Eric Bastow

Tombstoning

• the transition to Pb-Free (higher reflow temperatures, and related flux issues)
• miniaturization (0201s and 01005s)

• The reflow oven operator can slow down the ramp rate. A slower ramp rate allows for more uniform warming of the PCBA.
• Another technique is to employ a "soak" just below the melting temperature (solidus) of the alloy. For example, for a SAC305 profile (217°C solidus), one may implement a "soak" at 205 to 210°C for 30 to 120 seconds. This allows for the cooler parts of the PCBA to "catch up" to the warmer parts. After thermal equilibrium has been achieved, one can spike the temperature up to the appropriate peak temperature (i.e. 245°C). This technique (depicted in the reflow profile shown at the right) allows for all of the solder paste deposits to melt and wet the component terminations at roughly the same time; thereby, mitigating tombstoning.

Phone: +1.315.853.4900
E-mail: ebastow@indium.com

• exhibit reduced viscosity
• spatter during reflow
• produce excessive oxidation of the solder joint

• 96.5%Sn 3.5%Ag (221C)
• 80%Au 20%Sn (280C).

ITO is one of the materials that makes the magic of flat panel displays (monitors, TVs, etc.) possible. When sputtered on in a thin layer, it acts as a transparent conductive film. However, the process of sputtering is very inefficient in its use of ITO. And, with the explosion of flat panel display sales, large amounts of ITO end up as "waste".

It is important to understand the economics of ITO. The main precursor of ITO is indium (metal). Indium is a semiprecious metal and trades on the open market like gold and silver. It is subject to price fluctuations just like the other precious metals. So, there is an economic impetus to reclaim as much of the unused ITO as possible.


As mentioned earlier, flat panel displays employ a sputtered ITO coating. The indium metal must first be converted to indium oxide and then blended with the appropriate amount of tin oxide. The result is a pale green powder.

In order for the ITO to be usable, it must first be compressed in to a sputtering target. Planar sputtering targets are the dominant form of sputtering target used for sputtering of ITO. The geometry of the ITO sputtering target is often a rectangle or disc. Compressing the powder causes it to take on a darker color.

Inherent to the sputtering process is the uneven erosion of the sputtering target. The target material erodes in a "race track" pattern. These images of a spent nickel-vanadium sputtering target show the classic "race track" erosion pattern (valley).

Environmental initiatives coupled with the recent run-up when oil achieved $150 per barrell has put alternative energy back on the table. Solar energy was readily considered given its very "clean" and available nature. However, how can the average person, in their day to day life, take advantage of solar energy. Visions of roofs covered with (expensive and perhaps unattractive) solar panels and apparatus come to mind.
Well, an article in Photonics Media called "Painting the roof with solar ink" by Anne Fischer puts an interesting twist on capturing the suns rays for the sake of generating electricity. A company in the San Francisco Bay area, Innovalight, manufactures a solar ink comprised of silicon nanoparticles that can be suspended in a liquid. The "ink" can then be applied to flexible or rigid surfaces by means of "spray painting". Therefore, rendering virtually any surface that is exposed to the sun, a solar collector. Efficiency is said to be on the order of 18% currently. A company in China, JA Solar, has plans to commercialize this technology.
One of the founders of Innovalight, Brian Korgel, is trying to apply this approach to CIGS. Work done at the University of Texas has achieved 1% efficiency. The goal is 10% before going commercial and is anticpated to take 3 to 5 years

Once the flux has been formulated and scaled up it then must be mixed with solder powder. The mixing procedure and equipment must be capable of providing a homogenous product batch after batch after batch. However, because certain aspects of the solder paste must perform in a certain manner an optimum viscosity must first be determined by finding the proper powdered solder to flux ratio, or the “metal load”. Metal load is expressed in percent by weight.

Particle size impacts viscosity. Therefore the metal loading must be adjusted accordingly. The appropriate particle size for a paste is determined by the aperture sizes for stencil printing and the gauge (inner diameter) of the needle for dispensing applications. (Simply put, smaller particles are needed to fit through smaller holes.)

Each flux vehicle, solder alloy and particle size will have its own unique optimum metal load. The metal load is also application dependant. The optimum metal load for stencil printing will be higher than the optimum metal load for dispensing.

It just so happens that the optimum metal load (for stencil printing), expressed by weight percent, often equates to about 50% metal and 50% flux by volume.

Example Solder Paste Metal Load:

*The dispensing metal load is often not as sensitive to particle size as stencil printing. It could very well be that a metal load of 85% with type III may also be appropriate with type IV powder as well. The amount of powder has already been reduced enough to offset compaction due to smaller particle size.

All solder pastes have a shelf life. As mentioned earlier the flux can begin to react with the solder powder when both are in direct contact with each other in the paste form. The flux itself, often containing organic ingredients, can degrade over time. At a point, the paste will no longer be usable. The best way to optimize the shelf life of a paste is to keep it refrigerated.

The second main ingredient in solder paste is the flux (vehicle). Flux is a very complex group of chemicals/materials that must be able to do a number of things, some of which must happen simultaneously (a partial list is below). This requires the knowledge of experienced chemists and material scientists.

1)     The flux must not react with the powdered solder while in storage (shelf life). This is aggravated by the wide variety of solder alloys available in paste form. Each alloy family typically requires it own unique flux formulation.

2)     The flux must effectively remove any surface oxides present on the solder powder itself and mating surfaces prior to and during the melting of the solder.

3)     It must effectively prevent re-oxidation of the solder powder and mating surfaces at the elevated temperatures associated with reflow soldering, especially when performed in an air environment (air is ~21% oxygen).

4)     The performance of the flux must be unaffected by a wide range of temperature and humidity conditions.

5)     If the flux is “no-clean”, the residue must be non-corrosive and non-conductive per J-STD-004.

6)     If the flux is RMA, the residue must be easily cleaned with commercially available chemicals.

7)     If the flux is water soluble/washable, the residue must clean thoroughly with heated and pressurized deionized water.

8)     In a stencil printing application the flux must adequately fill and release from the apertures of a wide variety of sizes, shapes and stencil types, not stick to the squeegee, not dry out too quickly on the stencil, retain a brick like shape both at room temperature and elevated reflow soldering temperatures (minimize slumping), provide sufficient tack to hold components in place, not spatter during soldering (boiling of flux solvents), not outgas excessively (voiding)  and provide effective wetting of the solder to a wide variety of board metallizations, component lead platings and package bumps.

9)     In dispensing applications, the paste must dispense smoothly and consistently through a variety of needles sizes either through manual application or a variety of dispensing equipment types and technologies without clogging (and do many of the things listed in item 8).

10) Provide a cosmetically appealing solder joint and flux residue (if “no-clean”).

Once an alloy has been made it then needs to be fashioned into very small spherical particles, often referred to as solder powder. There are a number of different techniques for making solder powder, all of which require significant amounts of capital equipment and engineering expertise. The solder powder must then be sorted according to size, often referred to as “mesh size” or “type”. Here again a significant amount of equipment and labor is required. These sizes or types are defined in J-STD-006.

The medium in which the powder is made must be inert to prevent oxidation. Similarly, powder must also be stored in an inert environment until its time of use.

Much to the surprise of the SnPb and SAC alloy consuming world, there are a number of alloys available in solder paste, each with their own unique melting points and soldering and mechanical properties. Below is a table containing some of the alloys possible in solder paste (not exhaustive).

None of these individual metals are found pure on the planet. They have to be mined, refined and then alloyed to the proper proportion with the other metals to create these solder alloys. Oftentimes a wet chemical technique is required after alloying to validate that the alloy meets the allowable tolerances and purities spelled out in J-STD-006 or a customer specific specification.

For many in the electronics assembly world solder paste is often treated as a near commodity item. However, from within the walls of a solder paste manufacturer it becomes quite obvious that it is a highly engineered and intricate material.

The simplest answer to the question “What is solder paste?” is “It is a mixture of powdered solder alloy suspended in a flux (vehicle).” But the real answer is deeper than that very cursory response.

Later, we will discuss these topics in detail:

• Alloy
• Powder
• Flux
• Mixing and Metal Loading

That grey “sludge” that you use for soldering did not come out of a tar pit deep in a jungle somewhere. It is a sophisticated material that requires intelligent engineering and chemistry, equipment and proper handling at all levels of manufacture