Optical communications device having electrical bond pads that are protected by a protective coating, and a method for applying the protective coating
A protective coating encapsulates bond pads disposed on a substrate of an optical communications module and extends in between the bond pads. The protective coating has characteristics that (1) increase the dielectric resistances between adjacent bond pads on the substrate, (2) protect the bond pads from moisture in the environment, and (3) prevents, or at least reduces, ion migration between adjacent bond pads. In this way, the protective coating prevents, or at least reduces, corrosion growth that can lead to impedance degradation and electrical shorts between adjacent bond pads.
1. An optical communications module comprising:
a substrate having at least first and second electrically-conductive bond pads extending upwardly from a surface of the substrate such that a channel is formed between the first and second electrically-conductive bond pads, the first and second bond pads dimensioned such that the width of each of the first and second bond pad is greater than a spacing between the first and second bond pad, the first and second bond pads being electrically coupled to respective electrical conductors disposed in or on the substrate;
at least one integrated circuit (IC) having at least first and second electrically-conductive bond pads disposed thereon, the first IC being attached to a surface of the substrate or to a surface of a mounting structure mounted on the substrate;
at least first and second electrically-conductive bond wires, each bond wire having a first end and a second end, the first end of the first bond wire being attached to the first bond pad disposed on said at least one IC, the second end of the first bond wire being attached to the first bond pad extending upwardly from the surface of the substrate, the first end of the second bond wire being attached to the second bond pad disposed on said at least one IC, the second end of the second bond wire being attached to the second bond pad extending upwardly from the surface of the substrate; and
a protective coating encapsulating at least the first and second bond pads extending upwardly from the surface of the substrate and filling the channel formed between the first and second bond pads such that the protective coating electrically isolates the first and second bond pads extending upwardly from the surface of the substrate from each other.
2. The optical communications module of claim 1 , wherein the protective coating also prevents moisture in the environment from coming into contact with the first and second bond pads extending upwardly from the surface of the substrate.
3. The optical communications module of claim 2 , wherein the protective coating also prevents ions from migrating between the first and second bond pads extending upwardly from the surface of the substrate.
4. The optical communications module of claim 1 , wherein the protective coating comprises an epoxy.
5. The optical communications module of claim 4 , wherein the epoxy has a dielectric constant that is sufficiently high to prevent electrical shorting from occurring between the first and second bond pads extending upwardly from the surface of the substrate.
6. The optical communications module of claim 5 , wherein the epoxy has filler particles that are smaller than a pitch between the first a second electrically-conductive bond wires.
7. The optical communications module of claim 1 , wherein the spacing between the first and second bond pads extending upwardly from the surface of the substrate is between about 15 and 40 micrometers (microns).
8. The optical communications module of claim 7 , wherein the spacing between the first and second bond pads extending upwardly from the surface of the substrate is about 20 microns.
9. The optical communications module of claim 1 , wherein the optical communications module is a parallel optical transmitter module having multiple transmit channels.
10. The optical communications module of claim 1 , wherein the optical communications module is a parallel optical receiver module having multiple receive channels.
11. The optical communications module of claim 1 , wherein the optical communications module is a parallel optical transceiver module having multiple transmit and receive channels.
12. The optical communications module of claim 1 , wherein the first and second electrically-conductive bond wires are part of an inter-integrated circuit (I 2 C) interface.
13. The optical communications module of claim 1 , wherein the first IC is a laser diode driver IC.
14. The optical communications module of claim 1 , wherein the first IC is a receiver IC.
15. The optical communications module of claim 1 , wherein the protective coating also encapsulates the second ends of the first and second electrically-conductive bond wires.
16. The optical communications module of claim 1 , wherein the protective coating has qualities that cause the protective coating to adhere to the first and second bond pads extending upwardly from the surface of the substrate.
17. The optical communications module of claim 1 , wherein the protective coating has qualities that cause the protective coating to increase an electrical resistance between the first and second bond pads extending upwardly from the surface of the substrate to prevent electrical shorting from occurring between the first and second bond pads disposed on the surface of the substrate.
18. The optical communications module of claim 1 , wherein the protective coating has qualities that allow the protective coating to flow in the channel formed between the first and second bond pads extending upwardly from the surface of the substrate without disturbing the first and second electrically-conductive bond wires.