To comply with RoHS standards, a manufacturer that produced sensors for the oil and gas sector changed to a lead-free approach. However the manufacturing team faced adhesion problems which caused board failures.
Printed circuit board assembly (PCBA) materials have been driven to higher temperature capabilities due to RoHS requirements. The materials’ high glass transition temperature (Tg) was designed to handle the higher reflow temperatures of lead-free solders, but these materials decreased the adhesion of conformal coatings to them. The manufacturer detected defects in its PCBA process and quickly started to look 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, then scored in a cross hatch 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 forms an essential part of the manufacturer’s assembly process as the boards will be used in a rugged environment. However, the conformal coating was not sticking to the boards and the coating that did stick was not even. The manufacturer acknowledged that the failures were because of a wetting problem which led to a poor conformal coating process.
Dewetting, or lack of adhesion, is a process of applying a liquid coating on the surface of a package, printed circuit board, or device, and before it cures or dries, the liquid moves away from a contamination area. During the manufacturing process of circuit boards, solder is applied to the circuit board, a component is positioned on the solder, and the solder is sent through a reflow oven where it is subjected to heat. This melts the solder and fuses the component to the PCBA substrate.
In high-temperature metal joining processes, flux is applied to stop oxidation of the base and filler materials. Epoxy bleed-out, mold release compound, residual flux, or even fingerprints from handling can cause contamination. Theoretically, no-clean flux is consumed during the wave soldering process, however it does not really remove all of the contaminants.
Figure 2. Removal of organic flux residues improves coating coverage
As long as the dewetting issue was present, the PCBA line had to be shut down due to the failure rate being too high. Research was conducted and the conclusion was that plasma treatment before conformal coating may be helpful. When a neighboring board assembly company was visited, it was noticed that Nordson MARCH plasma treatment systems were being used. The applications team at Nordson MARCH were contacted.
Testing the Defective Boards
To analyze the problem and better identify the cause of the defective boards, Nordson MARCH performed a regular contact angle test using a contact angle measuring goniometer to calculate the board’s surface tension. The better the surface tension, the better the wetting.
In order to execute the test, a controlled water droplet is placed on the end of the needle and lowered until the droplet makes contact with the surface of the board. The needle is retracted and the board is tested to check if the drop remains.
If it beads up, that points to a high contact angle. If there is both 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 revealed by the degree of the contact angle - the greater the angle, the higher the contamination. The failed PCBA analyzed by Nordson MARCH for the manufacturer had a huge contact angle of nearly 90°. A suitable contact angle is lower than 20°. It was now clear as to why the conformal coating would not adhere.
Figure 3. Contact angle measurement test
The Plasma Treatment
Plasma is ideal for optimizing adhesion by boosting the materials’ surface energy, and it is also useful for cleaning contaminants that can unfavorably affect the adhesion of conformal coatings. Plasma treatment is suitable for a various surface activation, cleaning needs, and adhesion improvement applications in microelectronic packaging and assembly, semiconductor manufacturing, as well as for life science and medical device manufacturers.
Figure 4. Conformal coating enhances surface energy, and thus surface wettability
A system treated with plasma interacts with the PCBA surface in two different ways, chemically and physically. The physical interaction occurs through ion bombardment of the surface at a nano level. When the surface is impacted with energetic ions, it causes the contaminant material to be dislodged.
This is normally carried out using an inert gas such as argon. Chemical interaction with the surface uses active species developed inside the plasma, such as oxygen radicals that are highly reactive with organic material contaminants. Both mechanisms are frequently present during a plasma treatment and the dominant mechanism can be manipulated through process parameters such as power, location, pressure, as well as chemistry.
Usually, a plasma treated surface will give rise to a high energy surface state because of surface activation during the cleaning procedure. High energy surface states are favored for improved bonding, whether it is wire bonding, adhesive bonding, or, as in the case with the sensor PCBAs, conformal coating or lamination bonding. Untreated (low surface energy) surfaces normally display hydrophobic characteristics; while plasma treated surfaces are normally hydrophilic.
During the tests performed at Nordson MARCH, an AP-1000 batch processing plasma system was used to treat the PCB assemblies before conformal coating. This system was chosen because once the tests were finished, the same parameters could be easily scaled up to any one of multiple size AP-Series systems according to capacity requirements, as well as an automated in-line, high-speed plasma treatment system if production demands require a non-stop line-flow approach.
Plasma Treatment Results
Plasma was used to treat boards from two different product lines using a batch process and each one was confirmed by performing the goniometer test again for contact angles. Improvement was noticeable even after a short treatment.
In order to observe the effectiveness of using a plasma treatment system, the boards should be run in a regular PCB assembly process. The team visited the company many times. Establishing the ideal parameters by Nordson MARCH application engineers was key to the process.
The original tests were conducted in the Nordson MARCH facility at sea level, but the sensor company is located over 4700 ft above sea level. To confirm the settings, further runs using Nordson March’s AP-1500 plasma treatment system that has a larger capacity and can manage high volumes, were conducted at the manufacturer’s unit together with Nordson MARCH’s application team to observe the effects of altitude on the process.
It was observed that the altitude difference certainly had an impact. The flexibility of the plasma system’s control attributes enabled recalibration to reach the ideal results at the higher elevation.
Figure 5. Nordson MARCH AP 1500 plasma treatment system
When the plasma-treated PCB assemblies were returned to the assembly line for conformal coating, there was significant reduction in defect rate in all the cases. The company was able to resolve the coating-related failures 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.