Studying On The Coatings Of QFN EMI Shielding By Using Electroless Cu Plating

The process of electroless copper plating on the epoxy resin used to electronic packaging is studied in this thesis. The results show that the electroless copper plating bath have good stability and long periods, and the Cu deposition have lower surface resistance, good uniformity, better EMI shielding ability and well adhibition to the substrate. It can well fit the QFN packaging process. The optimized process of oil removing, coarsing and electroless copper plating bath are get. The electroless copper plating bath can work at lest five periods with the average deposition velocity about 1.88μm/h. The mixture ratio of the coarsing solution compounds have very little influence on the bonding force between the Cu deposition and substrate except the coarsing temperature. Longer coarsing time and higher coarsing temperature can enhance the bonding force, increase the surface roughness and change the surface appearance from "V" shape to "symbiotic ball-gap" shape. The surface appearance is much important than the modified surface to increase the bonding force. The best result can be get for coarsing 20～30 minutes under 85℃. The stability of electroless copper plating bath can much be increased by using multi stabilizers. The ethanol can not only increase the stability of the bath, but also the deposition velocity when its concentration is less than 20mL/L. According to the electrochemistry mixed potential theory, the linear sweep voltammagram method is used to study the cathod and anode processes of the stabilizers. The results show that α-α' dipyridyl can decrease the deposition velocity by inhibit the anodic oxidation of formaldehyde, and potassium ferrocyanide can stabilizing the bath by inhibit the deoxidation of Cu²⁺. The ethanol can not only inhibit the anodic oxidation of formaldehyde, but also accelerate the deoxidation of Cu²⁺ in suitable concentration. The mixed potential of the electroless copper plating process are also studied. they show that the induction time elongate with the increasing of the concentration of α-α' dipyridyl or formaldehyde. The ethanol can shorten the induction time in low concentration, reversely with the concentration more than 20mL/L. The surface resistance can decrease much with the increase of the Cu deposition thickness. After the thickness reach 1μm, the decrease value become little. The bonding force between the Cu deposition and substrate can reach 5B grade even after the thermal shock test and the cut by using high-speed grinding wheel of the QFN process. The theoretical calculation results show that the EMI shielding ability can reach 60dB with the Cu deposition thickness more than 2μm.