Publication date: 21 January 2010
It is widely known that the use of conformal coatings in the electronics manufacturing process helps eliminate airborne contaminants such as dust and moisture as well as creating an effective barrier against chemical ingress. However, there are other sources of contamination that can adversely affect performance reliability, most notably ionic and non-ionic contamination.
Reliability problems stemming from ionic and non-ionic PCB contamination have traditionally been considered a problem in the high reliability, safety critical sector. However, such contamination is becoming a growing problem in more mainstream manufacturing. This is due to the relentless trend towards miniaturisation in PCB geometries and component packaging styles, and in particular lower operating voltages. Keeping contamination at safe levels on an assembled board, however, demands a high degree of process control and optimisation and a thorough understanding of contamination test methods.
The reliability demanded by safety critical applications, where lives are literally at risk if electronics assemblies fail, is way beyond that considered acceptable for everyday products, such as PDAs or mobile phones. To achieve near perfect field reliability levels, however, manufacturers of high reliability assemblies have long employed ionic and non-ionic contamination testing to finely tune and maintain optimised manufacturing processes.
Although these methods were once considered esoteric and excessive for non-safety critical assemblies, the fine pitch, fine line geometries of many modern boards and the growing use of ultra miniaturised and complex component packages, such as µBGA and flip chip, has made this type of testing increasingly mainstream.
In addition, the current economic climate has served to focus the attention of manufacturers on rework and repair costs generated by insufficiently optimised processes and unreliable build quality. Particularly, the cost of field failure: the most expensive and reputation damaging place of all for a product failure to occur and be rectified.
During its manufacture from bare board to loaded assembly, a PCB can easily undergo several tens of process stages, each introducing the risk of both ionic and non-ionic contamination.
Common sources of ionic contamination include etching, plating, tinning or levelling residues, poor soldermasks, under-cured permanent or temporary solder masks, dust, moisture, oil pollution from finger prints, component packaging materials, flux from solder paste and residues from misprinted boards, and machine maintenance oils (especially from wave soldering conveyors).
If this ionic contamination comes into contact with any form of moisture (e.g. due to a high humidity operating environment) a chemical reaction can occur on the surface of a powered-up assembly. Namely: an electrochemical migration between conductive tracks (or pads) on an assembly that is designed to be electrically separate. This is because, as the two tracks pass each other, they can act as anode and cathode, resulting in an electrical field. This can lead to the formation of metal ions which, under the influence of the electrical field, will migrate across the surface of a board from one electrode to another, whereupon they give up their charge and deposit as a metal.
As the process continues there is a build-up of metal atoms at the second electrode that steadily grows backward towards the electrode from which they originated. This results in the formation of a tree-like branching structure - or dendrite - that can present a path of lower electrical resistance and thereby promote current leakage across the PCB.
If the dendrite grows to completely close the gap between the tracks (or pads) it can cause anything from a short circuit between neighbouring conductors (the more closely spaced, the more likely this will happen due to the smaller distance the dendrite has to bridge) to corrosion and catastrophic (total) field failure. Equally, scientific studies have revealed that lower operating voltages (another common trend in electronics generally) actually promotes the formation of dendrites far more readily than higher operating voltages, contrary to intuition.
Common sources of non-ionic contamination include levelling agents used in solder resists, wetting agents in fluxes (in particular spray fluxes), and cleaning formulae that contain surfactant additives (primarily those that are glycol-based and whose low surface energy makes them particularly able to penetrate board laminates and thereby promote the formation of sub-surface dendrites).
Although non-ionic contamination in itself should not pose a threat to operational reliability, it can act as an agent for ionic contamination and thereby further promote all of the problems listed above.
Because ionic and non-ionic contamination can be caused by such a large number of sources, it is an extremely prevalent problem within PCB assembly. In addition to long term reliability issues, it can also cause major process problems such as unpredictable variability and an increase in defect levels.
To prevent this, there are primarily three well-established test methodologies that PCB assemblers can consider. The challenge is that no one single method can be considered truly comprehensive. Manufacturers must therefore select a combination of overlapping test strategies in order to continuously monitor (and thereby optimise) their assembly process and keep contamination levels within acceptable quality control limits. This demands an understanding of the advantages and disadvantages of each test method.
The first test method deals with answering the perennial question with regard to long-term PCB reliability: how clean is clean? This is done by the use of ionic extract cleanliness (IEC) testing - commonly referred to as Solvent Extract Conductivity (SEC) and Resistivity (or Resistance) of Solvent Extracted (ROSE) testing.
In its simplest form, IEC testing involves washing a component or assembly with a test solution of isopropanol and de-ionised water, generally in a volumetric ratio of 75:25 to dissolve the contaminants, and measuring the resistivity of the collected washings.
The change in resistivity of the test solution can be related (by a complex curve fitting algorithm) to the equivalent average weight per unit area of sodium chloride (NaCl) - or salt - that must have been present on the surface of the specimen immediately prior to testing to produce that change.
Such testing can be performed before manufacturing to test the cleanliness of incoming boards and components, during manufacturing as a process control tool, and after manufacturing at the final assembly and cleaning stage to monitor overall process and cleaning quality. In addition, bare board manufacturers commonly test after the fabrication process prior to product shipment.
The drawback with this is twofold: first, prevailing specifications that stem from the US military many years ago suggest a pass fail level of 1.5µg per cm2 of NaCl equivalents. The problem is that this implies that it is acceptable to leave up to that level on every square cm of an assembly. However, with modern miniaturised circuitry this level would almost certainly be far too high. As a result, the pass-fail criterion becomes empirical and subject to thorough testing on an individual assembly basis. As a rough guide, a figure of around 0.2µg per cm2 (i.e. 7.5 times lower) is commonly used.
The second drawback with this method is that it is testing only for ionic contamination and won’t detect non-ionic contaminants. To detect the latter, another technique that may be employed is ion chromatography.
Ion chromatography allows an analysis to be made of precisely what types of contaminants are present on the surface of an assembly. A specialised resin is used to remove contaminants from a test solution taken from a prepared sample. These contaminants are then analysed to reveal the trace elements that are present.
The drawback of ion chromatography is that while it will reveal exactly what contaminants are present on a board, it will not facilitate a determination of whether the end product will be reliable, because some contaminants may not be harmful to operational reliability and can be safely left on a board. It is also a very exacting scientific method that demands expensive equipment and requires a lot of training on the part of the user in order to be able to interpret the test result data accurately.
A third technique is Surface Insulation Resistance (SIR), which has now evolved via thorough scientific research such as the European project headed by the world renowned UK National Physical Laboratory (NPL). This technique has also evolved to a new process characterisation specification now included in the new draft IEC 61189-5 specification.
SIR testing is usually performed on completed assemblies over-mounted on industry-standard test board coupons containing patterns, typically interdigitated combs, designed for the purpose. This procedure has been found to most closely represent an actual manufacturing process.
In operation, the insulation resistance of the test assembly pattern is monitored at pre-set (specification defined) intervals for typically 72 to 168 hour durations, as temperature and humidity are varied. Monitored resistance levels may range from 106LogΩ to 1014LogΩ for applied test voltages (again specification dependent) ranging from 5 to 100V, with a +5 to -50V bias.
If the test substrate has a low ionic content, then the measured SIR will remain ‘acceptable’. If the ionic content is high, then ‘unacceptable’ leakage currents, corrosion and metal migration, or dendritic growth, will occur.
Each SIR test method, standard or specification (which in addition to the draft IEC 61189-5 includes ISO 9455-17, J-STD-001C, IPC-TM-650 2.6.3 and 2.6.3.3, and Bellcore) defines what is ‘acceptable’ and ‘unacceptable’.
If SIR testing is used it is easy for a manufacturer to assess how different process chemistries at each stage of a manufacturing process react with each other - in other words their synergistic compatibility - and to ensure that these remain within acceptable limits.
It is important to remember that conformal coatings are designed to protect against the ingress of external sources of contamination such as moisture, dust and chemicals. However, for sources of contamination that are already present and coated over, the coating will definitely retard the formation of process residues such as dendrites and tin whiskers, it is unlikely that any coating will be able to prevent them from forming in the long term.
With the increasing use of no-clean, miniaturisation and low solvent coatings, this will become more of an issue and so pre-coating cleanliness will become a key determinant of long term reliability. By adopting clean manufacturing processes and employing contamination testing, manufacturers can ensure process stability by a process of on-going optimisation and fine-tuning. Moreover, they can quickly identify the manufacturing stages where most ionic contamination is being introduced and rectify the problem. In doing so they can maximise their manufacturing yields and minimise the cost of rework, repair and ultimately, premature field failure.
For further information about HumiSeal’s products please contact:
Phil Kinner, HumiSeal
T: 001 508-884-5025 E: PKinner@chasecorp.com
HumiSeal provides the widest selection of products to meet its customers’ tough electrical and environmental requirements. It manufactures over 60 coatings, thinners, strippers and masking materials and can also offer custom formulations to meet unusual specifications. HumiSeal products are qualified to MIL-I-46058C, IEC 60664-3, IEC 61086 and IPC-CC-830 standards.
HumiSeal is manufactured by Chase Electronic Coatings, an operating division of Chase Corporation www.chasecorp.com. Founded in 1946, Chase is a global manufacturer of tapes, laminates, sealants and coatings for high reliability applications. Chase maintains facilities in Evanston, IL, Pittsburgh, PA, Albany, NY, Taunton, MA and Camberley, United Kingdom. Additionally, Chase Electronics Coatings has a license partner in Japan.
