High TG laminates refer to materials with a Tg value typically above 170°C; for example, common lead-free compatible laminates can have a Tg of around 185°C. Furthermore, the thermal decomposition temperature (Td) at which these laminates lose 5% of their mass is also higher—taking a standard FR4 laminate from a particular brand as an example, its Td is approximately 305°C, whereas that of high TG laminates can reach 359°C.
This enables high TG laminates to demonstrate superior thermal stability when subjected to the high temperatures of lead-free soldering, effectively reducing issues such as cracks in the connections between PCB traces and pads, and pad deformation caused by temperature fluctuations, thereby improving PCBA production yield.
However, high TG laminates still face a certain risk of delamination during the lead-free soldering process. The causes and countermeasures are as follows:
Main causes of delamination risk
1.Resin properties
The resins used in high TG laminates have high viscosity and poor flowability at elevated temperatures. In the high-temperature environment of lead-free soldering, if lamination process parameters are not properly controlled (e.g. excessive heating rates or inappropriate timing of pressure application), the resin may fail to flow sufficiently, making it difficult to fill the voids and interfaces within the laminate, ultimately leading to delamination during soldering.
2.Internal Stress
high TG laminates have a relatively low coefficient of thermal expansion (CTE). However, during complex manufacturing processes such as multilayer lamination and etching, differences in CTE between material layers, combined with the high curing temperature of the high-TG laminate itself, can lead to the accumulation of significant thermal and residual stresses within the laminate. When these stresses exceed the internal bond strength of the laminate, delamination is likely to occur under the high temperatures of lead-free soldering.
3.Moisture Absorption
Although high TG laminates generally absorb less moisture than standard FR4, they will still absorb a certain amount of moisture in humid environments. Upon entering the high-temperature phase of lead-free soldering, this moisture rapidly vaporises and expands, generating significant internal pressure that causes the laminate to delaminate—a phenomenon commonly referred to as ‘laminate bursting’.
4.Surface Treatment and Bond Strength
The surface activity of high TG laminates differs from that of standard FR4. When performing surface treatments such as inner-layer oxidation or browning, the process results are difficult to control. If handled improperly, this can lead to insufficient bond strength between the inner copper foil and the prepreg. Under the high-temperature stress of lead-free soldering, delamination is highly likely to occur in areas with weaker bonding.
Key Strategies to Mitigate Delamination Risks
1.Optimising the Laminating Process
Controlling the heating rate: Appropriately reduce the heating rate during lamination to allow sufficient time for the resin to flow and fill the voids, thereby preventing premature curing or inadequate flow caused by excessive heating rates.
Adjusting pressure parameters: Based on the specific characteristics and thickness of the board, precisely set the timing and magnitude of pressure application to ensure uniform resin distribution under pressure, thereby enhancing internal bond strength.
2.Reducing Internal Stress
Material Matching: When designing multilayer boards, prioritise the use of materials with similar CTE values to minimise thermal expansion differences between layers, thereby reducing internal stress at the source.
Optimising Processing Workflows: Implement precise management of key processes such as etching and baking. For example, control etching rates and temperatures to prevent stress concentration caused by over-etching; set baking temperatures and times appropriately to help eliminate some residual stress.
3.Strict Humidity Control
Storage Environment: Store high TG boards in a dry, well-ventilated environment at a suitable temperature. Where conditions permit, use a desiccator for storage and strictly control the duration of storage to avoid prolonged exposure of the boards to humid air.
Pre-baking treatment: Pre-bake the boards prior to surface-mount assembly to remove internal moisture. Baking temperature and duration should be set according to the board’s characteristics and the manufacturer’s recommendations; typically, baking at 100–150°C for 4–8 hours is sufficient.
4.Enhancing Surface Treatment and Bond Strength
Process Optimisation: In light of the characteristics of high TG laminates, adjust the process parameters for surface treatments such as inner layer oxidation and browning (e.g. by appropriately increasing temperature and duration, or enhancing surface roughness) to strengthen the mechanical interlocking between the inner copper layer and the prepreg, thereby improving bond strength.
Quality Inspection: Strengthen quality inspections of laminates following surface treatment. Advanced equipment such as X-ray photoelectron spectroscopy (XPS) and contact angle measuring instruments may be employed to analyse the chemical composition and wettability of the laminate surface, ensuring that the surface treatment meets the required standards.
Although high TG laminates present a risk of delamination during lead-free soldering of PCBs, the probability of delamination can be significantly reduced through a series of targeted measures, including optimising the lamination process, reducing internal stress, strictly controlling humidity, and enhancing surface treatment and bond strength. This will fully leverage the performance advantages of high TG laminates, improve the quality and reliability of PCBs, and meet the electronics manufacturing industry’s growing demand for high-quality electronic products.
