The packaging reliability of substrates is determined by the bonding strength of various interfaces. Packaging substrates are composed of organic materials (insulating resin, solder mask) and inorganic material (copper). Substrates manufactured by Semi-Additive Process (SAP) contain multiple heterogeneous interfaces: copper-insulating dielectric interface, copper-solder mask interface, dielectric-dielectric interface and dielectric-solder mask interface. Interfacial delamination is the most common failure during substrate reliability testing.

Taking the copper-dielectric interface as an example, surface treatment is implemented on insulating dielectrics before copper circuit fabrication to improve surface roughness and modify surface chemical properties for reliable adhesion. Meanwhile, to satisfy the fabrication of ultra-fine circuits (line width/space ≤10 μm/10 μm), excessive surface roughening of dielectric is forbidden. Common modification methods for epoxy dielectric include wet chemical etching, plasma treatment and UV-ozone surface modification.

Plasma Surface Treatment

Plasma bombardment produces microscale roughness on material surface and generates mechanical interlocking force at bonding interfaces. Higher surface roughness leads to higher bonding strength, because crack propagation needs to consume more viscoelastic and plastic energy on rough surfaces. Besides, plasma treatment induces new chemical components and oxides on material surface to form stable chemical bonds, further improving adhesion performance.

Plasma Treatment Process for ABF Resin

Figure 1(a) shows the ground ABF surface without plasma treatment, and Figure 1(b) shows the ground ABF surface after plasma treatment. Plasma is an effective method to clean resin surface. Specifically, nitrogen, oxygen and carbon tetrafluoride are ionized into plasma under high-voltage electric field. High-speed moving plasma particles physically bombard ABF surface to realize cleaning. In addition, ionized oxygen radicals, fluorine radicals and free electrons chemically react with surface resin of ABF to generate carbon dioxide and water, achieving chemical cleaning simultaneously.

By comparison, the untreated ground ABF surface is covered with abundant residual resin wrapping partial silica fillers and presents a relatively flat morphology. After plasma treatment, most surface resin is removed and plenty of silica fillers with different sizes are exposed, forming uneven and rougher surface.

Figure 1 Micro-morphology of ABF surface and laminated ABF interface before and after Plasma treatment; Figure 1(c) and 1(d) show laminated ABF interfaces without and with Plasma treatment respectively

Figures 1(c) and 1(d) display the laminated cross-sections of ABF before and after plasma treatment respectively. An obvious dividing line exists between two adjacent untreated ABF layers, indicating poor interfacial bonding. No dividing line can be observed between adjacent plasma-treated ABF layers, which proves complete resin fusion and favorable interlayer adhesion.

Figure 2 is the schematic diagram of boundary formation between two adjacent ABF layers with or without plasma treatment before build-up lamination. For untreated ABF, spherical silica fillers are wrapped by surface resin whose property may be damaged by mechanical abrasion during grinding; accordingly, the so-called "cream layer" resin fails to fully fuse with the overlaid ABF during lamination. In contrast, uneven topography formed by fully exposed silica fillers of different sizes on plasma-treated ABF can be fully filled by the cream layer of laminated ABF resin, realizing complete integration between overlay resin and base-layer fillers.

Figure 2 Schematic diagram of boundary formation between two adjacent ABF layers with/without Plasma pretreatment before build-up lamination

Figure 3 lists the surface roughness of ABF and corresponding copper peel strength before and after plasma treatment. Untreated ABF has an Ra of 0.22 μm with copper peel strength of only 1.5 N/cm; after plasma treatment, Ra rises to 0.36 μm and peel strength increases to 4.2 N/cm. The improved adhesion mainly comes from physical anchoring effect caused by increased roughness.

Excessively high surface roughness will cause open or short circuits at the bottom of fine traces and deteriorate line profile control. Therefore, plasma process should be optimized to eliminate interlayer boundary and improve copper-ABF bonding without impairing fine-line manufacturability. In conclusion, plasma treatment on ground ABF eliminates interlayer delamination risk, strengthens circuit adhesion and enhances the anti-delamination performance of finished substrates.

Figure 3 Surface roughness of ABF and peel strength between ABF and copper circuit before and after Plasma treatment