Coatings critical to electronics integrity
AT A GLANCE
- The performance of a conformal coating is reliant on both the material and how well it’s applied
- For the low-volume, high-mix manufacturer, the most economic and practical means for maintaining conformal coating quality with reasonable production rate is either via batch dip coating or batch spray coating
- Sometimes dip coating is not practical so hand spraying is necessary. However, because many conformal coatings contain hydrocarbons and other VOCs that are potentially detrimental to operator health, fume extraction is required
- A UV cabinet recommended to ensure a consistent inspection regime for the conformal coating. Uncoated areas show up black under the UV lamps
ARTICLE
Conformal coatings are a proven method for protecting sensitive electronics and solder joints from corrosive electrochemical effects. Originally used almost exclusively for aerospace and military applications, the use of these coatings is multiplying as electronics assemblies increasingly pervade modern life – from toys to cars and medical to white goods – and are more frequently used in what can be considered ‘hostile’ situations in terms of such parameters as vibration, thermal cycling and exposure to moisture.
This presents a major challenge for electronics manufacturers, because to apply a durable conformal coating is not trivial. Simply manually dipping the board into a pot of coating is likely to result in a thick layer at a glacial production rate. High-volume manufacturers can justify the cost of automated conformal coating application systems as they provide the best opportunity to apply coatings accurately and consistently at high production rates. For low-volume, high-mix environments, however, semi-automated equipment alternatives for dip or spray coating are a good – and much less expensive – alternative. Fig. 1 shows a PCB suspended above the dip tank on one type of machine. However, there are many machines available, so when drawing up a shortlist, it’s important to understand which technical attributes directly affect the application process quality – and hence protective ability – of a conformal coating.
Figure 1: Dip coating machine
Less is more
The performance of a conformal coating is reliant on both the material and how well it’s applied. Conformal coatings come in two major categories: solvent based and 100% solid (despite the name, these materials are still liquids, but are solvent-less and hence more environmentally friendly).
Coatings are further classified by basic resin types. They comprise: acrylic (AR), urethane (UR), epoxy (ER), silicone (SR) and poly-par-xylylene (XY). Solvent-less coatings usually come in the form of silicones, however, some epoxies and urethanes are also available in solvent-less form, typically UV curable.
A few hybrids consisting of blends of more than one type are also available. Each type has a different combination of durability, mechanical and abrasion resistance, cost, and ease of application and rework.
However, whatever the choice of coating and perhaps somewhat counter intuitively – the maximum performance is achieved by using as little as possible. Coating thicknesses between 25 to 75 μm (0.001 to 0.003-in) are normally all that’s required. (See IPC-A-610D Acceptability of Electronic Assemblies or IPC J-STD-001D Requirements for Soldered Electrical and Electronic Assemblies for conformal coating thickness criteria on assemblies.)
In addition, the coating needs to be free of defects to ensure complete protection and long-term durability. There are standards available to guide manufacturers in the production of quality conformal coatings. For example, in addition to the thickness criteria, IPC-A-610D (February 2005) defines good conformal coating coverage for IPC Classes 1, 2, 3 (where Class 1 are “low consequence” products and Class 3 are “high reliability and military” products) and recommend:
- No loss of adhesion;
- No voids or bubbles;
- No dewetting, mealing, peeling, wrinkles, cracks, ripples, fisheyes or orange-peel.
While a skilled operator can apply an acceptable coat of conformal coating with a brush, this is practical only for the smallest batches, and like any manual production process is subject to repeatability challenges. For larger batches, the increased control provided by semi automated equipment is needed to maintain control and repeatability.
Coating with control
For the low-volume, high-mix manufacturer, the most economic and practical means for maintaining conformal coating quality with reasonable production rate is either via batch dip coating or batch spray coating.
Dipping machines employ a tank of coating material in which the assembly is immersed and then retracted, usually at a controlled rate. Properly controlled, dip coating not only simultaneously covers both sides of the board but also penetrates underneath components for a greater degree of protection. Because the machine features an open tank, solvent-based conformal coatings will gradually evaporate and thus the viscosity characteristics of the material will continually tend to increase. Constantly monitoring the viscosity is therefore important as it directly affects the quality of the coating. Solvent can be added to lower the viscosity.
Immersion speed and board orientation are both critical to correct coverage. The immersion speed should typically be less than 30 cm (12-in) per minute. Look for a machine that offers good control and repeatability of immersion speed (see Fig. 2). This immersion rate is slow enough to force air from beneath components providing that the board is orientated such that no traps are present. Extending the dwell time at the point of maximum immersion will allow for remaining air to come to the surface.
Figure 2: A dip coater should allow precise control of parameters such as dwell time and withdrawal rate to ensure a quality coating
To ensure consistency of immersion depth the coating level in the reservoir should remain constant. Some machines do this by pumping the fluid constantly into a reservoir - with the overflow spilling into a holding tank - maintaining a known depth. Simple tooling then permits assemblies to be coating up to a fixed level. This can be used to the manufacturer’s advantage by eliminating masking if the board design is such that all components that shouldn’t be coated are sited above a defined line. Otherwise masking will be required which can be a very time-consuming and labour-intensive process.
The withdrawal speed needs to be close to the cascade effect (i.e. the speed of flow due to gravity) of the coating material. This varies according to material viscosity but is generally in the 100 to 150 mm (four to six-in) per minute range. The speed should be consistent during withdrawal to minimise the so-called “wedge effect” whereby the coating is thicker at the bottom than the top.
A coating cycle of this duration is almost impossible to achieve by hand. And a quick handheld “dip-and-dunk” is liable to apply far too much coating resulting in drips and runs unacceptable under the guidelines detailed in IPC-A-610D.
The dip coating process is the most cost effective application method for batches from one to more than 600 per hour.
The cost per board at this production rate can be estimated by the following calculation:
Operator - £15 per hour/600 = £0.025 Machine - £19,000/(600 x 40 hrs per week x 50 weeks per year x 2 years) = £0.008 Material - £13 per l/257 PCBs per l* = £0.05
Total = £0.08 per board.
(*Assuming a conformal coating of 37% solids, 0.05 mm coating thickness, PCB surface area of 465 mm2.)
Spraying in safety
Sometimes dip coating is not practical - for example when applying two-part coatings – so hand spraying is necessary. However, because many conformal coatings contain hydrocarbons and other volatile organic components (VOCs) that are potentially detrimental to operator health, fume extraction is required. The extraction system should draw the fumes downwards and away from the operator to minimise wastage and shouldn’t be “wet-backed” (i.e. filtration at the back of the booth is a water cascade) as this can badly affect the coating finish.
Spraying on a turntable-mounted board and rotating 900 ensures thorough coverage. Rotating the board between each spray encourages even coating and penetration under components. A full 3600 rotation represents one application, and most coatings, depending upon their dilution, require two to three applications to achieve a desired film thickness of 25 to 75 μm (0.001 to 0.003-in).
The coating is applied using a high-volume, low-pressure (HVLP) spray gun (see Fig. 3.) to avoid turbulence and uneven coating deposition. (The profile of a typical circuit assembly resembles a city skyline in miniature, which causes turbulence and hence uneven coating deposition if the spray pressure is too high.)
Figure 3: Spray coating uses a high-volume, low-pressure (HVLP) spray gun to avoid turbulence and uneven coating deposition
Filtration reduces coating material build-up in the exhaust system. Without proper maintenance, this can rapidly impair the exhaust efficiency and compromise operator health and safety. Moreover, as most conformal coatings use flammable hydrocarbon solvents, any electrical equipment - such as lighting - in the spray chamber must be “explosion proof”-rated (or Ex ‘d’).
It’s advisable to consider a spray booth that’s suitable for use with the new water-based coatings and UV curable coatings. The best system is one that handles all types of conformal coatings and has large capacity pressure pots for the coating and a purging solvent. The pressure pots are pre-filled with the pre-mixed coating or solvent, which can be sufficient for several days of coating work without refilling. Due to the aggressive nature of many solvents, check that the seals and O-rings used on these pots are of a variety which, although expensive, withstand constant re-connections.
Finally, consider investing in a UV cabinet (Fig. 4) to ensure a consistent inspection regime for the conformal coating. Uncoated areas show up black under the UV lamps.
Figure 4: UV cabinet eases conformal coating inspection
FOR MORE INFORMATION
If you can’t find the information you need at Gen3 Systems’ website at www.gen3systems.com, then please contact Gen3 Systems direct on:
Telephone: +44 (0)1252 521500,Fax: +44 (0)1252 52 1112, or e-mail.
ABOUT THE AUTHOR
Graham Naisbitt (e-mail) is Managing Director of Gen3 Systems Limited (founded on Concoat Systems) and is a member of the IEC’s TC91 WG3, the working group that formulates test standards for the assembly industry. Naisbitt is also Leader of Solderability Testing Standard IEC 60068-2-69, Co-leader of Solderability Testing Standard IEC 60068-2-54, and Member of IPC-J-STD 002 and IPC-J-STD 003.
Download the PDF (4180 KB)8th Aug 2008