Introduction to Black Pad and Phosphorus-Rich Layers

Printed circuit boards (PCBs) are essential components in most modern electronic devices. They provide the physical platform to mount and interconnect electronic components using conductive copper traces etched from copper sheets laminated onto a non-conductive substrate.

However, during the operational lifetime of a PCB, the solder joints connecting components can deteriorate, leading to reliability issues and even catastrophic failures. Two common solder joint reliability issues seen in PCBs are:

• Black pad phenomenon – The copper pad underneath the solder joint becomes corroded and takes on a blackened appearance. This inhibits solder wetting during rework/repair and affects long-term reliability.
• Phosphorus-rich layers – Phosphorus from solder leaches into the copper pad, forming brittle phosphorus-rich intermetallic compounds. This leads to poor solderability and crack propagation.

Understanding the root causes and mechanisms of these phenomena is key to mitigating their effects and improving overall PCB reliability.

What Causes Black Pad Formation?

Black pad formation is the result of corrosion of the copper pads/traces caused by mass transport of copper atoms under high temperature and humidity conditions. Several factors contribute to black pad occurrence:

High Lead Solder

Traditional tin-lead solders are more prone to causing black pads versus lead-free solders. Lead alloys corrode the copper oxide layers on the PCB pads during soldering. Bare copper exposed to humidity/moisture permits increased mass transport of Cu atoms.

High Temperature Operation

High temperatures accelerate the diffusion of Cu atoms, especially when combined with humidity ingress into the PCB package. Common high temperature drivers:

• Power chips/devices with high thermal output.
• Poor thermal management or hot spots in the PCB.
• Long-term functional testing at high temperatures.

Humidity and Contaminants

Moisture and humid environments provide the electrolytic medium for mass ion transport. Contaminants like chlorine ions also take part in the corrosion reactions.

Sources of moisture include:

• Ambient humidity entering the PCB enclosure.
• Moisture absorption by components and the PCB substrate.
• Residual board moisture in organic substrates.

Design and Manufacturing Factors

• Fine line traces are more susceptible to corrosion.
• Voids, cracks or imperfections in solder mask coverage expose copper.
• Solder mask material interfaces can increase corrosion.
• Plating quality affects oxidation rates.

Mechanism Behind Black Pad Formation

The mechanism behind black pad formation involves galvanic corrosion accelerated by humidity and contaminants:

Galvanic Corrosion

The solder forms a large cathode while the smaller copper pad acts as the anode. The large cathode has faster oxygen reduction kinetics. This galvanic couple causes rapid dissolution of the anodic copper into Cu ions when electrolytic paths form.

Electrolytic Path Formation

Moisture and contaminants from the environment enter the PCB package and form continuous electrolytic paths between the anode and cathode. This allows current flow to support the transport of Cu ions.

Cu Ion Migration

The Cu ions migrate away from the pad/trace and react with chlorides from the solder flux residue to form copper chloride complexes. These complexes have high solubility and migrate across the PCB surface.

Oxidation and Reduction

The copper chloride complexes are reduced back to copper metal in oxygen rich areas. The regenerated Cu atom deposits on the PCB surface, causing spread of black copper oxide deposits around the pad.

This cyclic corrosion process continues during PCB operation, gradually corroding the copper pad.

Effects of Black Pad on PCB Performance

The black pad phenomenon can lead to several issues that impact PCB functionality and long-term reliability:

Poor Solder Joint Integrity

The corroded copper underneath does not allow good wetting of fresh solder. This leads to poor adhesion between the component lead and pad, causing unreliable solder joints prone to failure under thermal cycling or vibration loads.

Increased Electrical Resistance

Corroded pads have higher contact resistance to components. This may impact high frequency signals or very sensitive analogue circuits.

Loss of Solderability

In severe cases, the pad may be too corroded to allow solder wetting at all. This results in open solder joints causing circuit failure.

Cracking Under Stress

Brittle intermetallic compounds in the blackened layer are prone to cracking under mechanical or thermal stresses. These cracks propagate and cause solder joint fractures.

Difficulty in Rework/Repair

Blackened pads do not wet properly during rework to replace faulty components. This makes repair or rework of affected PCBs much harder.

Root Causes of Phosphorus-Rich Layers

Phosphorus-rich intermetallic layers develop during soldering of component leads to the copper traces of a PCB. The main factors responsible are:

Phosphorus Content in Solders

Most modern solders contain 0.5-1% phosphorus which acts as a mild oxidant during soldering to enhance wetting. Eutectic Sn-Pb solders have about 0.25% P content. The source is the tin ore used to produce solder.

Intermetallic Compound Formation

When solder alloys melt and interact with copper pads, intermetallic compounds (IMCs) like Cu3Sn and Cu6Sn5 form. Phosphorus migrates into these IMCs, forming brittle Cu-Sn-P layers near the pads.

High Soldering Temperature

High soldering temperatures above 245°C accelerates the reaction kinetics between Cu and molten solder. This provides the thermal activation energy for increased phosphorus diffusion into the copper.

Excessive Soldering Time

Prolonged contact between molten solder and the copper pad allows more time for phosphorus to diffuse. Typical causes are repeated rework/repair cycles or hand soldering.

Solder Volume and Design

Excessive solder with a high Cu/Sn interface area exacerbates IMC growth during soldering. Insufficient solder masking also plays a role.

Effects of Phosphorus-Rich Layers on Reliability

Phosphorus-rich brittle IMC layers lead to various failure modes and reliability problems:

Reduced Shock/Vibration Resistance

The brittle phosphorus compounds are prone to cracking under mechanical shock or vibration stresses. Cracks can propagate through the IMC layer, causing fracturing within solder joints.

Poor Thermal Cycle Reliability

Coefficient of thermal expansion mismatch between phosphorus IMCs and copper/solder induces stresses during thermal cycling. Cracks initiate and propagate over time leading to joint failure.

Increased Electrical Resistivity

Higher contact resistance occurs between chip leads and copper traces due to the resistive phosphorus layer. This causes problems in sensitive circuits.

Loss of Solderability

Severely phosphorized pads become impossible to solder. Rework and repair of affected boards becomes very difficult or impossible.

Increased Corrosion and Pad Cratering

Phosphorus IMCs increase anodic dissolution of copper during exposure to humidity and contamination. This worsens pad cratering and black pad issues.

Strategies to Prevent Black Pad Formation

A variety of design and manufacturing approaches can help reduce the incidence of black pad occurrence in PCBs:

Use of Lead-Free Solders

Lead-free solders like SAC305 are much less prone to causing black pads versus traditional tin-lead solders. Hence, switching to lead-free alloys vastly improves reliability.

Solder Mask Over Bare Copper (SMOBC)

Having solder mask overlap the copper pads and traces better seals and protects the conductors from corrosion as compared to solder mask defined (SMD) designs.

Conformal Coating Application

Applying parylene or urethane conformal coatings protects the entire PCB from humidity and contaminant ingress that drives copper corrosion.

Improved Thermal Design

Adequate heat sinking, ventilation and thermal management ensures components and traces do not exceed maximum temperature limits during operation. This inhibits thermal driving forces for Cu diffusion.

Moisture Barrier Protection

Baking PCBs to remove residual moisture followed by application of moisture barrier bags/encapsulation prevents humidity-induced galvanic corrosion reactions during storage and field use.

Cleaner Soldering Practices

Use of “no clean” fluxes and avoiding excessive flux residues on the PCB surface minimizes contamination and corrosion risks.

Tin Immersion Plating

Immersion deposition of 2-5 microinches of tin dramatically improves solderability while also protecting the copper traces underneath.

Methods to Control Phosphorus in PCBs

Various manufacturing processes and material selection criteria can help reduce phosphorus-related reliability risks:

Lower Phosphorus Solders

Using solders with <0.5% phosphorus content limits the amount available for diffusion into copper. Some phosphorus is still needed for good solderability.

Ni/Au Pads With Lead-Free Solders

Nickel/gold plated pads do not form phosphorus-rich IMCs with Sn-based solders. This prevents pad embrittlement while permitting lead-free use.

Optimized Solder Volumes

Stencil/printing process controls should regulate solder paste deposits and volumes on pads to prevent excessive solder and lengthy reflow times.

Lower Soldering Temperature/Times

Minimizing time above the liquidus temperature of the solder alloy reduces IMC growth kinetics during reflow soldering.

Dummy Solders First

“Dummy” solders with minimal/no phosphorus can be deposited first to consume some of the Cu/Sn interfacial area. This reduces the area for diffusion of the primary phosphorus-containing solder.

Conformal Coating

Protective acrylic, urethane or parylene coatings act as diffusion barriers against phosphorus migration into copper traces.

Plating Chemistry Controls

Adjusting the strike deposition process for Cu and Ni/Au platings modifies grain size and minimizes phosphorus contamination.

Conclusion

Black pad formation and phosphorus-rich intermetallic layers are two significant reliability threats in PCBs using Sn-based solders. Increased understanding of the root causes and mechanisms behind these phenomena enables applying targeted mitigation strategies during design and manufacture of boards. As PCBs become smaller and more complex, it becomes increasingly critical to prevent factors inducing solder joint failures. Adopting the right combination of material selection, plating technology, design rules and process controls is key to eliminating black pad and phosphorus-related issues.

This enables improved PCB reliability and service lifetimes to meet the quality and durability demands of electronics consumers. With diligent engineering and manufacturing discipline, black pads and phosphorus can be overcome as technology hurdles in the ongoing evolution of electronics PCB technology.

Frequently Asked Questions

What is the key difference between black pad formation versus creation of phosphorus-rich intermetallic layers in PCBs?

The main difference is that black pads are caused by corrosion of copper pads on the PCB surface, while phosphorus-rich layers are intermetallic compounds that form when phosphorus from solder diffuses into the copper pads. However, both phenomena affect long term solder joint reliability.

How does humidity drive black pad formation?

Humidity provides the electrolytic medium for ion transport that enables galvanic corrosion reactions to occur between solder cathode and copper anode. This leads to dissolution and migration of copper ions away from the pad.

Do Ni/Au plated pads fully prevent phosphorus contamination?

Nickel/gold pads prevent formation of Cu-Sn-P IMCs because tin in the solder does not react with nickel or gold. However, some phosphorus can still diffuse through the Ni layer and make it brittle. So Ni/Au plating helps but does not completely eliminate the issue.

Can black pads be effectively repaired after they form?

Black pads can be difficult to repair reliably after significant corrosion has occurred. The affected area often has poor adhesion and lacks solderability. Rigorous rework processes like completely removing the pad/trace copper and replating may salvage the pad. But often, the PCB needs to be replaced when black pads are detected.

How does conformal coating help prevent both black pad and phosphorus issues?

Conformal coatings like parylene and urethane seal the PCB surface from humidity, contamination and phosphorus diffusion. This inhibits the galvanic and chemical processes that cause copper corrosion and phosphorus-rich IMC formation.