With the continuing improvement of component packages in both weight and size, the need for an improved conformal coating increases for preventing contamination and moisture ingression. Reductions in overall component dimensions and pitch have increased the probability of dendritic growth and corrosion if an adequate conformal coating is not applied - and applied correctly. Where a conformal coating provides excellent protection for one component package, it does not always provide the same protection for another. A comprehensive test protocol including multiple types of component packages is needed to evaluate and select a conformal coating for your printed circuit board (PCB).
A test specimen should be used that replicates the hardware configuration of the PCB that is to be coated. Current test substrates available only contain a comb pattern for determining the presence of contaminants as indicated by a drop in surface insulation resistance (SIR). However, many of the substrates designed today include a mixture of surface mount components such as chip components, MELF components, SOIC’s, TSOP’s, QFP’s, and BGA’s of various dimensions and pitches. One test specimen designed by Soldering Technology International, Inc. (STI) incorporates a variety of daisy-chained components to test for dendritic growth and corrosion. The PCB contains two 0.5 mm pitch QFP’s, two 0.5 mm pitch TSOP’s, four 1.27 mm pitch PBGA’s, and twelve 0805 chip resistors. The QFP and TSOP each contain a double-daisy chain for applying a voltage bias, determining insulation resistance, and monitoring for intermittent failures such as shorts and opens while the PBGA contains four perimeter row daisy chains. The manufacturing process employed when assembling the test specimen should be matched as closely as possible to that of the actual hardware.
The first step in the test protocol is ensure that proper manufacturing processes and handling do not introduce any ionic contaminants to the PCB. If a cleanliness level is not specified when procuring circuit boards, PCB’s from a lot should randomly be ionograph tested for approved cleanliness levels. Results above the approved microgram NaCl/sq.in. level may require an additional cleaning step of bare circuit boards prior to assembly. An additional ionograph test should be conducted post-assembly before applying a conformal coating to ensure that ionic contaminants will not be trapped by the applied coating.
In order to evaluate the ability of a conformal coating to prevent moisture ingression, act as a barrier to surface contaminants, and provide surface insulation, standards have been developed for electrical and environmental screening. Several test methods are already available for determining the effectiveness of applied conformal coatings on a PCB. IPC, JEDEC, and Mil-Specs are available for electrical and environmental testing. STI prefers a combination of approved EIA/JEDEC and IPC standard test methods. IPC-TM-650 Method 2.6.3.4 outlines a testing procedure for determining moisture ingression and SIR values of conformal coatings. It also outlines a procedure for determining the susceptibility of the conformal coating to electrochemical migration (ECMR) by placing a voltage bias on adjacent conductors. STI incorporates the procedure outlined in this IPC test method with the temperature range outlined in JESD22-A101-B, Steady State Temperature Humidity Bias Life Test. STI’s modified test protocol follows the procedure below for determining the insulation resistance and level of moisture ingression of conformal coatings under accelerated temperature and humidity conditions.
STI Conformal Coating Evaluation Procedure:
Condition the specimen at 50°C with no added humidity for a period of 24 hours to completely dry the test assembly prior to electrical and environmental testing.
STI Conformal Coating Evaluation Procedure (Cont.)
2. Allow the specimen to cool prior to recording an initial insulation resistance measurement. Record the insulation resistance by applying 100 VDC to the test specimen as shown below. The minimum allowable resistance is 100 MΩ.
3. Place specimen in a humidity/temperature cycle chamber and apply 10 VDC bias to alternating daisy chains. The presence of corrosion/dendritic growth shall be noted by a voltage drop across the 10 kΩ resistors placed in series with the grounded connections. An approximate 9 VDC drop will indicate a 1000Ω resistance dendritic growth/corrosion between adjacent rows.
4. Expose test specimen to 10 cycles of temperature while maintaining 85%RH. Polarization voltage shall be maintained throughout the entire 10-cycle period. One cycle is as follows:
A. Start test at 25° C and raise the temperture to 85° C over a time span of 2 hours.
B. Maintain temperature at 85° C for 4 hours.
C. Lower the temperature from 85° C to 25° C over a time span of 2 hours.
D. Maintain temperature at 25° C for 4 hours.
E. Complete sequence A-D a total of 4 times for completion of 1 cycle.
Note: There shall be no delay between cycles.
5. A data acquisition system shall monitor the voltage drop across each resistor to record any occurences of this during humidity/temperature cycling. Voltage data shall be acquired every 60 seconds.
6. Every other thermal cycle (2, 4, 6, 8) the 10 VDC bias shall be disconnected and an insulation resistance test be conducted with the application of 100 VDC to the STI test specimen. Upon completion of the 10 thermal cycles, the test specimens shall be maintained at 25°C, 50% RH before the final insulation resistance test.
STI’s proposed test protocol combines the strengths of both the IPC and the JEDEC standard test methods. The IPC test method provides validation of the conformal coating to act as a protective barrier to moisture and contamination under a cyclic thermal and humid environment, as well as promotes electromigration with the application of a 10 VDC bias on alternating daisy chains. In addition, the JEDEC test method promotes condensation due to the elevated temperature range of 85°C. The combined test protocol provides a more comprehensive evaluation of a conformal coating’s ability to prevent electronic failures initiated by dendritic growth/corrosion. STI’s designed test specimen and modified test protocol together offer a better screening method for choosing the proper conformal coating for a PCB.
Conformal coating described in detail below:
Conformal coating is a protective non conductive dielectric layer that is applied onto the printed circuit board assembly to protect the electronic assembly from damage due to contamination, salt spray, moisture, fungus, dust and corrosion caused by harsh or extreme environments.
It is usually used in products that are used in outdoor environments where heat and moisture are prevalent. Coating also prevents damage from rough handling, installation, reduction of mechanical and thermal stress. It also prolong the life of the product during its operation.
At the same time, it helps to increase the dielectric strength between conductors enabling the design of the PCB to be more compact and small. It also acts to protect circuitry and components from abrasion and solvents.
When coated, it is clearly visible as a clear and shiny material. Some coatings are hard, while others have a slightly rubbery texture. Most coatings include a marker that appears greenish white when view under UV light. This marker enables easy inspection of the coating thoroughness checking during production.
In the past, coatings are only applied to military and life/medical products as the cost and the process of doing this was high then. In recent years, the development in material and new processes has enabled consumer electronics products to be coated as well. This will be becoming more common and will become a norm as circuitry and electronic components continue to shrink in size and dimension.
Types Of Conformal Coating
Commonly used conformal coatings are silicone, epoxy, acrylic, urethane and Paraxylene. The chemical and physical properties of the materials differs and therefore offer different degree of protection. The basic characteristics of the materials are described below.
Silicone
Silicone coatings range from elastoplastic which is a tough, abrasion-resistant to soft, elastomeric materials. Silicone are typically used in high temperature environments. It has good moisure and humidity resistance.
It has good thermal shock resistance due to its flexibility and is also easy to apply and repair. Its moisture resistance is similar to urethane and acrylic and Dielectric withstand is lowered than for the other coatings (1100 volts/mil). Flexibility of coating allows for much thicker film build than comparable acrylic or urethane coating. Its typical temperature range is -65 °C to 200 °C.
Epoxy
Epoxy coatings are very hard, usually opaque, and good at resisting the effects of moisture and solvents. Epoxy is usually available as a two part thermosetting mixture and shrinks during curing leaving a hard difficult to repair film. It possesses excellent chemical and abrasion resistance but can cause stress on components during thermal extremes. Epoxy is quite easy to apply but nearly impossible to remove without damaging the components.
Acrylic
Acrylic coatings are typically solvent based. Thermoplastic lacquer base means that the coating is easy to be applied andrepaired. They are usually low cost, tough, hard, and transparent. It exhibits low moisture absorption and have short drying times.
However, this type of coating does not demonstrate resistance to either abrasion or chemicals especially petroleum solvents and alcohol. It typical dielectric withstand is greater than 1500 volts and has a temperature range of -59 °C to 132 °C.
Urethane
Urethane coatings are hard and durable that has excellent resistance to solvents. It has similar moisture resistance to acrylic and silicone. Shrinkages during curing and hard film may stress the electronic components. It is difficult to apply and hard to be removed. Temperature range is quite similar to acrylic. However, its lack of reparability often prevent their use.
Paraxylene
Paraxylene coatings are highly uniform and yield excellent pin coverage. Their limitations include high cost, sensitivity to contaminants, and the need for a vacuum application technique.
Coating process
Before coating a printed circuit board, it must be cleaned and de-moisturized within 8 hours of conformal coating. De-moisturizing can be done in an oven with temperature set to 88 °C to 98 °C for 4 hours. Methods of coating include spraying, brushing or dipping. Chemical vapor deposition is used to coat with paraxylene. Steps of a spray coating are as listed below.
a) Board is cleaned.
b) Protected areas like terminal pins, connectors are masked off or removed.
c) Coating is applied using a spray process on both sides of the PCB and its edges.
d) Coating is cured using oven according to the coating type.
e) Masking is removed and any removed parts are reassembled.
f) Board undergoes full production testing to ensure functionality of board is not affected by the process.
Non-Volatile Organic Compounds Coating
In recent years, it was discovered that the solvents that are used in most conformal coating are not environmental friendly and is also a cause of concern for the human health. Most solvents have Volatile Organic Compounds or VOC which evaporate at room temperature.
These VOCs caused the formation of ozone and smog hence having an effect on the growth of plants and vegetation. This is a serious air pollution that also affects the people living within the vicinity. It caused irritations if exposed continuously and may be carcinogenic.
Non-VOC coating based on polyurethane technology has since being developed to replace the solvent-based VOC coating. When coating of PCB need to be done in huge quantity, all effort should be taken to use non-VOC coating even though the processes may take longer and more costly to implement.
About PCBWAY
Since 2003 PCBWAY has been the leading PCB quick turn manufacturer specializing in both Prototype and Production quantities, Initially produced single-sided and double-sided printed circuit boards for the consumer electronics market. PCBWAY is ranked among the top 4 board fabricators in asia and is well-known for its expedited turn time capabilities and its reliable best on-time shipping record.
Today, we have over 450 operators with high modern facilities to manufacture multi-layer PCB up to 12 layers. Backing up with a group of professional engineers, and well established quality system. PCBWay has grown to become a major PCB manufacturer in Asia to serve in diverse customers base such as electronics appliance, communication, educational electronics, power supplies, Automationsetc.
Our mission is to become one of leading PCB manufacturer that provide in high quality product with total customer satisfaction.
