Optimizing Underfill Materials and Processes
High-volume Assembly Of Lead-free Area Array Packages
BY MALCOLM WARWICK AND STEVE DOWDS
Underfills are widely used when assembling area array packages such as ball grid arrays (BGAs) or chip-scale packages (CSP) onto laminate-based PCBs. They are effective in enhancing solder joint reliability by countering the effect of mismatches in the coefficient of thermal expansion (CTE) between the package interposer, the solder joints, and the PCB. But applying an underfill adds to bill of material costs and adds assembly steps to standard SMT manufacturing sequences.
Board-level reliability of assembled area array packages is heavily influenced by package design, ball pitch, ball diameter, and final solder joint height. Manufacturers of portable systems such as camcorders, cell phones, and digital cameras are using fine-pitch array packages to continue with miniaturization and weight reduction. The trend to higher-density packages and increasingly harsh environments (thermal stress, drop, bending, and twisting) are the driving forces for the industry to use underfill materials for area array packages to maximize lifetime. Chip-scale packaging originally was conceived to be an underfill-free technology, with the objective of eliminating these costs. In practice, however, many assemblers still prefer to use some kind of countermeasure against the effects of CTE mismatch. Data from experimental lead-free chip-scale packages indicates that this is a wise move - the reliability of lead-free solder joints appears lower than for lead-rich joints in packages where global strains are high. Factors that increase global strain include low stand-off height relative to overall package dimensions, direct attachment to the substrate by solder balls, and large in-use temperature changes. Rapidly shrinking interconnect pitches, increasing circuit board density, and rising penetration of electronics into increasingly harsh automotive, telecom, aerospace, and other environments means that manufacturers must quickly identify suitable CSP underfill materials and processes for lead-free assembly.
Figure 1. Capillary flow underfill materials can be used unchanged in emerging lead-free assemblies.
The action of an underfill, to increase the reliability of grid array interconnects by distributing stresses across the surface of the substrate rather than allowing them to become concentrated in the solder balls, is well understood. The most widely used are capillary-flow underfills (Figure 1). These typically are applied after reflow and are designed to cure over time at moderate temperatures (130°C for 30 min.) to avoid any damage to temperature-sensitive devices. Other formulations are designed to snap cure at elevated temperatures (150°C for 5 min.). The arrival of these rapid cure formulas allows capillary-flow underfills to address applications where process speed is of critical concern, such as production of cell phones, portable devices, and other consumer products. In either case, when cured, capillary-flow underfills are known to withstand lead-free reflow up to the generally accepted maximum 260°C without losing their underfill properties. They can, therefore, be used directly in a lead-free process.
Alternative Underfill Technologies
No-flow underfills are dispensed after solder paste printing, but before component placement. Curing takes place at the same stage as solder reflow. But when selecting an underfill for use in lead-free assembly it is important to consider not only the maximum temperature the compound is able to withstand, but also its cure behavior.
To allow the component being soldered to self-align, no-flow underfills are designed to remain liquid until the peak of the reflow profile. An underfill designed for SnPb reflow will cure too early if exposed to a lead-free profile. As a result, self-alignment will be lost and yield will fall. To be compatible with lead-free assembly, no-flow underfills must be reformulated to begin curing at a later point in the reflow profile, such as a higher temperature. The new formulas must also be requalified for use in new lead-free packages. This is best undertaken in partnership with the material supplier. An even better solution is for the underfill to be provided as part of a fully qualified material set, which saves time and qualification effort, and guarantees compatibility between all of the constituent materials.
A further variation on the theme of pre-applying stress relief compounds to simplify application and speed up the processing is to apply dots of adhesive at the corners of the CSP pad site, where induced stresses normally are the greatest (Figure 2). This speeds assembly by eliminating post-reflow underfill dispense and cure steps, and reduces costs by eliminating underfill dispense and curing.
Figure 2. Corner-bond adhesive is deposited where package stresses are highest, using SMT equipment.
This technique deposits enough adhesive to match the stress environment. Typically, the material used is a single-component liquid epoxy adhesive with thixotropic characteristics suitable for SMT dispensing. Like a lead-free, no-flow underfill, the adhesive has a high curing temperature to enable curing during the reflow process with self-alignment of surface mount components prior to adhesive fixturing. Unlike capillary flow and no-flow underfills, it does not completely eliminate the stress, but is fast and provides sufficient fatigue life improvement. Corner-bond adhesives currently on the market are designed so that the adhesive bond breaks down at temperatures exceeding the standard solder reflow temperature of 220°C, allowing for easy removal and replacement of the surface-mounted components. The adhesive is reworkable, so it allows for replacement of defective chips and also minimizes incidents where the entire board must be discarded.
Corner-bond technology is a viable solution to underfilling lead-free CSP packages. However, like no-flow underfills, the material properties must be suited to the higher reflow temperatures of lead-free.
As a potential alternative to the described underfill technologies, pre-applied underfill moves the process upstream. The component manufacturer would apply the underfill precursor ahead of the component placement, and then curing takes place during solder reflow. Board-level assembly will be become easier because underfill dispense is no longer required, which helps enable high throughput and lowers manufacturing costs.
Migrating fine-pitch CSP components to lead-free construction is more difficult in practice than on paper. In particular, the stresses arising from CTE mismatches have a much greater effect on lead-free solder joint reliability, because of the higher physical strength and processing temperatures associated with lead-free solder alloys. Far from being designed out, as most assemblers would like, measures such as underfilling are now more necessary than ever. Manufacturers can continue using the same versions of conventional capillary flow underfills for new, lead-free packages.
Pre-applied underfills or corner-bond adhesives that cure during reflow offer a simplified solution. They are faster to apply, can be dispensed using ordinary SMT equipment, and cure during reflow without requiring a dedicated curing oven. But new formulas must be perfected, evaluated, and requalified as part of a complete lead-free materials solution. This will enable CSPs to continue along their aggressive development path, and thereby sustain rapid growth in world markets for consumer electronic products.
MALCOLM WARWICK, global director of PCB Assembly RD&E, and STEVE DOWDS, global product manager, may be contacted at Henkel Technologies House, Electronics Group, Wood Lane End, Hemel Hempstead, Hertfordshire, HP2 4RQ, U.K.; 01442 278000.