Successful electronics manufacturing hinges upon control of surfaces and interfaces between dissimilar materials in more ways than many people realize. For example, modern electronics must perform in harsh conditions, so protective materials (conformal coatings) applied to the printed circuit boards (PCB) and components prevent the destructive intrusion of moisture, dust particles and other injurious substances that can electrically degrade the components and interconnects.
Many components are bonded to circuit boards using adhesives. Furthermore, the electronic device itself (a cell phone, car or medical device) is frequently assembled using adhesives in place of -or in addition to- mechanical fasteners. The initial adhesion of coatings and structural bonds, as well as the durability of these components depends intimately on the chemical state of the surface immediately prior to application. Better manufacturing
is enabled when the processes that establish the chemical properties of the
surfaces (e.g. cleaning, activation and other methods) are understood and controlled.
Understanding and controlling surface properties is of particular gravity in the design and manufacture of hi-rel electronics. These are the indispensable components that keep aircraft in the sky, prevent life-saving and life-preserving medical equipment from faltering, and are critical for the delivery of power to our connected world.
The standards of production must meet the standards of performance. To ensure that every chip and every board is fit to be used in a device or machine, adhesion of coatings and bonded components must be guaranteed. Cleaning, surface activation, and other surface preparation processes enable better manufacturing by helping ensure successful adhesion.
Coating, soldering, sintering, and any other adhesion processes need to be understood and controlled quantitatively, with eyes on all the interconnected Critical Control Points (CCPs) that impact the ultimate strength of the adhesion and, therefore, the reliability of the final product. Poor surface quality due to uncontrolled cleaning and treatment processes is the most pernicious root cause of failure, and gets routinely ignored due to the inability to accurately measure the surface condition.
The eBook describes how to increase electronic manufacturing quality through cleanliness.
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Some of the types of coatings frequently employed in electronic component assembly and protection include:
These coatings are integral components of their respective processes, some as a preconditioning adhesion promoter and some as a finishing touch on a mostly completed device. Others in the list can be contaminants if misapplied or if they migrate and must be suitably handled and cleaned to ensure subsequent coating, soldering and other adhesion processes are not impeded.
Cleaning has always played an important role in the manufacturing of electronics but the emphasis on what and how to clean has shifted over the years. The role of electronics in aerospace, automotive and medical device manufacturing, as well as the ever-decreasing size of processors and chips engendering the miniaturization of the most powerful computers ever conceived, has changed drastically over the past few decades and manufacturing processes have had to adapt and evolve for these new responsibilities.
Electronics assemblies need to maintain their conductive and thermal properties between components regardless of the environment they are used in. There are many adhesion processes within the manufacture of these devices beyond the coatings mentioned above. Each process below relies on a chemically clean surface to be created and maintained so that the adhesion inherent to these techniques is strong and the components are protected from failure.
A method of connecting integrated circuits and the board or device. Often aluminum, copper, gold or silver are used and a combination of heat, pressure and, often, ultrasonic energy are used to weld the metal wire to the device.
A process of creating a barrier between the components on a printed circuit board and the environment through either targeted or full encapsulation, usually with a polymeric film.
A low temperature, die-attach technique where a silver paste is applied to a substrate and then the die is placed upon the paste with a certain amount of pressure. One of the many benefits of sintering is the lack of curing or reflow required.
The most common die-attach method for electronic device assemblies, this high temperature process utilizes a metal alloy with a high melting point and bonds the components through a reflow process.
Ionic contamination is a major focus of cleaning concerns for manufacturers. Ionic residues can originate from the aggressive chemistries used in various processes like copper etching or from the use of water-soluble soldering chemistries. If ionic residues are not effectively removed via cleaning, the conductivity of these residues, when combined with absorbed moisture, presents the danger of dendritic growth, corrosion and electrochemical migration (ECM); all of which cause failures in the boards by disrupting or redirecting currents from their intended paths through the components on the boards.
The detrimental effects of ionic residues caused a shift away from the older types of high-residue fluxes and legacy cleaning procedures to new generation low-residue “no clean” fluxes. The introduction of component mounting processes that were, relatively speaking, less prone to failure from a small amount of flux residue generated a not totally warranted sense of security.
Hi-Rel electronics applications means any conformal coating failures, electrical shorts due to ECM events, or any contamination that puts the assembly at risk are no longer acceptable. The closer proximity of components on circuit boards has also made, essentially, any flux not cleaned off the surface a contaminant that makes the assembly vulnerable to failure. Precision cleaning processes are required to remove all possible risk of failures.
Ionic contaminants are usually accompanied by the presence of organic residues including oils, silicone, hydrocarbons, etc. which are present on the surface prior to coating or bonding components. They present a different sort of risk than ionic contaminants. Organic contaminants are not usually a direct threat to electrical performance, but interfere with coatings and adhesives bonding to the board which can result in board failures.
There are dozens of terms for the types of coating failures that are commonly seen on PCB. Many are variations on the same kind of issues. Each one is an adhesion failure that can lead to the board shorting due to corrosion, physical damage or other issues the conformal coating is supposed to protect against.
The detrimental effects of organic contamination include:
Another class of contaminant that can be particularly insidious are the silicones. Because they maintain desirable properties at high temperatures and under exposure to strong chemicals, in many ways they are ideal for electronics fixturing and handling. However, these same properties make them resistant to removal by most solvents and cleaning solutions. Silicones represent some of the most detrimental contaminants found on electronic components: one or two molecular layers of silicone can prevent coatings from evenly spreading, causing pinholes and fisheyes; similar amounts can cause delamination by completely disrupting adhesion of coatings and adhesives.
There are more contaminants preying on the quality of board surface than just the flux residues. This is the origin of the fundamental issue with “no-clean” fluxes. When you don’t clean flux residue off you also don’t clean any other contaminants. Ionic and organic contaminants have many sources and can make their way onto boards at several points throughout the production process. Cleaning the totality of contaminants prior to coating or bonding is crucial for achieving predictable adhesion and performance.
Conformal coating, encapsulation and electronic device packaging as a barrier
against harsh environments is beginning to move past the obvious applications,
like aerospace and medical devices, and into driverless vehicles and consumer
electronics. Ensuring boards and assemblies are clean enough for these high
reliability requirements is a challenge for all electronics manufacturers.
Resistivity of solvent extract (ROSE) testing is a relatively common board inspection method that involves submerging or rinsing boards in solvent and then measuring the conductivity of the liquid. The solution conductivity correlates to the level of ionic contaminants removed by rinsing.
ROSE testing has the narrowest scope of all the quantitative cleanliness tests. It only senses ionic contamination, and only those that are removed in this particular rinsing procedure. Because of this, it gives an incomplete picture of the state of the board. IPC specifically states that this test has very particular uses: “Please note that this method does not predict reliability and should ONLY be used as a process control tool.”
Compared to other tests, the ROSE test is quite cheap and quick but still requires an offline station and takes more than a few minutes to complete. Once the test is done you are really only given a small amount of information without understanding what species of ionic contaminants are present, what their origins might be, and where on the board they might be concentrated. It completely misses any non-ionic contaminants that can interfere with bonding of coatings and adhesives. As board components get smaller and closer together, cleaning becomes more difficult and a more precise test is required to investigate how well the boards are being cleaned.
Unlike ROSE testing, ion chromatography (IC) has the ability to identify specific ions found on the surface. IC can also flag the presence of weak organic acids (WOA’s) left over from flux that had not been sufficiently cleaned. Different fluxes used will have different levels of WOA’s, with “no-clean” fluxes typically having higher levels of WOA’s than most fluxes.
With IC, in a similar fashion to ROSE testing, the conductivity of a solution the boards have been submerged in is assessed and the ionic contamination level is deduced from what was extracted in the solution. An important deficiency of IC testing is its inability to detect organic contaminants, which almost always accompany ionic contaminants.
These testing systems can be very expensive and are also imprecise in their detection of the areas of higher or lower contamination on the board. The results of this test can tell you what is present but not where it is and can result in many runs through a wash process when a more direct cleaning method could be used.
Surface insulation resistance (SIR) testing is an intensive and thorough examination of residues left on board surfaces after processing. This test is not regularly used as a process control but often as a last resort since it takes weeks to perform and is essentially always outsourced. The process involves subjecting the boards to high temperatures that mobilize contaminants on the surface and then evaluate the decrease in the insulation resistance of the board.
This test is useful for predicting certain types of reliability of assemblies but is indirect, time-consuming and expensive.
Contact angle measurements are the only test in this list that can be done rapidly, with quantitative precision, and on very particular and tiny portions of boards. They can be readily implemented in-line allowing them to be used as a process control feedback for cleaning processes or plasma treatment. Each measurement takes about two seconds and provides objective insight into the quality of a surface before and after cleaning or before a bonding or coating procedure. This test can also be performed post-coating to assess the uniformity of a coating as a way to control the deposition technique used in applying the film.
A contact angle is the angle formed between a drop of liquid and a surface. If the liquid ‘wets’ the surface due to strong attraction, it spreads across the surface and makes a small contact angle. If the drop of liquid is repelled by the surface, it beads up (as you might see on a water resistant article of clothing or a waxed car) and the contact angle becomes larger. Low contact angles correspond to clean surfaces that have high surface energy; high contact angles correspond to contaminated, poorly cleaned or poorly treated surfaces. The relationship between contact angle and cleanliness or treatment level is quantitative.
This test is the only test suitable for rapid assessment of electronic components that is accurately predictive of adhesion. While other tests are adequate to determine if there are contaminants that could lead to imminent ECM or conductive anodic filament (CAF) failure, contact angle measurements indicate the presence of contaminants that will interfere with coatings, soldering, wire bonds and sintering which will ultimately lead to board failures of various types.
Contact angle measurements in and of themselves are a pass/fail test but when correlated to the level of contaminants and residues prevalent in electronic manufacturing processes they are a quick, objective definition of successful processes. The data from these tests provide predictive analytics that indicate the reliability of an assembly, and as such, can be used to design and build a process that outputs only qualifying products.
Groups like IPC, ASTM, SAE and IEEE have been creating universal standards and recommendations for manufacturers to follow in order to design production processes that reduce risk and optimize each operation components are subjected to. IPC (formerly known as the Institute for Interconnecting and Packaging Electronic Circuits) publishes the most widely used acceptability standards for electronic equipment and assemblies.
Industry standards are what manufacturers live and die by. Quality and performance standards give manufacturers a framework to operate within but they do not guarantee freedom from failures. In this sense they can be a crutch because they can inspire complacency. When manufacturers have a coating failure or dewetting it is often due to the presence of organic contaminants on the board; cleanliness standards in the electronics industry do not include testing for the presence of organic contamination. In high reliability applications, manufacturers cannot tolerate failure and need better methods to drive down the probability of failure and defects to zero.
Cleaning standards have had to modify and expand their definition of clean over the years to accommodate increased sensitivity to contamination and reduced tolerance for failures. When talking about ionic contamination, the IPC-TM-650 standard sets an acceptable range for sodium chloride (NaCl) of 0.1 µg/in2 for military applications and 0.65 µg/in2 for general applications. But as reliability expectations have risen this standard has had to be augmented. The WOAs and other resins left by so-called no-clean fluxes are not measured and accounted for in tests recommended by these standards.
Merely cleaning for ionic contamination is not enough to prevent failures, and predominant cleaning methods are not meeting the challenge of high reliability requirements.
We want to highlight just a couple of the IPC standards that need to be understood in order to design a production process that relies on predictive rather than preventative maintenance. The process control requirements serve to guarantee cleaning equipment health and close monitoring loops if implemented with holistic data capture at every Critical Control Point in the manufacturing process.
An amendment to the IPC J-STD-001G has included language that opens up the arena for acceptable cleanliness tests as process controls. Formerly, merely testing for ionic contamination was sufficient during quality assessments of cleaning operations. Since board failures due to coating and adhesion failures have not diminished, a more expansive understanding of cleanliness has been brought to bear in the discussion and quantitative tests for non-ionic residues are necessary to control the totality of contaminants that threaten electronic assemblies.
The amendment itself says, “…the use of the 1.56 μg/NaCl equivalence/cm2 value for ROSE testing, with no other supporting objective evidence, is not considered an acceptable basis for qualifying a manufacturing process…”
This makes it clear that if this widely applied measure of cleanliness is not augmented by or paired with additional quality assessments that can cover a broader range of residues and contaminants, then validation of the manufacturing process is incomplete.
Another Critical Control Point that is covered by IPC standards is the safe storage and handling of electronic components and assemblies to preserve their cleanliness and prevent contamination.
Moisture is a major nemesis of electronics manufacturers and IPC 1601 details standards for finding a sweet spot when baking boards to eliminate moisture without causing collateral damage to the components. As stated in the introduction, the guidelines in the document also include suggestions for “proper handling, packaging materials and methods, environmental conditions, and storage for printed boards. These guidelines are intended to protect printed boards from contamination, physical damage, solderability degradation, electrostatic discharge (ESD) (when necessary), and moisture uptake.”
There are control and verification methods mentioned, explaining a method of weighing the board before and after moisture removal and contamination testing such as SIR and IC. Contact angle measurements are a quick, non-destructive and accessible method of determining if boards and components are sufficiently contaminant-free before they are put into storage and have not been contaminated during their time in inventory. Surface treatment degradation, substance transfer from storage packaging materials and other threats to surface quality can all occur during time on a shelf.
Many contaminants found on electronic components and circuit boards can cause inconsistent coating coverage, poor coating adhesion, or can promote dendritic growth. The most common (and potentially very effective) cleaning method of choice, among the many options available, is an automated aqueous wash and rinse cycle. Soluble contaminants (like ionic species along with left over flux residues) and most insoluble debris can be efficiently removed by well-designed and well-maintained industrial wash systems. Less frequently, manual cleaning may be necessary to de-flux degraded residues that may have degraded and/or polymerized in the curing ovens. These residues are insoluble in water-based cleaning solutions and are essentially impossible to remove with a detergent solution impingement alone.
Product performance and reliability depend on thorough removal of organic contamination down to a sub-micron level. Meeting performance requirements for electronic components requires a chemically clean PCB surface which can be difficult to achieve with aqueous cleaning systems. Many detrimental contaminants may be resistant to these cleaning processes, however, a successful adjunct to aqueous and solvent-based cleaning is plasma treatment. A properly engineered plasma treatment process can effectively remove contaminants that are otherwise completely intractable.
Plasma treatment systems provide high precision, molecular level surface engineering of materials like circuit boards. This treatment uses an electric field to ionize a process gas; the excited gas molecules react with contaminants on the surface and oxidize organic materials to water vapor and CO2, effectively removing them. A well-implemented plasma cleaning process will also lightly oxidize the underlying board surface to provide chemical “teeth” for adhesion of conformal coatings and adhesives.
It needs to be understood that these processes are too slow to remove large amounts of contamination so precleaning in an aqueous system may be necessary. Furthermore, plasma cleaning will not remove ionic contaminants since they cannot be oxidized to H2O and CO2 like organic contaminants. For similar reasons, they will not remove particulate contaminants. They excel as a chemical conditioning process.
Because plasma treatments use a reactive gas to perform the cleaning action, the plasma is able to access and clean places on boards that are difficult to chemically clean with traditional methods. Traditionally, treatments are batch processes performed in vacuum chambers. These are preferable treatments of complex shapes, as almost any place the plasma gas can diffuse will be effectively cleaned and treated. More recently, several manufacturers are providing atmospheric pressure systems that can be performed in-line. These systems are line-of-sight and therefore not as effective for cleaning and treating unusual geometries, but the in-line nature makes them an excellent option for high rate, high reliability manufacturing.
Common plasma treatment processes being employed by electronics manufacturers include batch argon or argon/oxygen plasmas in low pressure vacuum chambers or atmospheric pressure plasma in air or He/O2 immediately before an automated pick and place die attach step, or for activating populated board surfaces before conformal coating.
The effects of plasma processes are ‘invisible’ to the eye: the effect takes place at the sub-micron thickness level. Process control and process validation is accomplished by various techniques such as dyne ink tests, water break tests or lab instrument inspections. Dyne solutions are destructive, imprecise, subjective and have trouble characterizing the cleanliness of populated boards. Likewise, water break tests are also subjective evaluations that are confounded by surfaces that aren’t flat and uniform. Lab instruments, such as x-ray photoelectron spectroscopy (XPS) or benchtop contact angle goniometers, can provide detailed analysis of surfaces but are not practical for use in qualifying real assemblies that need to be processed at high rates.
Begin treatment sequence with a surface quality
measurement to get a baseline quality evaluation.
Lab instruments certainly can have a strategic role in creating a baseline for a cleanliness and surface activation standard that needs to be met during production. However, there has to be a direct correlation between the tests conducted on the production line and those done in the lab in order to have confidence that the same parameters are being measured.
Initiate plasma treatment of PCB surface.
Tests done on real parts in manufacturing have especially tough requirements. They need to be fast, repeatable, and accurate. For cleaning and surface treatment processes, the validation tests must be sensitive to the molecular level chemical changes that occur during treatment and processing, and be related directly back to the testing done at the process design stage. When dramatically different quality tests are utilized in production from those used in development it becomes clear why the real world results don’t necessarily match up with predictive models designed to understand the performance output of manufacturing processes.
Conclude the treatment sequence with a surface quality
measurement to ensure the treatment was successful.
Our experience is that many manufacturers will install plasma treatment equipment as a ‘magic wand’ cure-all for contamination that could be more efficiently and reliably mitigated by well-designed and well-maintained traditional aqueous cleaning procedures. But, well-designed and well-maintained implies appropriate monitoring techniques for process measurement and control. Understanding proper surface activation level and uniformity, evaluation of decay rate of properly prepared surfaces and overtreatment prevention that can damage the board surface are all crucial roles of effective process monitoring. Without this monitoring, manufacturers don’t have fully realized treatment processes.
Plasma treatment station with automated, inline validation.
Watch the video here.
Inbound part variability due to vendors’ inability to control consistency and quality is an issue whether manufacturers are aware of it or not. Characterizing the quality of surfaces when they come in from suppliers is crucial to maintaining a cleaning and treatment process that outputs consistent products. If the cleanliness level of the input is in constant flux, then dialed-in cleaning parameters will produce irregular results. This is particularly critical for cleaning processes, and even more so for plasma processes, which are unable to cope with widely varying chemical composition of surfaces prior to treatment.
Visual tests, subjective and discrepant dyne tests, or performance tests done long after a connection could be made back to the vendor, are all lacking evidentiary heft.
A conclusive, reliable and quick quality test done on incoming material is insurance for manufacturers who can’t afford to send unexpected contaminants forward into their process. Quantitative data is vital for effective vendor compliance and root cause analysis. As an example, many anti-adhesion substances are migratory in a production setting and can transfer to surfaces fairly easily.
Parts that have been handled after the surfaces have been assessed for quality frequently have to be re-assessed prior to coating or bonding in order to ensure the state of critical surfaces have not been degraded by the handling process. Components may need to be stored in inert packaging to prevent transfer of even the slightest contamination to the boards. However, best practice frequently dictates the necessity of reassessing surface quality once they are removed from inventory to ensure that no meaningful change to the surface has occurred.
In-line, automated testing matches the requirements of high-volume PCB manufacturing and sets a clear bar for acceptable cleanliness while reducing costs for manufacturers in a multitude of ways. By needing to allocate less time solving persistent coating failures, more time can be allocated to R&D and product development or even resource recovery of sub-standard components.
High-reliability applications can allow absolutely no defective product out of the manufacturing facility. As a result, scrap rates can be a high percentage of total throughput. The cost of failed parts is well known to manufacturers and has to be factored into product prices.
It is ALWAYS more cost effective to prevent failure than to correct it later. This principle is illustrated in the over-simplified 1-10-100 rule. This rule explains how spending $1 on prevention is far better than spending $10 on correction and unquestionably better than spending $100 on failure. The farther down the process an issue is caught the more costly that issue is to correct. If that issue can be removed as a possibility within the equation then the $99 leftover can be put to better use.
All electronics manufacturers are aware of the companies entirely devoted to rework and repair of electronic assemblies. Relying on this service and the inevitability of failures doesn’t have to continue if holistic, process-wide quality systems are implemented.
Reduction of a 5% scrap rate to 3% may be impossible without a quantified assessment of where the current quality levels are. Without quantitative tests, there’s no reliable way of knowing whether or not process changes are having the intended effect on quality, that is, until looking at failure rates. But, by that point it’s too late.
There are many quality standards in place for a variety of aspects of electronics manufacturing, but an often overlooked area is surface quality. This is frequently due to a lack of appreciation as to how they pertain to the reliability of bonding, coating, sealing and soldering processes. Poor surface quality due to uncontrolled cleaning and treatment processes is the most pernicious root cause of failure, and gets routinely ignored due to the inability to accurately measure the surface condition.
Engineers are no longer perennial in-house figures that can be present at the point of inspection. The global nature of manufacturing has forever changed the way we communicate and conduct business between companies and even within organizations themselves.
The worldwide distribution of supply chains, broadening of team locations, outsourcing of critical functions within an organization, increasing automation and the digitization of data and communication has opened up the entire world to even small to medium-size manufacturers. These changes have occurred slowly over the years but have altered the way manufacturing can be done and offered exciting solutions to the way it is currently being done.
A common, quantitative language that can be translated into analytics, will make previously unknowable trends visible and make optimization decisions possible from anywhere. The shareability of data is critical to the functionality of our modern society and the same is true for the new phase of manufacturing we find ourselves in.
This is Industry 4.0, which is really just a catchy name for a highly connected and intelligent way of setting up and operating production processes. Allowing various pieces of interdependent equipment to communicate seamlessly within their network and instantaneously send detailed information to quality engineers means that the miles between departments and divisions melt away. Even extremely physical tests, like those that examine the vibratory impact on electronic components of real-world environments inside automobiles, airplanes, or satellite systems, can be assessed and calculated virtually.
Ensuring adhesion processes by applying this level of automation and computing power to surface quality management will render preventive maintenance obsolete in favor of more powerful, predictive maintenance protocol.
The eBook describes how to increase electronic manufacturing quality through cleanliness.
Learn new methods to improve electronics manufacturing efficiency and product quality.
BTG Labs is dedicated to helping electronics manufacturers gain the reliability they demand. Through holistic process optimization and highly sensitive monitoring that has the resolution to capture sub-micron changes in the chemical composition and properties of surfaces that actually pertain to good adhesion - implementing tools that deliver predictive analytics - calculable performance and product quality are absolutely possible to achieve.
Our approach begins with our Process Experts conducting a Process Walk: our experts examine the manufacturing operations to identify the Critical Control Points that affect product reliability and performance, and then make best practice recommendations about how to properly manage these areas. We then proceed by building a test plan alongside our customers to create actionable projects that address the needs of a proposed or existing production process.
Our team of Process Engineers and Material Science Experts have the equipment and experience required to execute extensive root cause analysis to determine the sources of variables that cause products to fall out of specification. They provide optimization strategies for getting the most out of cleaning and treatment equipment.
We help get new processes online by ensuring proper process and performance validation is available throughout the path from process design, scale up, and production. We can help determine surface quality specifications that need to be put in place at the inception of new processes, streamlining the design to production pipeline. We also help characterize performance of output and offer products that monitor and verify all of these standards at each Critical Control Point.
For more information about how BTG Labs works to give confidence and clarity to manufacturers, download the eBook: BTG Lab’s Guide to Adhesion Science for Flawless Manufacturing. To begin building reliable and predictable manufacturing processes, schedule a call with a BTG Labs’ Process Expert.