Plating and Its Applications in Heat Sinks

Introduction

Plating is one step in the manufacturing of some Torrey Hills Technologies heat sink products like WCu and MoCu. This application note briefly discusses plating and its application in Torrey Hills Technologies heat sink products.

Definition

Plating alters the surface of material by depositing metals chemically or electrochemically while maintaining its bulk properties to meet the functional and environmental requirements.

Purposes of plating

A. Decoration

Decorative plating is used for adding attractive appearance such as gold on jewelries and Olympic medals.

                        

Fig. 1 Decorative plating

B. Corrosion protection

Some devices have to serve in tough environment like high temperature, wet and salty atmosphere. Common technique to meet this corrosion protection requirement is zinc plating, Ni/Au plating and etc.

                             

Fig. 2 Corrosion protecting plating

C. Engineering functions

In electronic packaging, some major functional rules required are electrical conductivity, die attachability, and wire bondability.

Electrical conductivity is greatly improved when applied gold plating because of the superior conducting characteristics of gold. Typical solderability includes gold-silicon (94% Au-6% Si) eutectic for die attaching the silicon chips, gold-tin (80% Au- 20% Sn) for sealing lid assembly, and tin-lead(63% Sn- 37% Pb) for common soldering pads, lead frames, and pins to ensure the electrical connections. Wire bonding is done after die attaching by connecting the silicon chip to the bonding pads with pure gold or aluminum wire. Its mechanism is the intermechanical locking between the wire and the plated film.

Two types of plating

Electroplating

A. Electrolyte

Electrolyte is an essential ingredient for a plating operation. It is important in the following respects.

1). Source of metal—The solution offers the metal to be deposited.

2). Conduciveness—Good conductive media enhance the mobility of metal ions

3). Stability—Hydrolysis of metal salts [MX+H₂O→M(OH) +HX] usually results in insoluble substances. Proper electrolyte provides the additives to break or prevent the formation of these hydrolyse.

4). Buffering effect—This helps to stabilize the pH range of the plating solution.

5). Deposit modification—Plating solution with only metal ions gives rough and nodule deposit which is usually useless in practice. The deposition condition can be greatly improved by adding grain refiner in the plating solution.

6). Dissolving anode—Except for inert anode like Pt/Ti for gold plating, anode metal is a replenisher for metal ions into the solution. The chemistry of the solution is aided to dissolve the anode metals and continuously react as a passivation breaker.

B. Anode

As a part of a complete electrical circuit, the basic function of an anode is to introduce current into a bath. There are two types of anodes, namely soluble anodes and inert anodes. Table 1 compares these two anodes.

Table.1 Soluble anodes VS Inert anodes

	Soluble	Inert
Plating application	Cu, Ni, Sn, Sn-Pb	Au, Pt, Rh
Metal replenish	From anode	Require routine addition
Anode shape/size	Limited	Any size and shape
Anode maintenance	Need routinely	Not required

C. Power source

DC (Direct current) power supply is needed to constantly move metal ions for deposition.

Electroless plating (Chemical plating)

This kind of plating does not need outer electric energy. There are two typical types of electroless processes, namely, immersion and autocatalytic platings.

A. Immersion Process

Immersion plating is the deposition of a metallic coating on a substrate by chemical replacement from a solution of a salt of the coating metal. The following reaction occurs when immersing base metal (M1) into the solution which contains noble metal (M2) ions.

M1→M1n⁺ + ne^-    Ionization of M1

M2n⁺ + ne^- →M2    Deposition of M2

Total reaction

M1 + M2n⁺ → M1n⁺ + M2

The driving force of this chemical replacement comes from the different electromotive force of metals. The mechanism of immersion process is similar to wet corrosion.

B. Autocatalytic Process

In immersion process, the thickness and adherence of the film is very limited since the reaction ceases when the work surface is covered with more noble metal. An autocatalytic process reduces the deposited metal from its ionic state in solution by a chemical reducing agent.

In summary, electroless plating process has the following characteristics.

1. Uniform thickness distribution created by perfect throwing power.

2. No requirement for electrical equipments.

3. Chemical cost is much higher than in electrolytic process.

4. Good for products which have electrical connection difficulty.

5. Some deposits have unique properties.

Nickel plating

A. Necessary ingredients

a)  Source of nickel ion—NiSO₄, Ni(NH₂SO₃)₂

b) Anode activator—NiCl₂, Ni(BF₄)₂

c) pH buffer—H₃BO₃, CH₃COOH

B. Major reactions

a) Decomposition of Ni salt

NiSO₄→Ni²⁺ + SO₄^2-

b) Decomposition of Ni metal on the cathode

Ni²⁺ + 2e^-→Ni

c) Dissolution of Ni on the anode

Ni + SO₄^2- + 2e^-→ NiSO₄

d) Some possible reactions that can be prevented by the addition of anode activator and pH buffering agents are as follows.

2H₂O→2H⁺ + 2OH^-

2H⁺ + 2e^-→ H₂

Ni²⁺+ 2OH^-→Ni(OH)₂

SO₄^2-+ H₂O- 2e^-→ H₂SO₄+ O passivation

Gold plating

A. Necessary ingredients

a) Source of Au ion—KAu(CN)₂

b) Conductive salt—KCN, K₂HPO₄

c) pH buffer—K₂HPO₄

B. Major reactions

a) Decompositions of gold complex salt

KAu(CN)₂→K⁺ + Au(CN)₂^-                Au(CN)₂^-  → Au⁺ + 2CN^-      

b) Deposition of gold on the cathode

Au⁺ + e^-→Au     

c) Formation of KCN

K⁺ + CN^-→KCN      

Plating in WCu and MoCu heat sinks

WCu and MoCu thermal management materials of Torrey Hills Technologies enjoy properties such as high thermal conductivity and adjustable CTE (thermal expansion coefficient). With proper Ni and Au plating, they are widely used in RF and microwave packaging, laser diode packaging and etc.

                                

Fig. 3 Various heat sink products of Torrey Hills Technologies

Au is one of the best wetting materials. Au plating greatly improves the heat sink surface soldability. Au is also inert for oxidization, thus preventing corrosion of the inside material. What is more, Au is really soft. Microscopically, the two mating surfaces will make contact only on the high points. So in this heat sink, the soft gold plating surface can deform and increase the contact surface area.

Nickel works as a barrier between gold and copper layers. This Ni layer prevents the diffusion between Cu and Au layers.

For more information on heat sink products of Torrey Hills Technologies, please check http://www.torreyhillstech.com/heatsinks.html