Manufacturing / Production Technology, Hardware & Services


Selective conformal coating and focused heat cure

10 October 2018 Manufacturing / Production Technology, Hardware & Services

Conformal coatings, of all types, serve a very useful purpose of circuit protection in harsh environments. They can modify the surface insulation resistance and provide a barrier to chemical or moisture attack, assist with mechanical shock resistance and help to keep dendritic growth down to acceptable levels. They will never stop dendrites, nor tin whiskers, from growing in the first place, but they will limit their activity and reduce the incidence of cross-talk or short circuits.

Figure 1. Typical industry take-up of conformal coating.
Figure 1. Typical industry take-up of conformal coating.

There are many applications where conformal coatings have become the norm because of this protective property, but it has to be said that conformal coating introduces further steps into the process chain and every step in that chain carries a risk of error – even though that error might be infinitesimally small. The basic SMT process of print solder paste/place components / reflow solder is a three-stage process and there are therefore three areas of error risk.

So, what is an error? The old perceived wisdom suggested that it is anything to do with a defect in the production process. Things like DPMO (defects per million opportunities) definitely apply but we should also consider things like efficiency levels. After all, we are supposed to be ‘lean, mean and hungry’ and we use concepts such as lean manufacturing regularly. This is where selective conformal coating starts to make an impact. It is an efficient process.

There is no hard-and-fast rule that states that any particular material is the only coating material for a particular task. All coatings perform to the protective requirements mentioned above. So, for example, a silicone material is a rubbery compound that has a fair degree of flexibility whilst adhering fast to the surface. It is therefore used extensively in industries where flexibility is required, such as flex circuits or flex-rigids.

The downside is that silicone is very difficult to clean and equipment needs regular cleaning and maintenance. There may not be any errors in the product but the efficiency of using silicone may affect the overall factory efficiency. Other materials may need specific processes and so the choice of material boils down to protective requirements plus ease of use.

Broadly, there are six material types that are commonly used: acrylics, polyurethanes, epoxies, silicones, parylenes and ARUR - which is a mix of acrylic and polyurethane. It is not the purpose of this article to discuss these materials in detail. They all have characteristics with respect to ease of use, rework, protection, industry usage and so on. Nor will this article be a guide to which is the best material for any application. The main purpose here is to discuss selective conformal coating which is suitable for all of these materials, with the exception of parylene which requires a vacuum deposition process, not a spray process.

If the product and the end-use demand it, any industry can adopt conformal coating but certain industries have historically used coating techniques for a long time. These are typically aerospace/avionics, automotive, military and medical. The medical industry is the least prone to change because of the regulatory procedures that must be undertaken, but the other industries, and any others, tend to choose a material for a particular purpose and then only change if necessary.

From a production point of view, we must consider the technologies available to provide the production efficiencies we require. There are factors such as: volumes or throughput, cleaning and maintenance, masks, flexibility of use, tooling, and repeatability that must be considered. We also need to consider costs, quality issues and special requirements such as the vacuum system demanded by parylene.

Figure 2. The ECO99C focused heat curing oven’s batch magazine system has a floor area of 2,4 m (3,0 m) long x 1,2 m wide, excluding input or output magazine loaders. It can heat 3 magazines’ worth of product at one time and will often be faster than an in-line system in a much smaller floor space.
Figure 2. The ECO99C focused heat curing oven’s batch magazine system has a floor area of 2,4 m (3,0 m) long x 1,2 m wide, excluding input or output magazine loaders. It can heat 3 magazines’ worth of product at one time and will often be faster than an in-line system in a much smaller floor space.

Focused heat cure

There are essentially four types of curing techniques, depending on the chosen material: heat cure, UV cure, moisture as a catalyst cure and vacuum deposition. Most materials use either heat or UV and there are a number of solutions available to provide the required throughput. Materials that require moisture as a cure catalyst will require extra cost to provide the moisture control.

Vacuum deposition systems (parylene) will be very expensive and are only available as a batch process which means that throughput depends on the coating amount and the batch sizes. Parylene is widely used by the military and aerospace industries because it coats very thin layers all over the product – even underneath components – and batch sizes tend to be small.

UV curing can either be a dwell process or a constant-movement in-line process. A dwell process means that the product dwells on a conveyor under a UV source for whatever time period the material needs. Throughput depends on the cure rate per product.

An in-line UV cure system has a controllable conveyor with a UV radiation system along its length so that boards are passed into it from a coating system on a regular basis. The length of this oven dictates the throughput. UV curing does not suit magazine batch curing since the UV must ‘see’ all of the conformal coat material on the board in order to cure it. Consequently UV systems need to process one board at a time.

Heat curing, as its name suggests, requires the application of heat to polymerise the material to cure the coating. In-line reflow ovens, situated after the coating system, can be set up for a cure profile and are often used for this application.

There is a unique alternative to conventional in-line heating and this is a magazine based system with a cyclic-run-conveyor. This technique is a TTnS patented invention for curing and can offer advantages over conventional in-line heat curing because the floor area is usually much smaller and, more importantly, the need to load a magazine with product before the magazine transfers into the batch oven means that the product undergoes ‘relaxation’ time before heating. During this relaxation time, solvents (either aqueous or alcohol based) will start to evaporate naturally before higher-temperature curing takes place.

This relaxation time is extremely important from a quality perspective because this time period yields a reduced propensity for voids and/or bubbles to occur in the material. A typical in-line reflow system probably will generally not have sufficient relaxation time for most of the solvent to evaporate, therefore the probability of voids or bubbles in the coating using an in-line reflow system will be much higher than using a magazine based system. Voids and bubbles are a significant quality issue because they could lead to poor adhesion, they could yield a site for dendritic growth to begin, and they also reduce the dielectric properties of the board.

Visit www.dataweek.co.za/+k5084 for the full version of this article, which covers manual, semi-automatic, fully automatic and selective conformal coating application, as well as the issue of tin whiskers.



Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Thermal wire strippers
30 September 2020, Vepac Electronics , Manufacturing / Production Technology, Hardware & Services
Hakko’s FT-802 thermal wire strippers satisfy the demands of the military, medical and aerospace industries. The new model ensures the precise, clean-cut removal of wire insulation, even through highly ...

Read more...
Ask your PCB supplier about reliability, not just capability
28 October 2020, Elmatica , Editor's Choice, Manufacturing / Production Technology, Hardware & Services
Make sure to discuss combinations and how your solution affects risk of failure with the multiple heat cycles required to assemble the product, and the risk of failure in the final application.

Read more...
Europlacer garners SMT pick-and-place honours
28 October 2020, Truth Electronic Manufacturing , Manufacturing / Production Technology, Hardware & Services
Industry analyst firm Frost & Sullivan (F&S) has confirmed that Europlacer took the accolade of Global Company of the Year in the SMT pick-and-place sector for 2020. This marks Europlacer’s sixth Frost ...

Read more...
How you can help your EMS partner
28 October 2020, Jemstech , Editor's Choice, Manufacturing / Production Technology, Hardware & Services
If you want a good quality, reasonably priced product in a quick turnaround time, always communicate your assembly requirements effectively with your EMS partner.

Read more...
Practical experience with PCB robotic soldering processes
28 October 2020 , Editor's Choice, Manufacturing / Production Technology, Hardware & Services
Over the past few years there has been a lot of discussion over the need for higher temperature materials and expanding the use and knowledge of high temperature assembly techniques.

Read more...
Indium announces new high-reliability alloy
28 October 2020, Techmet , Manufacturing / Production Technology, Hardware & Services
Indium has released a new high-reliability alloy with enhanced thermal cycling performance specially formulated for harsh environments. Indalloy292 is an innovative alloy engineered to provide advanced ...

Read more...
Compact X-ray inspection system
28 October 2020, Techmet , Manufacturing / Production Technology, Hardware & Services
Nordson DAGE announced the launch of the Nordson DAGE Explorer one compact X-ray inspection system. Designed specifically for the electronics industry with Nordson DAGE’s unique integrated imaging technology, ...

Read more...
Automating material handling through robotics
28 October 2020, MyKay Tronics , Manufacturing / Production Technology, Hardware & Services
Stock accuracy, traceability, and timely delivery of parts are key aspects of a material handling system. Today, most factories still rely on manual handling of electronic components, in the best case ...

Read more...
Data sciences harness full value of factory floor data
30 September 2020, Omron Electronics , Manufacturing / Production Technology, Hardware & Services
Data is easily collected and stored, but in most cases the data pipeline stops here and there is hardly any value extracted from the data. The data pipeline is often not completed in a proper way so that ...

Read more...
Oven control system supports IPC CFX
30 September 2020, MyKay Tronics , Manufacturing / Production Technology, Hardware & Services
BTU International, a leading supplier of advanced thermal processing equipment for the electronics manufacturing market, announced that its new Wincon 7.2 supports IPC CFX (Connected Factory Exchange) ...

Read more...