Glass Transition Temperature and Flatness — A Breakdown

• Manufacturing and Reflow
• What is Tg
  • Amorphous Glass
  • What Impacts Tg
• PCB Warping
• Tg and Its Impact on Warping
  • Creeping Up
    • Why Creep Happens
• Flatness
  • Measuring Flatness in GD&T

Manufacturing and Reflow

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.

What is Tg

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).

Amorphous Glass

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.

What Impacts Tg

For background, the glass transition temperature of a material is influenced by several factors, including:

• Molecular Weight: Higher molecular weight polymers generally exhibit higher Tg values due to the increased entanglement of polymer chains, which impedes their mobility and necessitates higher temperatures to facilitate movement.
• Chemical Structure: The chemical structure of the polymer, including the presence of bulky or polar groups, can affect Tg by altering the intermolecular forces, chain mobility, and free volume within the material.
• Crosslinking: The degree of crosslinking in a thermosetting resin has a direct impact on the Tg. Higher crosslink densities can lead to increased stiffness, reduced molecular mobility, and higher Tg values.
• Plasticizers: The addition of plasticizers can lower the Tg of a material by reducing intermolecular forces and increasing free volume, enabling increased chain mobility at lower temperatures.
• Fillers and Reinforcements: The incorporation of fillers and reinforcements, such as glass fibers or inorganic particles, can alter the Tg of the composite material by affecting the interfacial interactions, stress transfer, and overall morphology of the system.

PCB Warping

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:

• Uneven Shrinkage: During the heating and cooling cycles in the manufacturing process, the board’s base material undergoes thermal expansion and contraction. If this expansion and contraction are not uniform across the board, it can lead to warping. Uneven shrinkage can be caused by various factors, such as non-uniform material properties, asymmetrical board layout, or temperature gradients within the reflow oven.
• Copper Imbalance: The presence of large copper areas on one side of the board and none on the other can create an imbalance in the thermal expansion and contraction of the material, leading to warping.
• Component Stress: Large and heavy components, such as connectors and transformers, can exert mechanical stress on the board during assembly or operation, causing it to warp.

Tg and Its Impact on Warping

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:

• Improved Thermal Stability: A higher Tg substrate maintains its stiffness and low coefficient of thermal expansion over a broader temperature range. This means that the material is less likely to expand or contract significantly during the heating and cooling cycles of the manufacturing process, thus reducing the potential for warping due to uneven shrinkage.
• Increased Mechanical Strength: Substrate materials with a higher Tg generally have higher mechanical strength. This added strength allows the board to better withstand the stresses induced during assembly and operation, such as those arising from large or heavy components, thereby reducing the chances of warping.
• Enhanced Resistance to Thermal Degradation: A higher Tg material is more resistant to thermal degradation, which can occur during the assembly process or the board’s operation. Degradation of the material can lead to changes in its mechanical properties, which can contribute to warping. By using a substrate with a higher Tg, the risk of thermal degradation and associated warping is reduced.
• Reduced Creep: With a higher Tg, the material exhibits lower creep, which is the time-dependent deformation under constant stress. This reduced creep contributes to the overall dimensional stability of the board, lowering the likelihood of warping caused by component stress or thermal cycling.

Creeping Up

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.

Why Creep Happens

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.

Flatness

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.

Measuring Flatness in GD&T

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.