Flip Chip Underfill

Underfills are used for mechanical stabilization and for the simultaneous compensation of material stresses in electronic assemblies, especially flip chips. To reduce the coefficient of thermal expansion (CTE), the adhesives are partly filled with nano fillers. This is especially important when soldering ball grid array (BGA) chips. 

Adhesives that are used as underfill for chips are usually epoxy resins with capillary flow behavior. This means they flow easily into even the thinnest gaps and can therefore be applied quickly and easily. The use of dual-curing adhesive systems is most effective: the adhesive is cured in the edge area using UV light and thus fixed; the shadow zones under the component are then cured in a second step using heat.

graphical representation of a PCB flip chip with adhesive as underfill

Adhesive as underfill for flip chips on PCBs

Sustainable thanks to reworkability and recycling

Reworkability makes it possible to process, repair or recycle products after assembly. This point is becoming increasingly important for manufacturers of electronic components as legislation and environmental associations continue to drive forward the minimization of electronic waste. One starting point for sustainable strategies is the reworkability and reparability of individual modules on printed circuit boards to counteract the scrapping of a complete component or module.

Some underfill adhesives from Panacol are specially designed for this "reworkability": they can be removed again when exposed to temperatures above the glass transition range of 150°C. The epoxy resins adhere reliably up to this temperature and application range. Only above this critical temperature threshold is it possible to process the product. To make the adhesives easily recognizable, the black adhesives fluoresce yellow when excited with short-wave light.

Reworkable underfill for flip chips

A black, yellow-fluorescent underfill under normal (left) and UV light (right) in comparison

The table below provides an overview of Panacol adhesives that are used as underfill.

To download the technical datasheets (TDS) please click on the adhesive name.

Adhesive/ Underfiller Viscosity [mPas] Base Curing* Properties
Vitralit® 2655 150-300 epoxy UV
secondary heat cure
capillary flow
high ion purity
Vitralit® 2667 3,000-5,000 (LVT, 25°C, Sp. 4/30 rpm) epoxy UV
secondary heat cure
Low thermal expansion
low ion content
No outgassing
Structalit® 5751 200-500 (Rheometer, 25°C, 10s^-1) epoxy thermal Black color, fluorescent yellow, jetable, low ion content, reworkable above 150°C
Structalit® 8202 300-400 (Rheometer, 25°C, 10s^-1) epoxy thermal Black, fast curing
Capillary flow behavior
Compatible with flux materials
Low CTE, high glass transition temperature
Low ion content (Cl- <900ppm)

*UV = 320 - 390 nm     VIS = 405 nm