Can a high-nitrogen-based material with special inorganic fillers stand up to Pb-free processing?

Some studies have found halogen-containing epoxy resin might produce hazardous carcinogenic gases, such as dioxin and furan, under certain combustion temperatures (i.e., <1000°C).¹ Halogen-free flame retardants, which exclude tetrabromobisphenol A (TBBPA), are becoming increasingly popular as a replacement. There are a variety of approaches to replacing TBBPA and other halogenated flame retardants. Among them, the majority of literature focuses on phosphorus-based products, which are predicted to be the largest growing share.² However, eutrophication of rivers or lakes due to the hydrolysis of the phosphorus-based retardants has raised another environmental issue. Studies conducted by the Institute of Microelectronics indicate that phosphorus-based laminates absorb more than two times as much moisture as conventional laminates.² Furthermore, phosphorus-based flame-retardants tend to form phosphoric acids under thermal stress, which might be a long-term reliability concern because of acidic degradation.²

With that in mind, we have developed a novel halogen-free, phosphorus-free material (named UP-160HPF) for PCB applications that can fulfill the environmental requirements, exhibit good characteristics and nonflammability without a high cost penalty.

Our objective was to develop a halogen-free, phosphorus-free resin system that contains a large quantity of nitrogen in the main molecular framework. By using a dual-hardener system, which acts as an incombustible-gas generator, the resin system exhibits inherent flame-retardant properties upon curing. Other beneficial characteristics include higher cross-linking density and low coefficient of thermal expansion (CTE). Inorganic fillers with a correct choice of particle size, surface modification and proper dispersion, which can generally display synergistic effects in fire retardancy, were adopted. The flame-retardancy mechanism is known as a heat sink. When exposed to heat, the inorganic filler will decompose to release water vapor, which will cool the system, dilute burnable gases in the flame and create an oxide layer at the interface. The remaining metal oxides form a protective barrier on the polymer surface, shielding it against further decomposition and reducing the amount of toxic gases released.² The combination of the formula has a flammability rating of UL-94-V0, without additional halogen-based or phosphorus-based flame retardants. The absence of antimony or phosphorus-based flame retardants also fulfills environmental requirements.

Novel Material Properties

Table 1 compares general properties of the novel material with those of conventional FR-4. The higher cross-linking structure of the novel materials results in a higher glass transition temperature (Tg). The near-zero shrinkage characteristic of the unique nitrogen-containing resin system and the dispersion in the resin of some amount of inorganic fillers restrain the z-axis expansion as well. This will lead to higher through-hole reliability.

Although the dielectric constant is slightly higher than that of conventional FR-4, since tan δ is smaller, according to Eq. 1, the transmission loss of conventional FR-4 is almost three times higher than that of the novel material.

α = K * f *ε1/2 * tanδ             (Eq. 1)

where

α = transmission loss
K = a factor
f = signal frequency
ε = dielectric constant
Tanδ = dissipation factor or loss tangent.

Thermal resistance. Decomposition temperature (Td) by thermal gravimetric analysis (TGA) is one of the most important measures for determining the ability to withstand Pb-free processes. The TGA of the new resin system (Figure 1) is 40° higher than that of conventional FR-4.

Time to delamination is a test to determine the elapsed time at an elevated temperature, when a sudden and irreversible expansion, indicative of a delamination, occurs. Time to delamination at 260°C (T260) is a common measurement used to assess base material performance. With Pb-free assembly, temperatures of 288°C (T288) and 300°C (T300) are now used to evaluate materials. The T288 results (Figure 2) and (Table 2) indicate the new material is Pb-free assembly capable.

PCB Reliability and Processing Characteristics

Solder float was performed at 288°C for 10 sec. and Pb-free reflow with a 260°C peak temperature up to 15 cycles, respectively. There were no cracks or delamination in the boards under microscope.

Conductive anode filament (CAF) is a conductive copper-containing salt created by electrochemical migration. It is a significant and potentially dangerous source of electrical failure in the PCB.3 The anti-migration test vehicle (Figure 3) was prepared, and the test conditions were temperature=85°C, relative humidity=85%, DC=50V. No CAF was observed even after 2000 hr.

Innerlayer peel strength measurement after oxide processes (with assistance from Jetchem) was conducted per IPC-TM-650 and compared with conventional FR-4 and high Tg material (Figure 4). The results show the novel material’s performance is equivalent to conventional FR-4 and better than high Tg material. No significant differences were observed on peel strength between brown or black oxide. (Oxide chemicals were supplied by Jetchem.)

Furthermore, desmear process weight loss compared with conventional FR-4 (Figure 5) also shows equivalent results. (Desmear process was done by Jetchem permanganate system with a 12 min. reaction time.) The equivalent results permit more freedom for PCB vendors to design the processes and manage the shop floors.

Conclusion

A novel halogen-free, phosphorus-free material that can fulfill current environmental requirements has been described. The new material has lower dissipation factor, lower CTE (z-axis), exhibits good thermal characteristics and non-flammability without a high cost penalty. It is Pb-free compatible, and the major PCB processing conditions are almost equivalent to conventional FR-4. The new material is patent-pending in the US, Taiwan, Korea, China and Japan.

References

1. G. Söderström and S. Marklund, “PBCDD and PBCDF from Incineration of Waste-Containing Brominated Flame Retardants,” ES&T, vol. 36, 2002.
2. Muriel Rakotomalala, Sebastian Wagner and Manfred Döring, “Recent Developments in Halogen Free Flame Retardants for Epoxy Resins for Electrical and Electronic Applications,” Materials, August 2010.
3. George Morose, “An Overview of Alternatives to Tetrabromobisphenol A (TBBPA) and Hexabromocyclododecane (HBCD),” 2006.
4. D. J. Lando, J. P. Mitchell and T. L. Welsher, “Conductive Anodic Filaments in Reinforced Polymeric Dielectrics: Formation and Prevention,” 17th Annual Reliability Physics Symposium, April 1979.

Christina Jien is a specialist and Johnson Chang is director, engineering & RD operations at Uniplus Electronics Co. (uniplus.com.tw); This email address is being protected from spambots. You need JavaScript enabled to view it..