Composite solder balls metallised on the surface and calibrated for the assembly of electronic boards

Method for manufacturing composite solder balls that are metallized on the surface and calibrated, of diameter Df, said balls comprising:

• a core consisting of a spherical support particle of diameter D0, made of expanded polystyrene (EPS) and having an intergranular porosity of at least 50%, and
• a shell that covers said support particle and is formed by a plurality of metallic surface layers, comprising a copper coating of thickness Ecu, at least one nickel layer of thickness ENi, and a gold top coat of thickness EAu, said method being characterized in that it comprises the following steps:
• A) a first step of providing support particles, followed by
• B) a first step of granulometric sorting of the support particles consisting in a step of physical and/or mechanical separation of the support particles having a diameter Do such that D0=Df−2*(ENi+Ecu+EAu), it being possible for Do to be selected so as to be between 200 μm and 1000 μm;
• C) a step of activation treatment of the support particles thus selected, in order to obtain activated support particles;
• D) a first step of metallization of said activated support particles by means of autocatalytic chemical deposition of one or more layers of copper, said step being repeated until a copper coating of thickness Ecu of copper of between 15 and 30 μm is obtained, in order to obtain copper-coated support particles;
• E) a second step of metallization of the copper-coated support particles by means of autocatalytic chemical deposition of at least one layer of nickel alloyed with phosphorous NiP having a mass percentage of from 7 to 10% phosphorous with respect to the total weight of said layer of NiP, said step being performed until a thickness ENi of the layer of chemical nickel of between 4 μm and 7 μm is obtained, in order to obtain support particles coated with chemical nickel;
• F) a third step of metallization of the support particles coated in chemical nickel consisting in a step of deposition of gold by means of a method of galvanic displacement implemented by immersion in an aqueous solution containing gold ions, so as to obtain composite solder balls that are metallized and coated on the surface with a gold top coat of a thickness EAu of between 0.05 μm and 0.12 μm;
• and
• G) a second step of granulometric sorting of the particles thus metallized in order to sort and select composite solder balls which are metallized on the surface and have a diameter Df, at a tolerance of +/−5%.