Paul Socha

With regard to soldering or wetting (coating) with indium, we are often asked to comment on the oxide formation of indium and how to remove it. We are also asked how long will it take for the oxide to reform on the surface. The procedure, below, will help you to better understand indium oxide, its removal, and how to handle it once it has been removed.

Indium is self-passivating. At room temperature, the oxide formation on the surface of the indium will be between 80-100 Angstroms thick.   Generally, this amount of oxide is not considered significant to hamper the wetting of the indium to a substrate, especially if a flux is used. Even if a flux is not used, the indium should not have any difficulty forming a joint or coating a surface.

If the application calls for an oxide-free joint and a flux cannot be used, the indium oxide can be easily removed following these steps:

·         Clean the indium in isopropyl alcohol or acetone to remove any surface organics. Allow to dry.

·         Etch the indium in 10% HCl for 1 minute to remove the surface oxides.

·         Rinse the indium in DI water to remove the acid.

·         Rinse the indium in isopropyl alcohol or acetone to remove the water.

·         Blow dry with dry nitrogen or allow to air dry.

While this etching procedure will remove the oxides, it has also opened up a whole new surface on the indium which will be prone to oxidation. Generally, the formation of oxide will begin on the surface of freshly etched indium as soon as it is exposed to air. At this time the thickness of the oxide layer is between 30-40 Angstroms. After 2-3 days of being exposed to air, the oxide has reached its passivating thickness of 80-100 Angstroms.

Note: 

Indium has the unique ability to cold weld to itself when the oxides have been removed. During the etching process, care must be taken to keep units of indium separated so they will not stick together. If they do stick, it is very difficult to separate them without distorting the indium.

If the etched indium is not going to be used immediately, storage in a nitrogen dry box is recommended . Alternatively, the etched indium can be submerged in clean acetone to prevent exposure to air.

FLUX:
To solder directly to stainless steel, Indalloy #2 Flux (activation range 100-371°C) must be used to remove the surface oxides, allowing a clean surface for the solder to wet. This flux is recommended for mechanical assembly joining only. Due to the corrosiveness, it is not recommend for electrical applications because, if the post reflow flux residue is not thoroughly removed using warm water with mechanical scrubbing, the joint will be compromised due to the potential for corrosion during its life. An alternate solution would be to nickel plate the stainless steel, so a weaker flux (RA, ROL1) can be used that is less corrosive and can be easily removed with an appropriate solvent.   

Another alternate solution is to use a forming gas consisting of nitrogen and hydrogen. This method of oxide removal is generally used when the soldering temperature can be above 350°C which is ideal for activating the hydrogen to reduce the oxides. With this method, there is no post-reflow flux residue to clean up.

SOLDER:
The solders usually recommended for stainless steel joining applications contain a considerable amount of tin; however, the actual solder choice has to fit the temperature range of the application. Generally, a low-temperature application may require Indalloy #1E (52In,48Sn) - 118°C (eutectic), while Indalloy #182 (80Au,20Sn)- 280°C (also eutectic) is a great solder choice for high temperature. If you are looking for a solder in the moderate range of temperatures, Indalloy #121 (96.5Sn, 3.5Ag); 221°C (eutectic) is an excellent choice as well as any of the SAC alloys in the same temperature range. There are also many other solders to choose from that will work equally as well. Please see our solder alloy physical properties chart or consult our Applications Engineering staff at Indium Corporation.

We are frequently asked if it is possible to solder to aluminum. The answer is yes, if the following guidelines are followed: 

FLUXES:
Because it is difficult to solder to aluminum, Indium Corporation developed Indalloy Flux #3 (activation temperature is 96-343°C) to remove the tenacious oxides that prevent the solder from wetting to the surface. This flux is very corrosive and is not recommended for electronic applications because, if any of the post-reflow flux residue remains after a warm water rinse with mechanical scrubbing, the joint may be compromised. This flux is recommended for mechanical assembly joining applications only. 

Another alternate solution is to use a forming gas consisting of nitrogen and hydrogen. This method of oxide removal is generally used when the soldering temperature is greater than 350°C which is ideal for activating the hydrogen to reduce the oxides. With this method, there is no post-reflow flux residue to clean up.

METALLIZATIONS:
An alternate to corrosive fluxes is to nickel plate the aluminum so a weaker flux (RA, ROL1) can be used. These fluxes are less corrosive and can be easily removed with an appropriate solvent.   There are many solder alloys that will wet to nickel. Check out our solder alloy physical properties table.

SOLDER ALLOYS:
The solders that are normally recommended for joining aluminum are:

• Indalloy #201 (91Sn, 9Zn); 199°C E
• Indalloy #176 (95Zn, 5Al); 382°C E. 

Integrated® Preforms are connected units of solder that can be placed in an application to provide the sole solder for the joint. They can also be used to add to the volume of solder, when used in conjunction with solder paste - to fortify the joint. Integrated Preforms are available in most alloys that are currently incorporated in common solder pastes.

The only difference between connected preforms and separate preforms is that the Integrated (connected) Preforms are attached to one another and can be placed in the application as one multiple unit of solder. Connecting each individual solder preform is a small strand of solder that is designed so narrow that it will separate during reflow (to prevent bridging).

When manually reworking boards to add additional solder to joints becomes too large of a task, Integrated Preforms can be incorporated prior to reflow so any rework is not necessary. In through-hole applications, the connected preform can be placed on the component and then inserted into the holes of the board that have solder paste placed on top of them. There is no need to flux the Integrated Preform because the flux vehicle in the paste will provide the fluxing.

When used as a stand-alone connected preform, or with solder paste to fortify the joint, Integrated Preforms can be reflowed using the same temperatures and profiles as for the paste by itself.

To learn more about Integrated Preforms and solder fortification, plan to attend the presentation of a paper on this topic at Apex 2011 (Las Vegas, NV, 10-14 April) given by Dr. Ronald C. Lasky (see sidebar →), Indium Corporation Sr. Technologist and Instructional Professor, Thayer School of Engineering, Dartmouth College.

The next post in this series will address the various shapes of Integrated Preforms and how they are packaged.

Integrated Preforms®, like separate solder preforms, are available in all of the common shapes. Since not all applications are alike, these connected preforms will vary, not only in the alloy, but also in the size and shape of the solder preform required to deliver the correct amount of solder to the joint.

Some Integrated Preforms are made into discs, squares and rectangles for surface mount applications. Generally, the most common shape is that of a washer - for attaching through-hole components to a printed circuit board. Regardless of the shape, each is designed with specific dimensions that will fit the application. The spacing between the washers also varies because the pins on through-hole components can have different pitches.

Integrated Preforms are manufactured in 5” x 9” sheets with the designed shapes positioned within the borders. In preparation for shipment, they are stacked with lint-free paper and backing board and placed in a vacuum-sealed bag for delivery to the customer.

The customer can easily cut the shapes from the sheets with scissors or an X-Acto® blade. Care must be taken to handle the solder preforms with gloves so as not to contaminate the sheets with unwanted surface organics which could lead to voiding.

The next blog in this series will address the actual use of the Integrated Preforms and the many ways they can be modified to fit any application.

 

Flux & Substrate Compatibility

Flux is a liquid, solid or gaseous material which, when heated, speeds up and/or promotes wetting of the base material(s) by the fluxs removal of any surface oxides on the base material. Flux will protect against further oxidation during the soldering process.

Flux is an important component of most soldering operations. Using the correct flux for the application is essential to insure that a reliable joint is made.

To the right is a chart to help you choose the correct flux type for your application. Most post reflow flux residues need to be removed either to avoid corrosion or for reasons of appearance.

Reducing Gas Atmosphere is a gaseous atmosphere comprised of one or more gases, generally hydrogen and inert filler gas, such as nitrogen. Used where standard fluxes are ineffective at removing and preventing surface oxides.Hydrogen, for example, reduces the oxides, while nitrogen remains inert. It is recommended that both gases be present to effect a fluxing action. About 350°C is best.

http://upload.wikimedia.org/wikipedia/commons/b/ba/Thermometer.jpg

Operational temperature

Metallizations
Flux usage
Reflow method
Temperature limitations
Special conditions

Whether the market is communications, automotive, energy, military, medical or aerospace, choosing the correct solder is essential to making a quality product. A solder joint plays a very important role in an electrical or mechanical assembly. How this solder is selected for this joint is extremely important to the application, so following a guideline will insure that this is accomplished.

Before choosing a solder for an application, the operational or service temperature must be known; i.e. the minimum and maximum temperatures the assembly will see, during testing and in service. Continuous operational temperatures above 125°C will require the use of precious metal solders or hard solders such as Au/Sn, Au/Ge, or Au/Si.

Generally, it is best with soft solders to choose one that will melt at least 50°C higher than the expected service temperature of the assembly. (This includes temperature cycling during test and burn-in). If the difference in temperature (Delta T) between operational and the solidus temperature of the solder is too small, this could result in excessive scavenging, leaching and intermetallic growth. These unwanted conditions could lead to problems and eventual joint failure. 

After the correct temperature range for the solder is determined, consideration needs to be given to what metallizations in the joint will interact with the solder.

Metallizations will be addressed next in our series of steps to consider when designing a trouble-free solder joint.

 

Indium Corporations industry leading semiconductor bloggers are hosting a Meet the Bloggers session on Tuesday, July 15, 2008 at Indium Corporation's Semicon West exhibit, booth #7834, from 2-3pm PST.

The technology experts will lead discussions on topics including:

• Flux deposit measurement using non-contact metrology

Two upcoming white papers (currently under development):

• Wafer Flux Spin-Coating Topography
• Wafer-Level Flux Printing

Recent hot semiconductor blog topics, including:

• Semiconductor assembly materials
• Future trends in first- and second-level assembly
• Halogen-free semiconductor assembly materials
• Engineered solders in MEMS assembly
• Thermal interface issues
• Solar device assembly

Indium personnel who will be discussing these topics include Jim Hisert, Paul Socha, Fez Sayed, Dr. Andy Mackie, and Rick Short.

All attendees are welcome to participate in, or observe, the session. Snacks will be served, too!

Indiums blogs can be seen at www.indium.com/blogs

Have you ever had to hand-place solder washer preforms on the pins of a connector and found it to be very labor intensive? Indium Corporation has a product called InTEGRATED Preforms which enable the user to place multiple preforms at one time in less than 15 seconds.

InTEGRATED Preforms are joined in a matrix by fine, precise strands of solder which, during the soldering process, melt and flow to adjacent pads to give complete preform separation.

In addition to time saved, the quality of the joint is assured because only one preform is delivered to each joint eliminating the chance that a pin will be missed or two washers placed on the same pin. Each scenario will create a quality problem that reduces the yield and increases the time spent on rework.

Unique preform designs and complicated configurations can simplify difficult soldering jobs. An example is a thru-hole connector with multiple rows of long pins that are difficult to reach in the center with hand soldering. InTEGRATED Preforms eliminate the need to reach these remote areas. The connected washers can be placed on top of the board, under the connector. After reflow, equal volumes of solder are delivered to each of the pins in the connector.

In order to get the full benefit of InTEGRATED Preforms they must be uniformly fluxed on both sides including the connecting strands. Also, they need to be flat in the application and uniform heat must be used to reflow them.

Many printed circuit boards have mixed technology. The surface mount components can be placed in the paste first followed by the thru-hole components using InTEGRATED Preforms. The board can then be reflowed and cleaned once, eliminating time consuming steps.

To learn more about this time saving product patented by Indium Corporation, please contact the Applications Engineering Staff at Indium Corporation.and we would be happy to design an InTEGRATED Preform to fit your solder requirements.

For additional information regarding reflow methods using InTEGRATED Preforms, check out the June 2007 issue of SMT Magazine.

Have you ever had that sinking feeling when you tried soldering to stainless steel and found it to be very draining? You can join stainless steel, you know, and here's how!

You need the correct flux, a solder with some tin (Sn) in it, a heat source, and the ability to clean the post reflow flux residue in warm water.

Indium Corporation's Indalloy Flux #2 is especially formulated for removing the oxides from stainless steel. It has an activation range from 100-371°C. For high temperature applications, a forming gas (nitrogen and hydrogen) can be used.

Depending on the operational temperature of the device being soldered, Indium Corporation has a variety of tin containing alloys that will fit the requirements of the application. Indalloys #121 (96.5Sn, 3.5Ag) and #182 (80Au, 20Sn) are excellent solders for joining stainless steel. For other alloy choices please see our Alloy Property Chart for a complete selection of our solders.

The chosen solder can be reflowed using whatever heat source is available, as long as it is capable of reaching a temperature that is 20-40°C above the liquidus temperature of the solder. If a forming gas is used, it is best to use a temperature 350°C and above to activate the hydrogen so it will reduce the oxides.

Indalloy Flux #2 is corrosive, so it is necessary to clean any flux residue from the device being soldered. Because of the corrosiveness of this flux it is not recommended for electronic applications because it could cause problems if any residue remains after cleaning. If the application is for mechanical attach and not electrical, in certain applications, it may not require a cleaning.

If you have any questions please contact the Applications Engineering staff at Indium Corporation.

Picture: Dongyuan Kitchenware Industrial Co., Ltd

Do you have a 2' x 3' area on the wall of your office or cubical for the latest new and improved Indium Corporation Wall Chart? Click here to email us your name and address so we can send you one.

We have updated the Table of Specialty Alloys and Solders, added new tables for fluxes and improved the graphics. And yes, the periodic table is still there, as well.

The wall chart is a ready reference for a quick check of the melting temperature of an alloy or one of its unique properties. The chart will help to compare different properties of alloys to narrow down your solder selection.

Indium Corporation is a quality supplier of preforms, Integrated Preforms, Informs, wire, ribbon and foil, as well as, solder pastes, liquid fluxes and epoxies. If you need assistance you can contact an Indium Corporation Applications Engineers anywhere in the world. Our location information is on the chart. Call us to discuss your application and we will help you determine which form of solder will give you the best performance in your application. We have very talented engineers that help dozens of companies each week with their soldering questions.

In the upcoming weeks, we will be discussing topics such as:

The proper alloy for the metallizations
Flux selection
Methods of reflow
Engineered solder packaging
Ways to improve quality and uniformity in your process
Step soldering
Indium as a cryogenic seal, as well as, vacuum applications.

Paul A. Socha
Principal Engineer
Engineered Solder Products