Issue link: https://iconnect007.uberflip.com/i/1150604
88 SMT007 MAGAZINE I AUGUST 2019 Such improvements further enabled capabil- ities of selective patterning or coating cover- age desired in various leading-edge packages where full conformal shielding is not required. Sprayed material's adhesion compatibility to different substrates was also required for spray shielding to be considered as a viable or supe- rior alternative to sputtering. EMI Spray Shielding Material Improvements Initially, EMI shielding materials for spray coating were silver-based conductive inks. These modified inks were adopted for mass production in a very limited market segment due to concerns with the material thickness required (≥ 25 μm) to achieve adequate shield- ing performance and lack of ability to ade- quately coat the vertical sidewalls of the sub- strates. However, new conductive material formulations have recently been developed to address adequate shielding performance and coverage consistency. These formulations were initially focused on providing superior conductivity perfor- mance compared to conventional materials. As seen in Figure 3, shielding performance was improved by up to 80% and allowed reducing from 25-μm thick coatings to coatings in the 3–6-μm thickness range while achieving com- parable shielding performance as sputtering. With these novel EMI shielding material for- mulations, spraying can now achieve compa- rable performance and thickness as sputtering. Another challenge with conventional spray coating materials was the tendency of the mate- rial to "slump" after being sprayed or applied to the sidewalls of the target substrate. These early materials would tend to run down the sides of component packages causing incon- sistent thickness and non-uniform coverage on the coated substrates. New material formulations of EMI spray shielding materials have eliminated slump- ing. In recent tests with this material, we were able to coat test samples with 100% sidewall thickness as top surface thickness (1:1 ratio). Through adjusting parameters of the spraying process, such as dispense distance from the substrate or motion-speed of the spray valve, the coating thickness and resulting sidewall- to top-surface thickness can be adjusted. Typi- cal sidewall to top surface ratio is greater than 0.7:1 (Figure 4). In addition to improved sidewall coverage, adhesion performance is a critical compo- nent of this solution. Industry standards call for a cross-hatch etch-and-peel test and coat- ings must adhere with a rating of 4B or 5B to pass (minimal or no loss of material apart from the etched areas). Figure 5 illustrates a limited set of the test results for a new mate - rial at time zero as well as after accelerated life cycle tests. Figure 3: Shielding effectivity comparison between EMI spray shielding material and conventional silver ink. Figure 4: Spray shielding uniformity for top and sidewall surfaces with the new material.