A new bonding agent for inner layer bonding applications is characterized by particularly good adhesive strength.
The portfolio of MKS Atotech, a supplier of specialty chemicals and system technology for the electronics and surface finishing industry, now includes the bonding agent BondFilm HP for PCB production. It was developed for inner layer bonding applications and enables adhesion between copper and dielectric layers, improved thermal resistance and a reduction in particle formation of up to 95%. The adhesion promoter is characterized by its compatibility with a wide range of substrates and common oxide replacement lines.
As the latest product in a long development history of its series, BondFilm HP comprises only three necessary processing steps:
- cleaning
- surface activation
- the creation of a roughened organometallic layer in an etching bath.
The etching bath promotes adhesion and eliminates the potential for delamination. An additive in the etching bath improves adhesion between the copper and dielectric layers and at the same time significantly reduces the usual formation of slag particles. This in turn prevents organo-copper compounds from being deposited on the conductor tracks and the substrate material and offering potential for high-voltage short circuits and dendritic growth.
BondFilm HP also improves the reliability of printed circuit boards under high stress. The oxide replacement offers stronger and more uniform adhesion due to the organometallic adhesion layer, which can withstand high temperatures, moisture and other environmental influences. This makes BondFilm HP suitable for applications in the automotive, aerospace and aviation sectors, for example.
Roughness SEM BondFilmIn order tointroduce the process without serious changes to production planning, compatibility with a wide range of substrates is a fundamental requirement. For this reason, extensive tests were carried out with all common MLB and HDI materials under production conditions. The result clarified the assumption that polyamide and BT materials, in addition to common materials, also exhibit the best adhesion.
As part of the consideration of ESG (Environmental, Social & Governance) objectives, emphasis was also placed on improved environmental compatibility. The slightly reduced etching depth, a concentrate version and simplified waste water treatment contribute to this. The existing global legal regulations are complied with.
Process sequence
The first step in the BondFilm HP process involves cleaning the PCB. It must be ensured that no critical residues have a negative impact on the activation and ultimately the etching of the copper surface. Contamination such as oxidation or fingerprints can significantly impair wetting in the etching bath as well as the etching process, which can lead to cosmetic problems or poorer adhesion.
The cleaning process consists of two baths: an optional acidic cleaner that removes moderate oxidation and a necessary alkaline bath that completely dissolves grease and dry film residues. Due to the chemical composition, these process steps remove approx. 0.2 µm of copper without negatively affecting the roughness.
The second step now activates the circuit board so that the surface tension is changed in such a way that the etching bath evenly and completely wets all copper surfaces and conductor tracks.
The BondFilm Activator G360, specially developed for this purpose, is suitable for all existing BondFilm processes and distinguishes itself by removing environmentally critical components without compromising on performance.
In the final step, the surface treatment takes place using an optimized brown oxide process, which creates a surface enlargement in the form of roughness that later ensures mechanical adhesion between the copper and resin. Specially developed additives give BondFilm HP specific properties such as high copper loading, particle reduction, uniform roughness and the familiar dark brown cosmetics. The copper loading could also be increased to up to 45g/L through continuous further development of the market-leading brown oxide, which ultimately leads to further savings through reduced dosing consumption.
By means of a balanced dosing window, the temperature or the dwell time in the bath, the etching rate and roughness can be precisely controlled as required and adapted to the respective production requirements.
Performance
Inner layer adhesion in PCB production is primarily evaluated by the adhesive strength, measured in N/cm. It should provide an indication of how well the inner layers of the PCB are bonded together and how high or low the chance of possible delamination is, i.e. breakage of the bond between the copper and the dielectric. This is particularly important if the etching depth is specified as only 1 µm as the optimum, in contrast to the 1.2-1.4 µm usually used in industry. Taking into account the clear correlation between roughness and adhesion strength, this is of particular importance in order to prove that, despite a lower etching depth, the adhesion can compensate for the reduced etching depth due to assumed factors of particle reduction or the higher actual adhesion surface enlargement (measured in %).
Diagram 1: Adhesion strengthForthe process evaluation, a standard production environment and dielectrics were selected, which experience has shown are frequently used for the HDI (High Density Interconnect) market. Measurement results were consistently found to meet or clearly exceed the industry requirements of approx. 4 N/cm.
The extent to which the adhesion promoter is suitable as a pre-treatment for laser drilling was also tested. The change in the surface insulation resistance (SIR) after the HAST stress test (Highly Accelerated Stress Test) and particle measurements of the etching bath and on the treated panel were also carried out. Excellent measurement results were achieved across the board.
To enable customers to see this for themselves, Atotech offers free test processing of samples, which is carried out under production conditions at its in-house TechCenter in Guanzhou, China. Surface analysis and roughness determination are also carried out as part of this process. This ensures the best possible match between material and process.
