The field of printed circuit board finishing and bonding methods has been growing and diversifying for decades. The design of PCBs depends heavily on the use and environment of the electronic package, and those design decisions include what Surface Mount Technology (SMT) is employed. SMT involves the finish, the bonding and soldering method, flux cleaning, reflow and curing, wire-bonding and any other operations that are used to apply components to the surface of circuit boards.
For high-reliability applications, such as electronics used in military and defense aircraft, Electroless Nickel Immersion Gold or ENIG is the most common finish utilized. The strength of this finish makes it ideal for preserving the life and performance of hi-rel electronics in systems like Long Range Surface to Air Missile systems and systems used to track projectiles, communicate and navigate.
Pretty much all PCBs are designed with copper finishes on their raw, unpopulated surface. Copper is a highly reactive metal and, if left exposed, the copper surface will oxidize and deteriorate, ruining the circuit board. The finish exists to create a protective barrier over the copper circuitry and to provide a solderable surface when assembling the components onto the board.
Choosing the appropriate surface finish, and ensuring that the finish itself is fully prepared to be bonded or soldered to, gives you a critical interface between the component and the PCB. Finishes are chosen with the end product in mind and some of the considerations in choosing the best finish include:
- Cost of metals or other materials
- Life span
- Harmfulness of materials involved (carcinogens, lead content, etc.)
- How flat the finish is
- Difficulty of application
ENIG is a flat, lead-free finish that has great durability, and as such, has become one of the most widely utilized PCB surface finishes. In order to mount components onto an ENIG surface you have to make absolutely sure that it is truly ready to be soldered to, bonded or coated. In this article we’ll explore some of the problems that can occur with ENIG and how to make sure they don’t appear in your PCB assembly process.
What is ENIG?
|Electroless Nickel Immersion Gold is a two-part finish applied to printed circuit boards and consists of a very thin layer of gold (about 2-8 μin thick) over a layer of nickel (about 120-240 μin thick). The nickel protects the copper on the board and the gold ensures the nickel is safe from corrosion during storage before assembly of the board.|
|Other common PCB finishes are:||Image courtesy of Epec Engineered Technologies|
- HASL (Hot Air Solder Leveling)
- Immersion Tin
- Immersion Silver
- OSP (Organic Solderability Preservative)
- Hard Gold
All of these solve the basic problem of copper wiring vulnerability and each require proper handling and treatment to ensure successful bonding of the components. ENIG, however, has been established as the preferable solder mount method for high-reliability electronics applications with complex circuitry design.
Download our checklist to diagnose the root cause of adhesion failures you’re experiencing in your PCB manufacturing process: Checklist: Adhesion Failure Root-Cause Analysis for Manufacturers
What Could Go Wrong?
Manufacturers of PCB generally take great care when handling boards and components to ensure no damage comes to them. Wearing gloves and using fully sealed packaging goes a long way to preserve the life and usability of electronic components. Protecting them from corrosion due to moisture, finger oils and other environmental contaminants is extremely important. All of these are possible root causes of contamination and problems down the line.
ENIG is also susceptible to a particular kind of corrosion called “black pad” where the nickel has broken down more than desired (some nickel oxidation is required for the gold to form a coating on the nickel) and increasing the amount of phosphorus. When the nickel is corroded due to over oxidation or dissolving during the solder reflow (oven curing) phase, the nickel appears darker and becomes brittle.
But there are a few areas of the circuit board assembly process that typically go uncontrolled and could allow adhesion failure like dewetting, non-uniform coatings, corrosion, dendrite growth, shorts in the board and many other unpleasant issues.
Control Your Surface Preparation Process
Once PCBs have had the ENIG finish applied they often go into storage in batches and await their chance to be populated with small components. These components will be soldered, adhered, wire-bonded, sintered and attached in other ways to the surface mount pads on the ENIG finished boards.
If you think it’s safe to merely unwrap the ENIG and go to town with the bonding, it’s a good time for you to get acquainted with the vacuum plasma process.
Plasma treatment or plasma cleaning is a way to activate a surface by engineering a chemically compatible assortment of molecules at the first few molecular layers of the materials being bonded.
Let’s put it another way. When the ENIG is sitting in storage (or anytime after it’s been applied to the PCB) the chemical characteristics of the very top few layers of molecules of its surface may not be particularly interested in adhering to the very top few layers of molecules of the silver, epoxy, wires or other materials that it is going to try to bond with once the assembling gets underway. So the molecules on those top few layers need to be changed or removed so molecules that are interested in bonding can make their home on the surface.
The role of plasma treatment role in this is to bombard the surface of that ENIG with a gas that alters the chemical makeup of the surface. Adhesion of any kind is a chemical phenomenon that requires a certain set of amenable molecules to exist on both surfaces coming together to form the interfacial bond. Vacuum plasma is a common treatment method because it is able to get into extremely tiny spaces for precision chemical cleaning. Chemical cleaning is what we call getting the chemical recipe of the surface just right.
Vacuum plasma treatment works by introducing an ionized gas to a chamber where a vacuum has been pulled. The PCB is placed in the chamber and the gas interacts with the surface to clean it of organic impurities and alter its chemistry.
It is extremely important to bear in mind that the kind of cleaning plasma does is not the kind of cleaning an ultrasonic bath does. If there is a substance, such as an oil or silicone, on the surface, the plasma will be interacting with that substance and not the surface the operators are hoping to clean. With enough power and time plasma can remove certain substances from a surface but it is always crucial to understand where that power should be concentrated. When you over-treat a surface you can volatilize the surface and cause damage that is not possible to bounce back from. In order to use plasma most effectively you need to know exactly what your surface looks like going into the treatment and what it looks like coming out.
Understanding how plasma treatment works and its effect on adhesion is the first step to controlling the outcomes. But to accomplish that more precisely, you need to be able to measure the surface cleanliness of your board before and after treatment. Getting a baseline before the boards go into the chamber and then getting a quantitative reading on the cleanliness once the boards have gone through the treatment gives you a thorough picture of how much the surface is changing.
Using surface quality measurement equipment as a process control method lets you know how efficacious your treatment process is. Like all quality control methodologies, developing specifications, metrics and standards creates predictable outcomes and higher quality products. Putting a specification on surface quality before and after treatment gives you the opportunity to optimize your surface preparation equipment.
Dialing in the proper cycle time and power of the plasma unit allows the treatment process to do its job - removing contamination and creating a bond-ready surface. It is also important to measure the cleanliness within the vacuum chamber to ensure there are no unintended contaminants being deposited on the surface of materials being placed inside the chamber. Chamber cleanliness is an often overlooked Critical Control Point in the adhesion process of PCB manufacturing. A Critical Control Point is any place where the surface of a material has an opportunity to change, for good or for bad. Every Critical Control Point needs to have a spec associated with it and a way to measure the cleanliness at that point.
Other Non-Obvious Variables That Could be the Root Cause of Adhesion Issues
When the components have been applied to the epoxy on the ENIG during the assembling of a populated circuit board, the whole package will be cured in an autoclave or oven. To ensure even curing and heat transfer, often a silicon rubber press pad will be laid on top of the components. This applies the right amount of pressure and is an extremely gentle process, but as the curing happens the silicone can become mobile, contaminating the board.
Silicone can be one of the most detrimental contaminants to adhesion. Many suppliers of these pads have designed them to be resilient to these high temperatures, but in order to be absolutely certain you’re not inadvertently introducing unwanted substances to your PCB, it is imperative that you have a way of measuring the chemical cleanliness of your board before and after curing.
We’ve talked a lot about the board, but one of the least controlled areas of cleanliness in electronics manufacturing is the surface quality of the components that populate the boards. These chips need to be chemically compatible with the adhesion requirements in the same way that the ENIG does. Not only do the surfaces need to be clean to bond with the board but the side that will eventually have a conformal parylene coating applied to it needs to be clean as well.
Manufacturers expect that the components they are receiving for suppliers are clean and ready for assembly. But without a surface quality specification on all in-coming parts and supplies, it’s anyone’s guess what contamination might be making its way into the process by way of the supply chain. One of the most pivotal Critical Control Points is before any handling or treatment is even done by the manufacturer. Measuring surface quality at the outset and requiring suppliers to provide assurances of the cleanliness of the parts they provide goes a very long way to building predictability into every downstream operation.
A Measured Process is an Effective Process
Manufacturing electronics involves a lot of bonding and coating. Every interfacial bond is vulnerable to contamination and therefore, the chemical cleanliness of every surface needs to be measured and understood throughout the entire adhesion process. This process begins before the cleaning and after the assembling. Every aspect is connected and in order to create a process that allows for high-reliability, you need to measure the surface quality and control the outcomes.
To learn more about diagnosing the root cause of adhesion failures in PCB manufacturing and how to control the outcomes of bonding to ENIG, download our eBook: Checklist: Adhesion Failure Root-Cause Analysis for Manufacturers
Thank you to EPEC: Engineered Technologies for use of their Images, all image rights reserved to EPEC.