A manufacturer producing sensors for the oil and gas industry shifted to a lead-free method in order to meet the RoHS standards. However, the manufacturing team experienced adhesion issues, which resulted in board failures.
The requirements of the RoHS standards have driven printed circuit board assembly (PCBA) materials to have higher temperature capabilities. The high glass transition temperature (Tg) of the materials was intended to manage the higher reflow temperatures of lead-free solders; however, these materials reduced the adhesion of conformal coatings to them. The manufacturer detected the flaws in its PCBA process and instantly began to search for a solution.
Figure 1. Adhesion testing results using the ASTM method D3350 or ISO 2409 Scotch tape test. To perform the test, the coating is deposited on the board surface, and then scored in a crosshatch pattern. Scotch tape is applied and then peeled off. The number of squares removed is then compared to an adhesion chart. The more squares removed, the less the adhesion, while the fewer squares removed, the better the adhesion.
Conformal coating constitutes an important part of the manufacturer’s assembly process since the boards will be employed in a harsh environment. However, the conformal coating was not attaching to the boards and even the coating that did stick was uneven. The manufacturer recognized that the defects were due to a wetting issue, which resulted in a poor conformal coating process.
Lack of adhesion, or dewetting, is a process in which a liquid coating is applied on the surface of a package, device, or printed circuit board, and before it dries, or cures, the liquid moves away from a contamination area. The process of manufacturing the circuit boards involves applying solder to the circuit board, placing a component on the solder, and then passing the solder through a reflow oven where it is exposed to heat. This liquefies the solder and joins the component to the PCBA substrate.
In high-temperature metal joining processes, flux is applied to prevent oxidation of the base and filler materials. Mold release compound, epoxy bleed-out, residual flux, or even fingerprints from handling can result in contamination. In theory, no-clean flux is used during the wave soldering process but it does not actually eliminate all of the contaminants.
Figure 2. Removal of organic flux residues improves coating coverage.
So long as the dewetting problem existed, the PCBA line had to be closed as the failure rate was very high. The study was carried out and the inference was that plasma treatment prior to conformal coating might be useful. When a nearby board assembly company was called on, it was noted that Nordson MARCH plasma treatment systems were being employed. The applications group at Nordson MARCH were contacted.
Testing the Defective Boards
To study the issue and better identify the source of the defective boards, Nordson MARCH carried out a regular contact angle test with the help of a contact angle measuring goniometer to estimate the board’s surface tension. With better surface tension, the wetting becomes better.
In order to carry out the test, a controlled water droplet is kept at the tip of the needle and is brought down until the droplet touches the surface of the board. The needle is taken back and the board is tested to verify if the drop stays.
If the droplet forms a bead, that indicates a high contact angle. If the board has good surface tension and therefore wetting, the droplet will lay flat on the surface of the board.
The amount of contamination on a board is dependent on the degree of the contact angle — with a greater contact angle, the contamination becomes higher. The failed PCBA examined by Nordson MARCH for the manufacturer had a large contact angle of around 90°. An appropriate contact angle is below 20°. It was now apparent as to why the conformal coating would not stick.
Figure 3. Contact angle measurement test.
The Plasma Treatment
Plasma is suitable for optimizing adhesion by increasing the surface energy of the materials, and it is also helpful in removing contaminants that can have an adverse effect on the adhesion of conformal coatings. Plasma treatment is ideal for a variety of surface activation, cleaning requirements, and adhesion enhancement applications in semiconductor manufacturing, microelectronic packaging and assembly, and also for life science and medical device manufacturers.
Figure 4. Conformal coating enhances surface energy, and thus surface wettability.
A system subjected to plasma treatment interacts with the PCBA surface in two different modes — physically and chemically. The physical interaction happens through ion bombardment of the surface at nanoscale. When the surface is hit by energetic ions, it causes the contaminant material to be displaced.
This is usually performed with the help of an inert gas like argon. Chemical interaction with the surface employs active species formed within the plasma, for example, oxygen radicals that are highly reactive with organic material contaminants. Both mechanisms are often present at the time of plasma treatment and the dominant mechanism can be controlled via process parameters like location, power, chemistry, and pressure.
Generally, a plasma-treated surface will result in a high energy surface state due to surface activation at the time of the cleaning process. High energy surface states are preferable for enhanced bonding, regardless of whether it is adhesive bonding, wire bonding, or, as in the case with the sensor PCBAs, lamination bonding or conformal coating. Untreated (low surface energy) surfaces usually exhibit hydrophobic traits, while plasma-treated surfaces are usually hydrophilic.
While the tests were carried out at Nordson MARCH, an AP-1000 batch processing plasma system was employed to treat the PCB assemblies prior to conformal coating. This system was selected since upon completing the tests, the same parameters could be easily scaled up to any one of multiple size AP-Series systems as per capacity needs, and also an automated in-line, high-speed plasma treatment system if production requirements demand a non-stop line-flow approach.
Plasma Treatment Results
Plasma was employed to treat boards from two distinct product lines through a batch process and each one was verified by conducting the goniometer test again for contact angles. There was an obvious improvement even after a short treatment.
The boards must be operated in a regular PCB assembly process to notice the effectiveness of using a plasma treatment system. The group visited the company several times. The key to the process was to set up the ideal parameters by Nordson MARCH application engineers.
The original tests were carried out in the Nordson MARCH facility at sea level; however, the sensor company is situated more than 4700 feet above sea level. To validate the settings, more runs using Nordson March’s AP-1500 plasma treatment system with greater capacity and with the ability to handle high volumes were carried out at the manufacturer’s unit along with Nordson MARCH’s application group to note the effects that elevation has on the process.
It was noted that the difference in elevation indeed had an effect. The flexibility of the plasma system’s control features allowed recalibration to achieve the appropriate outcomes at the higher elevation.
Figure 5. Nordson MARCH AP 1500 plasma treatment system.
When the plasma-treated PCB assemblies were put back to the assembly line for conformal coating, there was a considerable decrease in defect rate in all aspects. The company managed to overcome the coating-related defects and have the assembly line in operation.
This information has been sourced, reviewed and adapted from materials provided by Nordson MARCH.
For more information on this source, please visit Nordson MARCH.