Electromagnetic noise suppression of LSI packages using ferrite film-plated lead frame

A lead frame in a large-scale integration (LSI) package, which should be a path of electromagnetic noise conduction, was coated with ferrite-plated films in order to suppress the electromagnetic interference (EMI) found inside recent electronic devices. A lead frame with 80 pins and a pin pitch of 0.65 mm was mounted on a rotating table, onto which a reaction solution of FeCl ₂+NiCl₂+ZnCl₂ and an oxidizing solution of NaNO₂+CH₃COONH₄ were simultaneously sprayed at 90 °C. The ferrite film on every surface of the lead frame had nearly the same thickness of around 3 //m with a definite columnar structure aligned perpendicular to the film plane. This suggested that the aqueous solution sprayed on the top surface of each pin flowed down to reach not only the lateral sides but also the back side under the optimum plating condition. Noise suppression effects of the ferrite films were predicted using an electromagnetic field simulation. In order to examine the influence of permeability on the noise suppression effect, the measured permeability spectra of Films A (Ni _0.2 Zn _0.3Fe_2.2 O₄ ) and B (Ni _0.2 Zn_0.3 Fe_2.5 O₄ ) with natural resonance frequencies F_xS around 0.05 and 0.35 GHz, respectively, were employed for the calculation. The calculated loss power ratio P _loss /_in of the ferrite films exhibited similar frequency dependences to those of the product of the measured imaginary permeability μ' and the frequency f(μ' x f) for both films. The loss power ratioP _loss /p_in for Film B, which had a higher f_r, was higher than that for Film A above and around the cross point of theirμ×f. This indicated that magnetic loss was the predominant factor inP_loss /P_in. For Film B,P_loss /P_in was almost the same as that without ferrite films around 0.1 GHz, where P _loss /P_in began to rise. This meant that the insertion loss was negligibly small. In addition, the harmful effect of the ferrite films on signal transmission due to an increase of reflection parameter S₁₁ was weak enough for both films. The simulation results described above suggested that the lead frame equipped with a low-pass filter having an intended cut-off frequency can be obtained by changing the permeability spectrum, i.e., chemical composition of the ferrite film.