The International Magazine for Device and Wafer-Level Test, Assembly, and Packaging Addressing High-density Interconnection of Microelectronic IC's including 3D packages, MEMS, MOEMS, RF/Wireless, Optoelectronic and Other Wafer-fabricated Devices for the 21st Century

Feature Article
The Basics of Wafer-Level AuSn Soldering

By Robert S. Forman, Rohm and Haas Electronic Materials L.L.C., Freeport, N.Y., [rohmhaas.com], and Gerard Minogue, Surfect Technologies, Albuquerque, N.M. [surfect.com]

Optoelectronics, hermetic packages and fluxless solder joints all rely on soldering with a eutectic alloy of gold and tin (AuSn). This article reviews the current state-of-the-art of AuSn solder assembly and discusses how to create AuSn structures on electronics assemblies. Recommended methods of manufacturing are also examined.

Gold-tin soldering (80%Au: 20%Sn by weight percent) has historically been employed in the microelectronics industry for fluxless hermetic lid sealing and die attach applications.

Gold, nickel and platinum are the most common surface finishes selected to be soldered with eutectic AuSn. Stamped AuSn preforms are used to bond large, metal components together.

AuSn preforms are widely available from several sources, and can be fabricated in a large variety of shapes and gauges. Package lids, heat sinks, and die are joined by inserting an AuSn preform between the surfaces to be sealed, pressing them together and applying heat.

The AuSn soldering mechanism consists of the following steps:

1. Melting occurs as the temperature exceeds 278°C. The tin melts, and gold dissolves into the tin.

2. Intermetallic Formation occurs between the barrier metal, tin and gold.

3. Freezing takes place due to local depletion of tin at the barrier metal (due to continued formation of tin intermetallic compounds), enriching the local gold concentration, thus promoting freezing.

4. Solid State Diffusion occurs slowly between the various elements in the solder joint.

Electroplating or Evaporation Techniques

On smaller parts, where the use of preforms is impractical or impossible, the AuSn alloy can be created by electroplating or evaporation techniques

When it is not possible to fabricate a stamped preform that will meet assembly requirements, an in situ preform can be fabricated using photolithography and electroplating.

Gold, nickel and platinum are the most common surface finishes selected to be soldered with eutectic Au-Sn.

In operation, a metallized surface of one of the components is coated with photoresist and lithographically patterned using the phototool; it is then plated sequentially with gold and tin to create an AuSn solder alloy pattern.

After plating, the photoresist is removed, and the parts are cleaned, baked, aligned (assembled) and reflowed together to create the hermetic joint.

AuSn offers multiple advantages when making solder joints. The most important are high melting point, ability to solder without flux, formation of a hermetic seal, excellent thermal, mechanical and electrical properties and low intermetallic growth rates when used over Ni, Pd or Pt.

Figure 1. Phase diagram

AuSn Metallurgy

The AuSn phase diagram is a eutectic binary-phase diagram with two eutectic melting points, one at 80% Au (280°C) and one at 10% Au (217°C). It is the 80% gold alloy that is used for fluxless soldering.

The 80:20 AuSn eutectic consists of AuSn and Au5Sn intermetallic phases. The eutectic alloy is calculated to be 64.3% Au5Sn and 35.7% AuSn by weight. There is no free tin in the solder alloy, and the tin constituent is completely tied up as a mixture of the two intermetallic compounds.

Notice that this zone is defined by very steep liquidus lines on both sides of the eutectic melting point, indicating that enriching the composition by one percent of gold leads to an approximate 30°C increase in melting temperature, as shown in Figure 1.

For this reason, the creation of AuSn solder forms requires accurate control of both solder composition and bonding temperature. The other eutectic alloy, 10:90 AuSn, will not produce reliable solder joints, due to the formation of a brittle AuSn4 intermetallic within the solder joint.

Stamped AuSn Solder Preforms

To fabricate AuSn solder performs, gold and tin are blended as molten liquids to the 80:20 ratio and cast into ingots. A series of rolling operations then flattens the ingots into strips with finished thickness specifications ranging between 25 and 1500 microns.

Preforms are stamped or machined from the rolled strip based on customer requirements. The minimum preform size varies by vendor, however, preform thicknesses do not fall below 25 microns and preform dimensions do not fall below 1mm x 1mm due to difficulties in preform stamping and handling.

Vacuum-Deposited AuSn Preforms

Vacuum-evaporated or sputtered layers of gold and tin can be sequentially applied directly onto silicon or glass wafers. One approach is to alternate layers of gold and tin until the target deposit thickness and metal stoichiometry are reached.

Thin deposits are common (less than 1µm) and are usually made up of gold and tin layers from 0.1-0.5µm thick. Typically, gold is employed for the top layer to protect the tin from oxidation while ensuring solderability during reflow.

Paste Screening AuSn Preforms

Solder pastes are comprised of 80:20 alloy spheres with a thixotropic carrier material to support the spheres and fluxing agents.

Metal content is controlled to ±1% of the design composition and can be formulated within a wide range, such as 60-90 weight by percent. The solder feature size, volume, and placement accuracy are limited to the capabilities of the selected screen printing process.

A consequence of using AuSn alloys made from paste that contain flux binders is that this type of deposit eliminates one of the biggest advantages of using AuSn in the first place: namely, that it can be reflowed without flux. When using flux there is always the possibility of out-gassing volatile compounds that may condense on the device surfaces or be trapped inside the package.

Figure 2. A stack of alternating electroplated Au and Sn layers

Electroplated AuSn Preforms

In electroplating, a photoresist generally defines the shape, thickness and location of the desired solder deposit.

Gold and tin are then sequentially plated in steps alternating between gold and tin. Alternating Au and Sn plating layers, deposited in a 1.5:1 thickness ratio of Au to Sn, are necessary to achieve the proper 80Au:20Sn stoichiometric ratio.

The outer plated surface is always gold to insure proper wetting and reflow of the solder and substrate metalization during assembly. Figure 2 shows a stack comprised of alternating electroplated Au and Sn layers present in the correct thickness ratio so as to create a true 80%Au:20%Sn eutectic alloy upon reflow, as shown in Figure 3.

Comparing Deposition Processes

In general, the electroplated deposition of AuSn offers superior dimensional control at small feature size, tight pitch and high feature density compared to solder preform placement or solder paste screening. This is particularly important when creating large arrays of solder features at waferscale.

Figure 3. Reflowed eutectic alloy with melting point confirmation

Creating solder features by electroplating imposes significant demands on the plating chemistry, plating tool and the plating process. This is especially true of solder features deposited at wafer scale where the solder deposit must simultaneously meet stringent requirements for alloy composition, dimensional accuracy and consistency.

It is important to note that a compositional variation of less than 1.0 wt.% can produce an extreme shift in the liquidus onset rendering the solder joint unusable.

With the proper selection of plating tools and chemistry it is possible to deposit AuSn solder features with a liquidus onset consistency of ±1°C. Solder features as small as 20 microns, with pitches as close as 5 microns, can be deposited at wafer scale with dimensional variations of better than ±5.0 %.

Au Plating

Gold plated deposits for soldering applications must be of the highest quality. The gold must be free of porosity and organic contaminants that can adversely affect solderability.

A gold plating specification for substrate joining is published in Mil-G-45204, Type Ill, Grade A. One gold surface serves as the base metal for patterning and electroplating of the AuSn metal stack. It is common to add a diffusion barrier of electroplated nickel when one side of the bond is copper-based. (Figure 4 lists the advantages and disadvantages of using a gold plating bath.)

Gold Sulfite Baths
Pros: No cyanide compounds, compatible with many spin-on photo resists, commonly found in use by GaAs fabs
Cons: Unstable, purple colloid precipitates out readily; unpredictable hardness over life of bath; Expensive to use due to short bath life, Efficiency changes with age, Frequent dumps, low plating rates (0.25-0.5 u/min)
• Operates at a high pH (9-10)
Gold Cyanide Baths
Pros: Predictable hardness over life of bath (can alloy with metals to change hardness) InexpensiveĐ infinite life, consistent efficiency as bath ages, stable matrix, rapid plating rates (0.5-2 u/min)
Cons: Contains cyanide compounds, can damage photoresists
• Operates at a neutral pH (5-7)
Figure 4. Characteristics of gold plating baths

Twenty-four karat (pure) electroplated gold deposits that meet the requirements for making 80:20 solder forms can be produced from two different process solutions, potassium gold cyanide and potassium gold sulfite.

The two solutions differ primarily in how the gold is kept in solution. Both chem-istries have specific characteristics to consider when selecting a gold plating process.

Brighteners and other additives can affect the degree of solderability when present in these finishes, so using a gold plating process that imparts a very low carbon content into the plated deposit is highly desirable.

Plating Efficiency

High efficiency (>90%) gold plating baths will reduce attack on the photo resist during the plating cycle and produce uniform deposits. When these baths are employed at low cathode efficiencies, large quantities of hydroxyl (OH-) ions are produced at the plating surface, which causes a localized increase in the plating bath pH and negatively affects photoresist performance.

This condition leads to under plating, photoresist film lifting, and blistering. Hydroxyl ion generation cannot be completely prevented, but can be minimized by maintaining the gold content in the plating bath at the high end of the bath manufacturer's recommendation.

Plating efficiency can also be improved by vigorous agitation of the solution at the cathode. This agitation replenishes the gold that is depleted from the cathode diffusion layer and improves plating efficiency.

Agitation also moves the hydroxyl ions away from the photoresist-metal interface, reducing OH- concentration and minimizing attack on the photoresist sidewalls. A gold plating process operating at a neutral pH will give better results when building multiple alternating metal layers of gold and tin. For long plating cycles, minimizing attack of the photoresists by the plating chemistry is key.

Tin Plating

Tin is amphoteric, that is, it is soluble in acids and bases, so contamination of the gold bath with tin metal ions is a possibility if a neutral pH gold bath is not used.

This contamination may occur if the proper gold bath is not used and can dramatically shorten the useful life of the gold bath. As gold solubility in a tin plating bath is negligible, contamination of a tin bath with gold is not of concern.

There are many process compatible tin baths available. Methane sulfonate (MSA) baths that plate tin from its stannous state (Sn+2) are desirable. "Matte" baths possess lower carbon content in the deposit and reflow well with gold to make the desired alloy.

These tin baths are highly acidic, typically containing 200-250 grams/liter of acid, and are very efficient. Predictable plating rates and coverage can be expected using tin MSA baths.

Bonding Cycle

Soldering process parameters will vary by application. For illustration, a typical process for gold-tin solder joints follows:

The parts to be soldered must be clean and totally free of contamination. AuSn solder surfaces must be prebaked in vacuum at 240°C for 2-4 hours before use. This is particularly important for vapor deposited and electroplated AuSn solder.

Assemble the parts and apply weight if required to maintain intimate contact. Do not use flux. Vacuum bake at 240°C for two minutes, reflow at 305-325°C for 1-2 minutes using an Ar, He or Ni-forming gas or a vacuum atmosphere. Cool below 240°C and release gas or vacuum to the ambient atmosphere.

Solder Fillets

The resulting AuSn solder fillets should be smooth and exhibit a metallic silver color with a shiny luster. Dull, rough, gold-colored solder fillets are an indication that the soldering temperature is too high and that the dwell period at the soldering temperature is excessive.

If the gold coating is completely consumed during soldering, the AuSn solder comes into direct contact with the underlying basis metal. As in the case of nickel, the basis metal may not wet the molten AuSn solder, creating an inferior joint.

Optoelectronics, hermetic packages and fluxless solder joints rely on soldering with a eutectic alloy of gold and tin.

It is, therefore, necessary for some of the initial gold coating to remain during solder cycle. Controlling temperature and dwell time are very important.

When molten AuSn solder contacts a gold coating, the pure gold on the surface of the coating reacts with the AuSn solder to form the more gold-rich Au5Sn intermetallic compound. The solder is thereby depleted of AuSn and enriched with Au5Sn during the soldering operation.

The chemical composition and melting temperature of the resulting solder mass changes from the original eutectic values, and the solder becomes more gold-rich. The consequence is a strong solder joint with a resultant melting point somewhat higher than the original eutectic mixture.

Conclusion

Optoelectronics, hermetic packages and fluxless solder joints rely on soldering with a eutectic alloy of gold and tin. The 80%Au:20%Sn by weight percent alloy has historically been favored for fluxless hermetic lid sealing and die attach applications.

The bottom line: Strict process and materials control are essential to maintaining the requisite AuSn alloy composition as deposited and after reflow. By matching one of the previously described processes to your specific application, a perfect AuSn solder bond can be produced.

References

1. C.H. Lee, Y. M. Wong, J. Appl. Phys 72 (8), 3808, October 1992.

2. G. Minogue, "A Novel Au/Sn Seal Ring Tech-nology," presented at the IEEE-CPMT Technol-ogy Symposium, Phoenix, Apr. 20, 2004.

Mr. Forman is the sales manager for Semi-conductor Advanced Packaging, Rohm and Haas Electronic Materials, L.L.C., Packaging and Finishing Technologies. He received his bachelor's degree in chemistry from the University of Oregon. He began his career in the electronics industry at Tektronix in 1977. He has been involved with electronic materials engineering and manufacturing for the past eight years with Rohm and Haas Electronic Materials, L.L.C.[bforman@rohmhaas.com]

Mr. Minogue is chief scientist at Surfect Technologies. He was formerly senior scientist at Cookson Electronics Assembly Materials' global R&D center in Jersey City, N.J. Earlier, he was also manager of advanced technology for General Ceramics at Tokuyama Soda Ltd., Anaheim, Calif. He is a graduate of both Yale and Rutgers universities. [gminogue@surfect.com]

CSR Advertisers Stock Index
SymbolNameLastPct
Change
MMM3M Company158.310.16
AMKRAmkor Technology7.020.79
AMATApplied Materials23.872.78
EMNEastman Chemical86.760.42
HONHoneywell Intl.99.080.20
NEWPNewport Corp.17.86-0.56
NDSNNordson Corp.79.24-0.59
RTECRudolph Tech9.302.99
STSensata Tech49.95-0.20
SMGZYSmiths Group PLC18.730.27

To read or download the current issue, please enter requested information below:
(Fields with an asterisk (*) are required.)

Subscriber Information:
Mailing Address:
First Name: *
Last Name: *
Title: *
Company: *
Address: * Street Address / P.O. Box
Apt, Suite, Floor
City: *
State/Province: *
Province: *
Zip/Postal Code: *
Email: *
Phone: *
Fax: