Lead frame surface finishing
The present disclosure is directed to a lead frame design that includes a copper alloy base material coated with an electroplated copper layer, a precious metal, and an adhesion promotion compound. The layers compensate for scratches or surface irregularities in the base material while promoting adhesion from the lead frame to the conductive connectors, and to the encapsulant by coupling them to different layers of a multilayer coating on the lead frame. The first layer of the multilayer coating is a soft electroplated copper to smooth the surface of the base material. The second layer of the multilayer coating is a thin precious metal to facilitate a mechanical coupling between leads of the lead frame and conductive connectors. The third layer of the multilayer coating is the adhesion promotion compound for facilitating a mechanical coupling to an encapsulant around the lead frame.
1. A method, comprising:
forming a lead frame, including:
forming a main body and a plurality of leads of a substrate;
electroplating a first coating of copper or a copper alloy on the main body of the substrate;
forming a second coating of a precious metal on the first coating; and
forming a third coating of an adhesion promotion compound on the second coating by reacting a surface of the second coating with a reactive species.
2. The method of claim 1 , wherein the forming the main body and the plurality of leads includes rolling a sheet of copper or a copper alloy, the rolling the sheet including scratching a surface of the substrate.
3. The method of claim 1 , wherein the first coating is pure copper, the second coating is silver and the third coating is silver oxide.
4. The method of claim 1 , wherein the substrate has a first hardness ranging from 100 HV to 200 HV and the first coating has a second hardness ranging from 100 HV to 120 HV, the second hardness being less than the first hardness.
5. The method of claim 1 , wherein a thickness of the first coating ranges from 0.2 μm to 2.0 μm and a thickness of the second coating ranges from 0.01 μm to 0.1 μm.
6. The method of claim 1 , further comprising bonding a metal wire to the second coating.
7. A method, comprising:
forming a first coating of copper on a lead frame;
forming a second coating of a transition metal on the first coating; and
forming a third coating of an adhesion promotion compound on the second coating by reacting a surface of the second coating with a reactive species.
8. The method of claim 7 , comprising forming the first coating of copper with an electroplating process.
9. The method of claim 7 , further comprising placing an integrated circuit die on a die pad of the lead frame after forming the second coating.
10. The method of claim 9 , further comprising forming the third coating on the lead frame after placing the die on the die pad.
11. The method of claim 10 , wherein the third coating is an oxide of the second coating.
12. The method of claim 10 , wherein the second coating includes one or more of gold, silver, an nickel.
13. The method of claim 9 , further comprising forming an electrical connecter between the die and a lead of the lead frame.
14. The method of claim 13 , further comprising forming the third coating after forming the electrical connector.
15. The method of claim 14 , wherein the electrical connector is a bonding wire.
16. The method of claim 9 , covering the die in a molding compound.
17. The method of claim 9 , wherein placing the die on the die pad includes placing the die on the second coating on the die pad.
18. A method, comprising:
electroplating a first coating of copper on a die pad and a lead of a lead frame;
forming a second coating of gold or silver on the first coating on the die pad and on the lead frame;
placing a die on the second coating on the die pad; and
forming a third coating of an adhesion promotion compound on the second coating on the lead after placing the die on the die pad.
19. The method of claim 18 , wherein the third coating is an oxide of the second coating.
20. The method of claim 18 , further comprising forming the third coating on portions of the die pad outside the die.