Tutorial #16
Gerard Minogue
February 2002

OVERVIEW

This tutorial is excepted from Chapter 13, “Area Array Solder Spheres, Pastes, and Fluxes,” by Gerard Minogue of Alpha-Fry Technology.
It appears in Dr. Ken Gilleo’s book on flip chip and related technologies.
AREA ARRAY PACKAGING HANDBOOK — MANUFACTURING AND ASSEMBLY
K. Gilleo, Editor. Copyright 2002, The McGraw-Hill Companies, Inc., New York, NY

While solder spheres are frequently used in high volume wafer bumping, they can also be placed manually or with sphere-shooters onto wafers or single die. This tutorial is particularly intended for those lower-volume operations, whose perpetrators may not realize that, while solder spheres can be smaller than grains of sand, they can’t be handled like sand.

13.6.1 Static Electricity Issues

Static electricity is an issue that definitely must be factored into BGA sphere handling once the spheres are removed from the container. Electrostatic dispersive (ESD) packaging is recommended. The propensity of BGA spheres to charge is derived from the capacitance effect stemming from the growth of surface oxide on the spheres. The initial oxide growth is quite rapid, occurring essentially simultaneously with the production of the spheres from the molten state. Once the spheres have solidified and cooled, the oxide growth stabilizes and tends to become self-limited after several weeks.

Spheres stored in glass or conventional plastic jars will charge triboelectrically on agitation of the container. The net static charge in the spheres can range from tens to hundreds of volts. The natural surface oxide layer present on solder spheres following manufacture will make the spheres behave both as resistors and capacitors, the resistance being proportional to the stoichiometry and thickness of the oxide and the capacitance deriving from the dielectric properties of the oxide and the mirror charge induced in the spheres as they are agitated. Agitation of the spheres against one another and against the walls of a container during shipping not only will result in the darkening of the spheres from fret corrosion but also will increase the oxide thickness over time, making the spheres increasingly predisposed to static charging during handling.

The use of ESD containers is recommended for the storage and transport of solder spheres. However, the use of ESD containers does not always guarantee that all the spheres in a container will be static-free; since the oxide coat also has a characteristic breakdown voltage, the spheres in the center of the jar may retain a residual static charge even though the spheres in direct contact with the ESD material usually will discharge immediately.

The use of vibratory devices on feed funnels or hoppers for BGA sphere pick-and-place hardware should be avoided, if possible, or at least minimized. If a vibratory funnel or hopper is used, electrically grounded electropolished stainless steel is preferable to plastic. If plastic is used, a grounded dangler wire or strap should be positioned to discharge the spheres. Alternatively, an ionization gun-type device can be used to continuously discharge the spheres as they are fed into the pick-and-place hardware.

13.6.2 Denting

Denting of spheres during handling is an ever-present risk, particularly for high-lead alloys, which are softer than either tin-led eutectic or most no-lead alloys. Denting occurs commonly during sorting and handling operations conducted by the manufacturer; however, spheres dented during manufacturing are by design removed prior to shipment to the customer. The major risk for sphere denting occurs during mechanical handling by pick-and-place hardware. Vibratory feed apparatus should be adjusted so as to instill the minimum amount of mechanical energy into the spheres being fed. Sphere denting in itself will not necessarily result in an improperly formed BGA joint, but it can result in jams or misfeeds of the BGA sphere placement hardware. Dents that occur after manufacturing should be differentiated from surface artifacts that are normally a part of sphere cooling and solidification, particularly in the case of silver-containing alloys or no-lead alloys. In these cases, the specular reflectivity commonly associated with tin-lead alloys typically is not seen due to the differing cooling regimes and mechanics of alloy segregation on the sphere surface.

13.6.3 Oxidation

In the absence of extensive posthandling, the majority of sphere oxidation occurs in the first seconds to minutes after manufacture, at least in the case of tin-lead and other common alloy systems used in electronic assembly. The kinetics of oxidation for the tin-lead systems favor the initial formation of tin oxide.

13.6.4 Discoloration

The major discoloration phenomenon seen in BGA spheres, excepting the possibilities of alloy contamination or extrinsic coatings applied following the manufacture of the sphere, is sphere darkening due to fret corrosion. The degree of darkening or discoloration will vary in direct response to the degree of surface oxidation induced by agitation. Discoloration varies from slight dulling of surface specularity to near black in tin-containing systems. High-lead solders such as 10Sn/90Pb will develop a purplish discoloration on agitation that will turn black rapidly. In general, high-tin solders are more prone to rapid surface discoloration and darkening than are high-lead solders. High-tin lead-free solder systems tend to darken in a manner analogous to tin-lead eutectic solder. Solder spheres that are kept in a dry atmosphere (50 percent or les relative humidity) and are not mechanically agitated will keep their initial surface brightness for 6 months or more, based on tests conducted at Alpha Metals laboratories. Unless special dulling flux compositions are used, the joints created by reflow of BGA solder spheres are as bright as those formed by solder paste reflow.

Atmosphere. The standard processing atmosphere for both tin-lead and no-lead solder sphere reflow is nitrogen gas, with the ambient oxygen level ideally at 1000 ppm or lower. The driving force for reduction of oxygen during the reflow profile is the trend toward milder and milder activator systems in BGA fluxes, particularly the water-washable and low-residue no-clean classes of fluxes.

Storage atmosphere for BGA spheres prior to placement and reflow ideally should be in as cool and as dry a location as possible. If the spheres are not to be used for some period of time following receipt, the container should be left sealed. If opened for incoming inspection, it should not be left open. High relative humidity (>60 percent) exposure of solder spheres that are stationary in a storage container ordinarily will not cause extensive surface oxidation and solderability problems within a period of 2 days to 2 weeks, a typical residence period for a high-volume manufacturing environment. High relative humidity combined with high temperatures (>30° C) and mechanical agitation or vibration of the spheres can cause rapid acceleration of both surface oxidation and surface darkening. The agitation exacerbates the oxidation process by repeatedly cracking and fretting away surface oxide, exposing fresh metal for further oxidation. The fretted oxide removed from the spheres is occasionally visible as a fine black powder in the container holding the sphere product.

Temperature. Experimental evidence suggests that temperature excursions below those encountered in the assembly process itself do not appreciably affect BGA spheres unless combined with high humidity and mechanical agitation. In this instance, significant levels of sphere surface oxidation and darkening can be expected if the spheres are also stored above 30° C.

Storage Limits. Tin-lead spheres left intact inside sealed shipping jars may not retain solderability indefinitely, but they will definitely remain intact for periods of at least 4 to 6 weeks if not disturbed and if not exposed directly to high temperatures (>30° C) and high relative humidity (>50 percent). Spheres that have been opened for use may begin to experience solderability loss within days at high temperature and high relative humidity, but this does not usually present a problem in most manufacturing environments where sphere consumption is much greater than one jar every few days. If assembly operations are to be interrupted for more than several hours, solder spheres should be returned to their packaging and preferably stored in cool, dry conditions to maintain maximum solderability.

Assembly Machine Damage (Balling). The risk of assembly machine or pick-and-place damage comes from two possible sources: the mechanical energy imparted to the spheres by vibratory generators intended to keep the spheres feeding smoothly without bridging and the mechanical action of the vacuum pickup chucks or positioning plate depending on the assembly hardware manufacturer chosen. Spheres produced via conventional manufacturing technologies involving oil reflow of cut performs on occasion may demonstrate less of a tendency to bind or gall in assembly hardware than do jetted spheres, the tradeoff being that jetted spheres may have more of a chemically pristine surface than oil reflowed spheres by virtue of the manufacturing technology employed.

FOR MORE INFORMATION

This material was excerpted from Chapter 13, “Array Solder Spheres, Pastes, and Fluxes” by Gerard Minogue
in AREA ARRAY PACKAGING HANDBOOK — MANUFACTURING AND ASSEMBLY
K. Gilleo, Editor Copyright 2002, The McGraw-Hill Companies, Inc, New York, NY