Introduction
Printed circuit boards (PCBs) serve as the core component in virtually every electronic device. They provide the substrate to mount and interconnect electronic components using copper traces. To protect these copper traces from corrosion and ensure reliable solder joints, the copper pads are plated with solderable surface finishes.
ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) has emerged as an advanced plating finish for PCBs, providing excellent solderability while also resisting corrosion. This article provides a comprehensive understanding of ENEPIG plating technology, its properties, process steps, pros and cons, and applications.
What is ENEPIG Plating?
ENEPIG or electroless nickel electroless palladium immersion gold plating is a PCB surface finish comprising of three metal platings deposited sequentially on copper pads – nickel, palladium and gold.
- Electroless nickel – Corrosion resistant layer
- Electroless palladium – Barrier layer prevents nickel diffusion
- Immersion gold – Outermost layer provides solderability
The term ‘electroless’ refers to autocatalytic deposition without using electrical current. The metals deposit through a chemical reduction reaction. ‘Immersion’ uses a chemical displacement reaction.
This tri-metal finish provides excellent solderability while also resisting corrosion and oxidation. It is an alternative to conventional finishes like electrolytic nickel/gold, immersion tin and OSP (Organic Solderability Preservatives).
Key Properties
- Excellent solderability and wettability
- Low and stable contact resistance
- Corrosion resistance comparable to gold
- Good wirebondability
- Lead-free solder compatibility
- Halogen-free formulation available
Why Use ENEPIG Plating?
ENEPIG plating offers following benefits over other PCB finishes:
1. Reliable Solderability
- Gold outer layer provides excellent solderability, similar to immersion gold.
- The underlying nickel and palladium enhance adhesion of the gold layer.
2. Lead-free Solder Compatibility
- Gold allows reliable lead-free soldering, unlike tin finishes which are prone to tin whiskers.
- Palladium barrier layer prevents diffusion of nickel into solder joint.
3. Corrosion Resistance
- The nickel underlayer provides corrosion resistance comparable to gold.
- Palladium isolates the nickel from the solder joint.
4. Halogen-free Formulations
- Electroless nickel and palladium use halogen-free chemistries, avoiding issues with immersion gold.
5. Contact Resistance Stability
- The nickel and palladium base preserves the low contact resistance of gold finishes.
6. Wire Bond Compatibility
- The gold layer allows reliable wire bonding.
7. Self-Limiting Thickness
- Immersion plating process provides self-limited uniform thickness.
ENEPIG Plating Process Steps
ENEPIG finish deposition involves sequential electroless plating of nickel, palladium and immersion gold:
1. Electroless Nickel Plating
- PCB undergoes dilute acid cleaning and microetching.
- Activated in palladium chloride to initiate nickel deposition.
- Electroless nickel plating solution deposits nickel through an autocatalytic chemical reaction.
- Thickness of 5 to 8 micro-inches nickel is plated.
2. Electroless Palladium Plating
- Nickel layer is activated in an acidic solution.
- Electroless palladium solution deposits a thin layer of palladium, typically 0.2 to 0.5 micro-inches.
- Palladium prevents diffusion of nickel into solder joint.
3. Immersion Gold Plating
- PCBs are dipped in immersion gold solution which contains gold salts.
- The gold displaces palladium through a galvanic exchange reaction.
- Gold layer of 0.1 to 0.5 micro-inches is plated.
- Provides solderability and wire bondability.
Comparison of ENEPIG vs Other Finishes
| Parameters | ENEPIG | ENIG | Imm. Tin | OSP |
|---|---|---|---|---|
| Solderability | Excellent | Excellent | Good | Fair |
| Process Control | Moderate | Difficult | Easy | Easy |
| Lead-free solder compatible | Yes | Yes | Prone to whiskers | Yes |
| Corrosion resistance | Excellent | Moderate | Poor | Fair |
| Contact resistance | Low and stable | Unstable | Low | Moderate |
| Shelf life | 12 months | 6-9 months | 4-6 months | 3-6 months |
| Soldering heat resistance | Good | Fair | Excellent | Poor |
| Wirebond compatibility | Excellent | Excellent | Fair | Poor |
| Cost | Moderate | High | Low | Very low |
Pros and Cons of ENEPIG Finish
Pros:
- Excellent solderability and contact reliability
- Resists corrosion as effectively as gold
- Compatible with lead-free solders
- Allows wire bonding
- Provides stable low contact resistance
- Halogen-free formulations available
- Self-limiting and uniform thickness
Cons:
- More expensive than tin, silver, OSP finishes
- Requires precise process control
- Palladium is expensive
- Multiple plating steps increase cycle time
- Shelf life shorter than tin finishes
Applications of ENEPIG Plating
The key applications where ENEPIG finish provides benefits are:
1. Lead-free Soldering
Compatible with lead-free solders, unlike tin finishes prone to whiskering.
Withstands under-hood temperatures. Halogen-free for reduced outgassing.
3. Avionics and Aerospace
High reliability needed for extreme conditions.
4. Medical Electronics
Biocompatible finish.
5. Wireless and Portable Electronics
Supports lead-free soldering and tin whisker mitigation.
6. High Speed Digital Circuits
Gold provides low contact resistance stability.
7. Wire Bonding
The gold layer enables wire bonding.
ENEPIG Plating on Different PCB Pad Types
ENEPIG can be deposited on various finishes present on PCB pads:
| Pad Type | Process |
|---|---|
| Bare Copper | Directly applies ENEPIG finish |
| Immersion Tin | Pre-activates with palladium before ENEPIG |
| Immersion Silver | Masks pad surface before ENEPIG deposition |
| OSP | Removes OSP completely prior to ENEPIG |
| Electrolytic Nickel Gold | Strips nickel and activates for ENEPIG process |
| Immersion Gold | Strips gold, activates nickel, then follows ENEPIG steps |
| Solder Mask Defined | Selectively strip solder mask before plating ENEPIG |
Key Process Challenges with ENEPIG
Some process challenges associated with ENEPIG plating include:
- Uniformity: Electroless deposition depends on local chemical conditions. Careful monitoring and tank agitation is required.
- Palladium Activation: Insufficient activation can cause non-uniform nickel deposition and gold embrittlement.
- Bath Maintenance: Regular analysis and replenishment of electroless baths is critical.
- Solder Mask Adhesion: Compatibility between solder mask and ENEPIG chemistry must be ensured.
- Via Filling: Deposits thin coating only. For thicker coatings, additional electroless copper buildup may be required.
Frequently Asked Questions (FAQs)
Q1. Does ENEPIG finish contain any nickel on the surface?
No. The top surface is pure gold, with palladium and nickel below it. Palladium acts as diffusion barrier to isolate nickel from gold layer.
Q2. Can ENEPIG finish withstand multiple reflow cycles?
Yes, ENEPIG provides excellent soldering heat resistance. The underlying nickel and palladium enhance adhesion of the gold layer through repeated temperature cycling.
Q3. What is the typical thickness of ENEPIG finish?
Thickness values are: Nickel: 5-8 microinches, Palladium: 0.2-0.5 microinches, Gold: 0.1-0.5 microinches. Total thickness is usually under 1 micron.
Q4. Does ENEPIG allow both lead and lead-free soldering?
Yes, the gold outer layer of ENEPIG is highly solderable with both lead-based and lead-free solders. Underlying nickel and palladium enhance intermetallic formation.
Q5. Can ENEPIG finish withstand board assembly in wave soldering?
ENEPIG finish has adequate thermal resistance to withstand wave soldering processes, unlike PCB finishes like OSP which have poor heat resistance.
