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 FeCl2+NiCl2+ZnCl2and an oxidizing solution of NaNO2+CH3COONH4were simultaneously sprayed at 90degC. The ferrite film on every surface of the lead frame had nearly the same thickness of around 3mum 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 (Ni0.2Zn0.6Fe2.2O4) and B (Ni0.2Zn0.3Fe2.5O4) with natural resonance frequencies(frs) around 0.05 and 0.35 GHz, respectively, were employed for the calculation. The calculated loss power ratioPloss/Pinof the ferrite films exhibited similar frequency dependences to those of the product of the measured imaginary permeabilitymu''and the frequencyf(mu'' xf) for both films. The loss power ratioPloss/Pinfor Film B, which had a higherfr, was higher than that for Film A above and around the cross point of theirmu'' xf. This indicated that magnetic loss was the predominant factor inPloss/Pin. For Film B,Ploss/Pinwas almost the same as that without ferrite films around 0.1 GHz, wherePloss/Pinbegan 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 parameterS11was 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.