Underfill For BGA And CSP Drop Reliability
An area array joint is a small ball of solder between a package and a pad, and it is much stronger in shear than in tension. A drop, a bend, or a shock puts the joint into tension, and the failure occurs at the interface between the solder and the copper pad, which is the weakest link in the chain. Underfill addresses the problem by coupling the package mechanically to the board, so that the load is shared between the joints and the cured polymer rather than being concentrated on a single row of solder.
This article explains what an underfill does, when it is used, how the material is chosen and applied, and which defects appear in the process.
What Underfill Does
The material is a filled epoxy that is dispensed along one or two edges of a mounted package and drawn into the gap by capillary flow. After it cures, it forms a mechanical bridge between the package body and the board surface, and the individual solder joints are embedded in it. When the assembly is shocked, the underfill distributes the load over the area of the package instead of concentrating it at the joints, and the strain in the solder is reduced by an order of magnitude in a typical drop event.
The failure location moves rather than disappearing. With underfill, the package is mechanically coupled to the board, so a severe impact may crack the die, the package substrate, or the laminate instead of the solder joint. That is a trade rather than a pure improvement, and it is one of the reasons the technique is applied where drop and bend are the dominant risks rather than universally.
When It Is Used
Drop resistance, usually assessed with a drop test, is the classic application. A portable product that will be dropped, a large package on a thin board that will flex, and a chip scale package with a small number of joints that must carry the whole load are all cases where underfill improves the outcome. The geometry matters more than the package name: a fine pitch array with a small standoff and a large body has a higher strain per joint than one with a larger standoff and fewer balls.
Thermal cycling is the other application, although the mechanism is different. Underfill constrains the expansion mismatch between the package and the board, which reduces the fatigue of the joint and shifts the stress into the polymer, and it also protects the joint from the environment. Where the product sees both drop and thermal cycling, the material has to be chosen for both requirements rather than for one of them.
Materials And Properties
The coefficient of thermal expansion, and with it the CTE mismatch between the package and the board, is the property that matters most, and it is controlled by the filler content. A silica filled epoxy can be brought close to the expansion of the solder and the package, while an unfilled material expands several times more and can itself add stress during thermal cycling. The filler particle size matters for the process rather than for the properties: the particles have to pass through the gap between the package and the board, which means the gap must be several times the size of the largest particle.
The modulus and the glass transition temperature set the mechanical behaviour, the cure schedule sets the throughput, and the flow properties set how far the material travels before it gels. A snap cure material allows a short in line process, while a slower cure gives a longer flow time and a lower stress state. Reworkability is the last consideration: most underfills make a package impossible to remove, and a reworkable material trades some performance for the ability to replace the device.
Applying The Material
The board is normally baked before dispensing to drive off moisture, because any water in the laminate turns to vapour during the cure and produces voids in the underfill. The material is then dispensed along one edge of the package, or around two adjacent edges for a large body, in a pattern that gives a uniform flow front. The board is often heated to reduce the viscosity and to speed the flow, and the time allowed for the capillary fill is set by the distance the material has to travel.
The volume has to be enough to fill the gap and to leave a controlled fillet around the perimeter. A fillet that is too small leaves the edge of the package unsupported, and one that is too large flows onto a neighbouring component, into a test point, or onto a connector, where it interferes with the next assembly step. After the fill the material is cured, and the cure profile is taken from the supplier rather than from the reflow oven schedule.
Defects And Their Causes
Voids are the most common defect and they come from several sources: moisture in the laminate, air trapped as the flow front advances, an uneven dispense pattern, and a gelling time that is too short for the material to fill the gap. A void at the corner of a package is not necessarily harmless, because it is precisely the area that carries the highest strain during a drop.
Incomplete fill leaves part of the array uncoupled, which is worse than no underfill at all because the remaining joints carry more load than they did before. The causes are an insufficient dispensed volume, a gap that is blocked, and a flow path that is obstructed by vias or by a solder mask dam. Filler separation, where the particles settle and leave a resin rich region, follows from a long cure at a low temperature. The fillet size is a process control issue, and the inspection of the fillet, of the flow, and of the voids is usually done by X-ray or by scanning acoustic microscopy on a sample.
Design Considerations
The underfill flow path is a layout constraint. A neighbouring component placed where the material must travel will block the flow, and a component placed where the fillet will spread will be contaminated by it. The usual approach is to keep a keepout around a package that will be underfilled, sized from the fillet dimension, and to plan the dispense edge so that the flow path is as short and as unobstructed as possible.
Where the assembly has a second reflow after the underfill is cured, the cure temperature and the second profile have to be checked against each other, because a material that softens during a later reflow will not perform as designed. The choice between underfill and a coating is a different decision, with the coating providing environmental protection and the underfill providing mechanical coupling. The environmental side is described under board level protection, the alloy context under lead free and leaded soldering, and the broader framework under PCB design quality characteristics.
FAQ
Does underfill always improve reliability? It improves drop and bend performance and thermal fatigue, but it transfers the load to the package and the laminate, which can then fail instead. For a benign environment it adds cost and removes reworkability without a benefit.
Why must the board be baked before underfill? Because moisture in the laminate turns to vapour during the cure and creates voids in the material. The voids reduce the coupling exactly where the strain is highest.
Can an underfilled package be reworked? With most materials, no. The cured epoxy has to be removed mechanically and the site cleaned, which usually damages the board. Reworkable materials exist but trade some mechanical performance.
