Chip on Board - Die Attach
Chip on Board Contents
Adhesive:
Die Attach adhesive provides both the thermal and mechanical interface between the die and the substrate. Although the adhesive can be applied to either the substrate or the back of the die, most manufacturers employ paste adhesives applied to the substrate. Dispensing, stencil printing or pin transfer may be used depending on die size. Most utilize dispensing except for film type adhesive. Dispense patterns depend on die size and shape but all must ensure a void free bond line. For high thermal transfer applications, a thin bond line combined with silver (Ag) filled epoxies can meet the need.
Die Placement:
Die placement into the dispensed adhesive demands accuracy as well as proper orientation and planarity control. Adequate pressure to set the die into the adhesive will ensure firm attachment and establishes the bond line thickness. Orientation and accuracy of placement directly impact the bond pad to substrate wire length and the "keep out" or die spacing requirements.
The variety of substrates available for COB applications allows for a wide selection of electrical, thermal and mechanical properties tailored to specific circuit requirements. Ceramic and glass ceramic substrates offer stable dielectric constant, low dielectric loss tangent and good thermal dissipation. Organic substrates can provide low dielectric constant, lighter weight and, in many cases, lower cost. Ease of design, technology availability and time to market make COB the preferred choice for a high proportion of today's integration and performance module applications.
Clean:
Following die placement, adhesive curing exposes bond pads on both the die and the substrate to organic contamination by plasticizers and curing agents in the adhesive. This is particularly true when employing organic substrates. Plasma cleaning provides the best solution, although solvent cleaning with ultrasonic assist and CO2 snow blasting find application as well. Removal of any organic contamination from the bond pad surfaces is crucial to ensuring high wire bond yield and strength as well as process stability and reliability.