HAND SOLDERING METHOD BASIC INFORMATION AND TUTORIALS

The various soldering methods which are used with electronic assemblies differ in the sequence in which solder, flux, and heat are brought to the joint, and in the way in which the soldering heat is brought to the joint or joints.

With hand soldering, the heat source is the tip of a soldering iron, which is heated to 300–350 °C/570 660 °F. A small amount of flux may have been applied to the joint members before they are placed together.

The assembled joint is heated by placing the tip of the soldering iron on it or close to it. Solder and flux are then applied together, in the form of a hollow solderwire, which carries a core of flux, commonly based on rosin.

The end of the cored wire is placed against the entry into the joint gap. As soon as its temperature has reached about 100 °C/200 °F, the rosin melts and flows out of the solderwire into the joint. Soon afterwards, the joint temperature will have risen above 183 °C/361 °F; the solder begins to melt too, and follows the flux into the joint gap.

As soon as the joint is satisfactorily filled, the soldering iron is lifted clear, and the joint is allowed to solidify. Thus, with hands oldering, the sequence of requirements is as follows:

1. Sometimes, a small amount of flux.

2. Heat, transmitted by conduction.

3. Solder, together with the bulk of the flux.

Clearly, this operation requires skill, a sure hand, and an experienced eye. On the other hand, it carries an in-built quality assurance: until the operator has seen the solder flow into a joint and neatly fill it, he – or more frequently she – will not lift the soldering iron and proceed to the next joint.

Before the advent of the circuit board in the late forties and of mechanized wavesoldering in the mid fifties, this was the only method for putting electronic assemblies together. Uncounted millions of good and reliable joints were made in this way.

Hand soldering is of course still practised daily in the reworking of faulty joints. Mechanized versions of hands oldering in the form of soldering robots have become established to cope with situations, where single joints have to be made in locations other than on a flat circuit board, and which therefore do not fit into a wave soldering or paste-printing routine.

These robots apply a soldering iron together with a metered amount of flux-cored solder wire to joints on three-dimensional assemblies, which because of their geometry do not lend themselves to wave soldering nor to the printing down of solder paste.

Naturally, soldering with a robot demands either a precise spatial reproducibility of the location of the joints, or else complex vision and guidance systems, to target the soldering iron on to the joints.

LEAD FREE SOLDERING BASIC INFORMATION AND TUTORIALS

The issue of lead in electronics has gone through more than 12 years of deliberation and debate by legislative bodies, manufacturers, and individuals around the world. Various ideas have been exhibited, particularly in U.S., and individual opinions expressed by both supporters and oppositions have been eloquent.

On the global landscape, the tangible progress in technology and legislation differs in the three major continents—North America, the European Union, and Asia. Although a uniform consensus is still to be worked out, the technology has advanced, the business climate has changed, and, overall, the marketplace is striding into a highly environmentally-conscious playing field.

Various organizations have made dedicated effort to inform the industry about this pivotal issue. For instance, the Swedish Institute of Production Engineering Research (IVF) has developed the “Electronics Design-for-Environment Webguide,” which disseminates updated information to the industry regarding the development of legislation and technology.

The International Tin Research Institute (ITRI) launched the Lead-Free Soldering Technology Centre, and IPC initiated the Lead Free Forum on the Internet. Professional organizations such as Surface Mount Technology Association (SMTA) and International Microelectronics and Packaging Society (IMAPS) have organized symposia dedicated to disseminating knowledge and information.

Global legislations in the three regions are described separately, below. To producers and manufacturers, waste reduction, recovery, and recycling should be and inevitably will be treated as a long-term goal supported by an ongoing effort.

A product should be designed for minimal environmental impact and with the full life cycle in mind. Life-cycle assessment includes all the energy and resource inputs to a product, the associated wastes, and the resulting health and ecological burdens. Overall the goal is to reduce environmental impacts from cradle to grave.

SOLDER-JOINT INTEGRITY BASIC FAILURE PROCESSES TUTORIALS

Solder-Joint Integrity

Solder-joint integrity can be affected by the intrinsic nature of the solder alloy, the substrates in relation to the solder alloy, the joint design or structure, the joint-making process, and the external environment to which the solder joint is exposed.

Therefore, to assure the integrity of a solder joint, a step-by-step evaluation of the following items is warranted:

■ Suitability of solder alloy for mechanical properties

■ Suitability of solder alloy for substrate compatibility

■ Adequacy of solder wetting on substrates

■ Design of joint configuration in shape, thickness, and fillet area

■ Optimal reflow method and reflow process in terms of temperature, heating time, and cooling rate

■ Conditions of storage in relation to the aging effect on the solder joint

■ Conditions of actual service in terms of upper temperature, lower temperature, temperature cycling, vibration, and other mechanical stress

■ Performance requirements under the conditions of actual service

■ Design of viable accelerated testing conditions that correlate with actual service conditions

Basic failure processes

In the real world, solder-joint failure often occurs in complex mechanisms involving the interaction of more than one basic failure process. Although creepfatigue is considered to be a prevalent mechanism leading to the eventual solder joint failure, separate test schemes in creep and fatigue are often conducted to facilitate data interpretation and an understanding of the material behavior.

The basic processes or factors that are believed to contribute to solder failure during service are as follows:

■ Inferior or inadequate mechanical strengths

■ Creep

■ Mechanical fatigue

■ Thermal fatigue

■ Intrinsic thermal expansion anisotropy

■ Corrosion-enhanced fatigue

■ Intermetallic compound formation

■ Detrimental microstructure development

■ Voids

■ Electromigration

■ Leaching

LASER SOLDERING BASIC INFORMATION AND TUTORIALS

Two types of laser have been applied to solder reflow—carbon dioxide (CO2) and neodymium-doped yttrium-aluminum-garnet (Nd:YAG). Both generate radiation in the infrared region with wavelengths of about 10.6 μm from the CO2 laser and 1.06 μm for the YAG laser.

The wavelength of 1.06 μm is more effectively absorbed by metal than by ceramics and plastics; the wavelength of 10.6 μm is normally reflected by conductive surfaces (metals) and absorbed by organics.

The main attributes of laser soldering are short-duration heating and highintensity radiation, which can be focused onto a spot as small as 0.002 in (0.050 mm) in diameter. With these inherent attributes, laser reflow is expected to

■ Provide highly localized heat to prevent damage to heat-sensitive components and to prevent cracking of plastic IC packages

■ Provide highly localized heat to serve as the second or third reflow tool for assemblies demanding multiple-step reflow

■ Require short reflow time

■ Minimize intermetallic compound formation

■ Minimize leaching problems

■ Generate fine-grain structure of solder

■ Reduce stress buildup in solder joint

■ Minimize undesirable voids in solder joint

With these attributes in mind, laser soldering is particularly beneficial to soldering densely packed regions, where local solder joints can be made without affecting the adjacent parts, to soldering surface mount devices on printed-circuit boards having heat sinks or heat pipes, and to soldering multilayer boards.

In addition, it also provides sequential flexibility of soldering different components and enhances the high-temperature performance of adhesives used for mounting surface-mount devices.

With respect to reflow time, laser soldering can be accomplished in less than 1 sec, normally in the range of 10 to 800 ms. The laser can be applied to pointto- point connections through pulsation as well as to line-to-line connections via continuous laser beam scan.

The fine-pitch flat-pack devices have been connected to printed wiring boards using YAG continuous laser beam scans on each side of the package.

Both the use of prebumped solder pads and the direct application of solder paste are feasible. In directly reflowing solder paste, although using spattering and heat absorption problems have been observed, they are not incurable.

To eliminate these problems, the preheating and predrying step is necessary. Location of laser beam impringement is another factor. In addition, compatible properties of solder paste have be designed to accommodate fast heating in relation to fluxing and paste consistency, coupled with the proper design of the equipment and its settings.

SOLDER BEADING AND SOLDER BALLING BASIC INFORMATION AND TUTORIALS

Solder balling.

Elevated temperatures and excessive time at those temperatures during the warm-up and preheating stages can result in inadequate fluxing activity or insufficient protection of solder spheres in the paste, causing solder balling. In addition to the quality of solder paste, the presence of solder balls may be essentially related to the compatibility between the paste and the reflow profile. On the other hand, inadequate preheating or heating too fast may cause spattering, as evidenced by random solder balls. The two heating stages preceding the spike/reflow zone are primarily responsible for this phenomenon.

Solder beading.

Solder beading refers to the occurrence of large solder balls (usually larger than 0.005 in [0.13 mm] in diameter) that are always associated with small and low-clearance passive components (capacitors and resistors).

This problem will occur even when the paste may otherwise perform perfectly, i.e., free of solder balls at all other locations (components) on the board and with good wetting. The trouble with solder beading is that it may occur in most or all board assemblies, rendering the first-time yield to nearly zero. The current remedy on the production floor is to manually remove the beads.

The formation of solder beads near or under capacitors and resistors is largely attributed to paste flow into the underside of the component body between two terminations aided by capillary effect. As this portion of paste melts during reflow, it becomes isolated away from the main solder on the wettable solder pads, forming large discrete solder beads.

With other factors, reflow profile is a contributor to this phenomenon The practice of adopting a slower preheating rate and a lower reflow peak temperature can reduce solder beading. However, if the reflow profile is at its optimum, and the problem still persists, a new paste with a strengthened chemistry is the solution.

SAFETY USE OF SOLDERING IRON TUTORIALS

SOLDERING SAFETY TIPS

Soldering poses a few different dangers. (You might use solder to attach various pieces of your electronics project, such as soldering wires onto a speaker, microphone, or switch.) The soldering iron itself gets mighty hot.

The solder (the material you heat with the iron) gets hot. Occasionally, you even get an air pocket or impurity in solder that can pop as you heat it, splattering a little solder toward your face or onto your arm.

To top that off, hot solder produces some nasty fumes. Soldering itself takes experience to get right. Your best bet is to have somebody who is good at it teach you.

Here are some soldering safety guidelines you should always follow:

1. Always wear safety glasses when soldering.

2. Never solder a live circuit (one that is energized). Soldering irons come in models that use different wattages. Use the right size soldering iron for your projects; too much heat could ruin your board or components.

3. Solder in a well-ventilated space to prevent the mildly caustic and toxic fumes from building up and causing eye or throat irritation.

4. Always put your soldering iron back in its stand when not in use. Too, be sure that the stand is weighted enough or attached to your worktable so that it doesn’t topple over if you should brush against the cord.

5. NEVER place a hot soldering iron on your work surface. You could start a fire.

6. Give any soldered surface a minute or two to cool down before you touch it.

7. Never, ever try to catch a hot soldering iron if you drop it. No matter how hard you try, you are very likely to grab the hot end in a freefall. Let it fall; buy a new one if you have to — just don’t grab!

8. Never leave flammable items (like paper) near your soldering iron.

9. Be sure to unplug your soldering iron when you’re not around.

Don’t put your face too close to the soldering site because of the danger of stray hot solder and those horrible fumes.

Instead, use a magnifying device to see when soldering teeny-tiny components to a board.

You can buy clampon magnifiers that keep your hands free for soldering.

SOLDERING IRON BASIC INFORMATION AND TUTORIALS

WHAT IS SOLDERING IRON? HOW TO USE SOLDERING IRON?

SOLDERING IRON

Soldering is used in nearly every phase of electronic construction so you’ll need soldering tools. A soldering tool must be hot enough to do the job and lightweight enough for agility and comfort.

A temperature controlled iron works well, although the cost is not justified for occasional projects. Get an iron with a small conical or chisel tip.


Soldering is not like gluing; solder does more than bind metal together and provide an electrically conductive path between them. Soldered metals and the solder combine to form an alloy.

You may need an assortment of soldering irons to do a wide variety of soldering tasks. They range in size from a small 25-watt iron for delicate printed-circuit work to larger 100 to 300-watt sizes used to solder large surfaces.

If you could only afford a single soldering tool when initially setting up your electronics workbench than, an inexpensive to moderately priced pencil-type soldering iron with between 25 and 40-watt capacity is the best for PC board electronics work.

A 100-watt soldering gun is overkill for printed-circuit work, since it often gets too hot, cooking solder into a brittle mess or damaging small parts of a circuit. Soldering guns are best used for point-to-point soldering jobs, for large mass soldering joints or large components.

Small “pencil” butane torches are also available, with optional soldering-iron tips. Butane soldering irons are
ideal for field service problems and will allow you to solder where there is no 110 volt power source.

Keep soldering tools in good condition by keeping the tips well tinned with solder. Do not run them at full
temperature for long periods when not in use. After each period of use, remove the tip and clean off any scale that may have accumulated.

Clean an oxidized tip by dipping the hot tip in sal ammoniac (ammonium chloride) and then wiping it clean with a rag. Sal ammoniac is somewhat corrosive, so if you don’t wipe the tip thoroughly, it can contaminate electronic soldering.

Place the tip of the soldering iron into the “Tip Tinner” after every few solder joints. If a copper tip becomes pitted, file it smooth and bright and then tin it immediately with solder.

Modern soldering iron tips are nickel or iron clad and should not be filed. The secret of good soldering is to use the right amount of heat.

Many people who will have not soldered before use too little heat dabbing at the joint to be soldered and making little solder blobs that cause unintended short circuits.

Always use caution when soldering. A hot soldering iron can burn your hand badly or ruin a tabletop. It’s a good idea to buy or make a soldering iron holder.