the thermosetting resins found in FR-4. Unlike crystalline materials, which exhibit a sharp melting point, amorphous materials undergo a gradual change in their properties as the temperature increases. Glass transition is the temperature range where the material transitions from a hard, relatively brittle state (glassy state) to a softer, more flexible state (rubbery state).1 It is essential to note that the glass transition is not a phase change; instead, it is a second-order thermodynamic transition2 characterized by a continuous change in the material’s molecular mobility and mechanical properties.
For background, the glass transition temperature of a material is influenced by several factors, including:
Warping is a common issue in printed circuit boards that can adversely affect the functionality and reliability of electronic products. It refers to the deformation or distortion of the PCB from its intended flat shape. Some of the main causes of warping in PCBs include:
A higher glass transition temperature can help reduce warping in PCBs by improving the material’s thermal and mechanical stability during the manufacturing process and the board’s operation. Here are some ways in which a higher Tg can decrease warping:
Creep is an interesting phenomenon that occurs at the molecular level in materials under constant stress or load. The underlying mechanism of creep involves the rearrangement of atoms or molecules in the material, driven by thermal energy and applied stress. As PCBs usually do not undergo constant stress by mounting (at least, they shouldn’t), creep usually comes in through the forces that cause warping above, rather than an extra mechanical load.
In amorphous materials, such as the resins found in PCBs, creep occurs primarily through diffusion processes. Diffusion is the random movement of atoms or molecules in a material, driven by thermal energy. The rate of diffusion depends on temperature, activation energy, and the material’s molecular structure. Creep in these types of materials is closely related to the molecular mobility and free volume within the material. At low temperatures, the molecular mobility is limited, and creep deformation is minimal. However, as the temperature increases, the molecular mobility and free volume increase, allowing for a greater degree of rearrangement under applied stress.
Now that we know what Tg is and how it can affect warp, we need a way to discuss flatness better than “somewhat flat.” We can utilize Geometric Dimensioning and Tolerancing (GD&T), which is a standardized system used to define and communicate the allowable variation in a part’s geometry, including flatness. Flatness is typically communicated in length units, such as micrometers (µm) or inches (in), depending on the industry and regional standards. The flatness value represents the maximum allowable deviation of a surface from an ideal flat plane. A smaller flatness value indicates a tighter tolerance, implying that the surface must be closer to perfectly flat.
In GD&T, flatness is a form control tolerance that specifies the allowable variation of a surface without considering the orientation or location of the part. Flatness is defined by a tolerance zone, which is a region between two parallel planes. All points on the surface must lie within this tolerance zone for the part to be considered acceptable. To communicate flatness in GD&T, the flatness symbol (a parallelogram) is used, followed by the tolerance value. This information is typically placed in a feature control frame, which consists of one or more compartments that convey the geometric tolerance, material condition, and any applicable modifiers.
A common method for measuring flatness is using a surface plate and gauge blocks. The part is placed on a highly accurate, flat surface plate, and gauge blocks are slid underneath the part at various locations. The maximum variation in the height of the gauge blocks represents the flatness deviation. Alternatively, if you have a large amount of money and space at your disposal, Coordinate Measuring Machines (CMMs) are your friend. CMMs are advanced measuring devices that use a probing system to collect data points from the surface of the part. These data points are then used to create a three-dimensional representation of the surface, from which flatness deviations can be calculated.
The above article was proofread by ChatGPT using the GPT-4 model. All information has been independently fact checked and verified and citations are provided where deemed appropriate.
ISO 11357-2: Plastics – Differential scanning calorimetry – Part 2: Determination of glass transition temperature (1999). [Back]
Defined as when the Gibbs free energy of a material exhibits continuous first derivatives but discontinuous second derivatives. This can be easily visualized through the Gibbs free energy diagram - where the solid, liquid, gas transition of most frequently first-order and any any secondary state transitions are second-order. [Back]