Soft Gold plating stands as the top choice for surface finishing in Gold ball wire bonding because it delivers both precision and reliability which are essential in such applications. At RUSH PCB Inc., we offer state-of-the-art soft Gold plating services that provide exceptional bond strength, superior conductivity, and long-lasting reliability for your critical electronic assemblies.
Delve into Soft Gold Plating
Soft Gold plating is an electroplating process tailor-made for applications demanding the highest wire bonding performance. It is a premium surface finish that involves the deposition of a highly pure Gold layer (99.9% pure Gold) over a nickel barrier layer on top of copper traces.
Standard Plating Stacks
- A nickel underlayer for diffusion barrier
- Soft Gold topcoat electroplated to the desired thickness (typically 0.5-1.5 micrometers)
- Suitable for fine-pitch applications like demanding Gold ball bonding
The carefully engineered Gold layer provides an ideal surface for wire bonding operations. While this approach does come with a higher price tag compared to traditional finishes, it offers unparalleled performance for fine-pitch Gold ball bonding where reliability is of the utmost importance.
Important Note: Solder Compatibility
However, there is an important material compatibility consideration to bear in mind: standard tin-based solder alloys tend to dissolve Gold during the solder reflow process. This can lead to a variety of problems:
- Mechanical damage to Gold traces
- Brittle solder joints that are prone to fracture
- Accelerated fatigue cracking due to thermal cycling
Recommendation
In cases where there are soft Gold regions and soldered connections on the same PCB, it is generally better to use indium-based solders instead. Gold is only sparingly soluble in indium, which can lead to more reliable solder joints and a longer lifespan.
Soft Gold is Superior to Other Bonding Techniques
Standard Bonding Issues
The standard wire bonding approach involves bonding Gold wires to aluminum or copper pads, which introduces a range of reliability issues such as:
- Intermetallics at Gold/Al or Gold/Cu interface that are brittle and can affect bond integrity
- Thermal cycling issues due to mismatched coefficients of thermal expansion in dissimilar metal bonds
- Native oxide film present on Al and Cu pads and must be removed more aggressively during bonding, possibly leading to pad damage
Advantages of Soft Gold
The soft Gold approach eliminates these concerns:
- Bonding is homogeneous, and Gold wire bonds to a Gold surface, avoiding the formation of brittle intermetallics
- Improved yields due to more consistent bonding
- Absence of oxide issues: Gold does not form an oxide layer and hence can be bonded without the need for more aggressive scrubbing
- Smaller pads: The improved bonding capability allows for the use of smaller pads, which enables smaller chips and higher I/O density
- Process is more predictable, which makes it easier to validate the parameters
Standard Electroplating Process
Standard nickel and Gold electroplating processes have been in widespread use in the electronics industry for many years as surface finishes for PCBs. The typical standard process is shown in the figure. Let’s first discuss the standard process to understand the limitations.
Standard Approach Drawbacks
Typically, the standard electroplating sequence involves the following steps:
- Nickel electroplating through a photoresist mask on top of copper pads
- Photoresist stripping and copper etching
- Passivation layer of polyimide to protect copper sidewalls from corrosion
- Immersion Gold plating on top of nickel to increase surface wettability
- Bake at 360°C for 1 hour to cure the passivation layer
The problem in this process is the bake at 360°C for 1 hour causes the nickel to diffuse into the Gold layer and significantly harden the Gold surface. The hardened surface has a lower wire bondability, and thus the standard approach cannot be used for a reliable wire bonding pad.
Improved Selective Plating for Wire Bonding
In this section, we will discuss how RUSH PCB Inc. employs an improved selective plating technology to be able to create reliable wire bonding pads.
Optimized Process Sequence
The optimized selective plating sequence is shown in the figure. The Ta/TaN liner is used as the conducting layer:
- Ta/TaN liner deposition: This can be done by sputtering a conductive Ta/TaN layer on top of copper
- Photolithographic patterning: Pattern the pad geometry using photoresist
- Selective copper etching: Electrochemical etching to remove unwanted copper between the pads
- Photoresist stripping: Strip the photoresist
- Ni and Au electroplating: Plating through the Ta/TaN liner directly onto the copper pads
Layer Thickness
The layer thicknesses that we typically get are as follows:
- Nickel (diffusion barrier): ~1 micrometer
- Soft Gold (bonding surface): ~0.5 micrometers
Note that we have copper fully encapsulated and protected under the Ni and Au layers
The final stack completely encapsulates copper and provides full protection from corrosion. In addition, the topmost soft Gold layer has all the properties necessary for a good, reliable wire bonding pad.
Industries & Applications
Soft Gold plating finds crucial applications across a wide range of high-reliability electronic products:
- Aerospace & Defense: Flight control systems, avionics, radar assemblies
- Medical: Implantable devices, medical imaging equipment, patient monitors
- Telecom: High-frequency RF modules, base station components, satellite communication systems
- Automotive: Engine control units (ECUs), ADAS, powertrain management systems
- Industrial: Process control, instrumentation, sensor systems
Specifications & Quality
Quality is of the utmost importance to RUSH PCB Inc. throughout the soft Gold plating process:
- IPC standards are strictly followed with regard to Gold thickness uniformity
- The thickness is measured to the part using X-ray fluorescence (XRF)
- Gold purity is also verified (min. 99.9% for soft Gold)
- Wire bond pull strength testing
- Visual inspection for defects in the plating
Start Your Soft Gold Plating Project
We also provide advanced PCB technologies such as:
- High Layer Count: Supports up to 50+ layers for complex circuit designs
- Exotic Materials: Rogers, polyimide, ceramic, and more
- Laser Drilled Micro-vias: Enables high-density interconnect (HDI) technology
- Blind/Buried Vias: Supports non-traditional via structures
- Controlled Impedance: Design and manufacture for specific impedance requirements
- Soft Gold and ENEPIG Surface Finishes: Provides the highest level of reliability for wire bonding
RUSH PCB Inc. provides top-notch manufacturing for these advanced PCB technologies for the following applications:
- Medical Device
- Automotive
- Industrial Automation
- Telecom/RF
- Defense & Aerospace
Why RUSH PCB for Your Soft Gold Plating Needs?
- Experience: Our team of engineers have several decades of experience in advanced surface finishing and can guide you towards the right solution for your design.
- Control: Our plating equipment and process control allow us to maintain a consistent, repeatable process that can meet the stringent requirements of demanding applications.
- Turnaround: Our manufacturing processes have been optimized to provide fast turnarounds to help you prototype and manufacture your designs.
- Price: Our price is competitive and, because of our high processing efficiency, volume-based, which can make your project cost-effective without sacrificing quality.
Contact RUSH PCB Inc
If you’re ready to start your soft Gold plating project, get in touch with us at RUSH PCB. We offer:
- Free DFM Review of your design
- Consultation on soft Gold plating specification
- Competitive Quotation
- Technical recommendation for optimal wire bonding results
Give us a call today and let’s start working towards a solution to your wire bonding challenges.
