Ball-Attach Flux

The ball-attach process for BGA and PGA packages uses a flux that is usually applied via pin-transfer from a dipping tray. Solder spheres (solder balls) are then placed into the deposits and the whole assembly is reflowed. BGA fluxes are usually water soluble, while PGA fluxes are often very reactive no-clean materials.

Indium Corporation most popular ball-attach fluxes include:

• WS-446HF: is a robust, halogen-free, water-wash flux that was designed to provide one simple solution to complicated applications, especially those with a single cleaning step for both BGA ball-attach and flip-chip processes. It has a powerful activator system to promote good wetting on even the most demanding substrate metalizations such as Cu OSP, ENEPIG, and ENIG. Its rheology is suitable for dipping flip-chip applications, as well as pin transfer or printing BGA ball-attach applications, for sphere sizes 0.25mm and above. WS-446HF helps to improve production yield by minimizing non-wet-open defects, missing balls, and electrochemical migration (ECM).
• WS-575-C-RT: a halogen-compliant, room temperature stable, one-step flux that eliminates the need for pre-fluxing
• WS-3600: a robust, halogen-containing flux that works well under challenging conditions
• NC-585: designed for PGA applications; eliminates issues caused by cleaning while still ensuring good
  solderability

Flip-Chip Flux

The flip-chip process involves taking the singulated die from a wafer mounted on a wafer dicing tape, inverting ("flipping") them and placing them onto a substrate. The substrate may be a printed circuit board, a ceramic substrate, or (in the case of 2.5D and 3D assembly) an interposer.

Copper pillar/solder microbumps are emerging as a standard flip-chip solder bump replacement in many parts of the semiconductor assembly industry, from standard chip-attach to power devices using flip-chip on leadframe as assembly technologies. For logic and similar devices, substrate metallization (landing pad) technology has also shifted from solder-on-pad (SoP), manufactured from printed and reflowed and cleaned solder paste, to individual solder balls. The technology is now moving to simple organic solderability preservative (OSP) on copper.

Water-soluble fluxes, which may be applied by dipping or spraying, have long been used in flip-chip assembly, but their long history of utility is facing multiple challenges as a number of factors makes the move to no-clean inevitable in the high-volume sub-130micron copper pillar era.

The use of copper pillar instead of solder bumps has meant that chip-substrate clearances and finer pitches do not necessarily move in lock-step with each other. However, copper pillar heights of 40-60 microns (and even shorter than this in the near future) combined with fine pitch, makes cleaning extremely complex. Aqueous (water-based) cleaning becomes more complex with fine pitch flip-chip attach as it becomes increasingly difficult to get the cleaning solution under the chip to dissolve the residues, and then to carry the residue-containing solution out from under the chip.

A recent emerging failure mode caused by cleaning involves solder joint damage during aqueous jet impingement. This failure mode is believed to be driven by a combination of: very small diameter microbumps; the move from hemispherical to very thin solder microbumps on copper pillar (reduced joint compliance); and low CTE, low-cost, and low layer count (thinned) substrates. In these circumstances, a move towards an ultra-low residue no-clean flux is inevitable, although new failure modes continue to appear, being driven by the smaller dimensions. For this reason, Indium Corporation has introduced a series of ultra-low residue (ULR) fluxes called NC-699, NC-26-A, and NC-26S for both current and emerging applications.

TCB & Copper Pillar Flux

Cu-Pillar Flip-Chip Flux is a dipping flux designed for use in thermocompression bonding flip-chip copper pillar applications. Its rheology and chemical design enables its use with dipping depths down to 10 microns or less.

The amount of flux deposited on the substrate can be optimized by changing equipment parameters. Key variables include temperature, copper-pillar dimensions, shear speed, time of shearing before dipping, dwell time in flux, and depth of immersion. The flux rheology can be optimized for the desired application by shearing to achieve the desired viscosity.

Packaging is available in 10cc and 30cc syringes.

Wafer Bumping Flux

Wafer bumping flux, which is also referred to as bump fusion flux, is a low-viscosity flux spin coated onto solder-bumped and copper-pillar or solder-capped wafers. The flux is designed to remove oxide and other contamination during reflow and cleaning steps. It converts rough, non-spherical, plated or wafer probe-dented solder bumps into shiny, oxide-free bumps. These fluxes can be either water-soluble or solvent clean.

Standard products include (chart):

Flux Type	Flux Application Method	Description	Cleaning Method	Halogen-Free	Material
SC	Dispense/Spin Coating	High Pb, SnPb eutectic, and SnAg solder bumps	Solvent or aqueous-based chemistry	YES	SC-5R
WS	Dispense/Spin Coating	20-65 micron pitch copper-pillars with SnAg or Sn100 microbumps	Warm DI water	YES	WS-3401 WS-3543

Considerations	Solder Paste Printing	Plating	Flux/Solder Ball Printing	C4-NP (Suess/IBM)
Used In High Volume Manufacturing?	Yes	Yes	Yes	Yes
Alloy Restrictions	All solder alloys, as long as powder can be made	Binary alloys only (Sn/Pb, Sn/Ag, Sn/Cu etc.) due to alloy control issues	All solder alloys, as long as sphere can be made	Probably limited to binary alloys
Bump Size	Down to around 125micron pitch only. Only 45% of paste volume is metal	Down to 2micron bumps feasible: possibly less	60micron bump diameter in mass production, but most commonly used for wafer level CSP manufacturing	Unknown
Bump Uniformity	OK: can vary significantly with age of solder paste and print process variables	Good	Good with appropriate tight tolerance solderspheres	Good
Voiding	Common	Little or no with a controlled plating process	Little or no	Little or no
Cost Comparison	Low Cost	More Expensive	Low Cost	High Capital Cost
Prototyping	Fairly Easy	Complex	Easy	Complex