Introduction to Soldering
Soldering is an often found activity when building a space payload. Solder is a fusible metallic alloy used to join metallic surfaces. A variety of solder alloys are available and are designed for melt temperature and to control the alloy layers in the final solder joint interconnection. Solder joints primarily provide a conductive connection for electrical circuits but, in concert with electronic part leads or attachment pads, must also perform reliably under induced and exposed environmental conditions such as mechanical shock, vibration, temperature induced tensile stress, and vacuum.
Figure 1: Soldering Iron, Solder, and Desoldering Braid
The below videos provide a sense of the basics of soldering.
Hand soldering a leaded, through-hole part:
Hand soldering surface mount parts:
Hand soldering two wires:
Process Engineering Considerations for Quality and Reliability
Solder joints will fail when a pull or bend force on the solder joint overwhelms the tensile strength of the solder itself. The more likely cause of solder joint failure is the growth of small defects in the joint into a large defect which appears as a crack between the two conductors that are intended to be connected by the solder joint. Cracks can create a permanent and sustained disconnect or can make an intermittent electrical connection that appears normal at times but then, with vibration or expansion and contraction with temperature, makes the connection appear to fail or to change its electrical resistance. Defects can exist at the crystal lattice level or the macro level. The time it takes for defects to coalesce and form a crack all the way through the joint is called the time-to-failure and is usually measured in the number of thermal cycles (of a particular temperature range) or time exposed to vibration (of a certain range of superimposed frequencies). Engineering evaluation of the reliability of solder joints based on the type of solder used, the shape of the part lead, the shape of the solder joint, the metallic composition of the formed joint, or other parameters, requires the testing of hundreds or thousands of joints to predict their life expectancy.
Reliability studies are typically performed for solder joints of a particular design to discover their time-to-failure for a temperature cycling range or vibration range of interest (e.g., for use in cars, or use in spacecraft, or use in medical equipment). Reliability is always relative to the environment of interest and the intended useful lifetime (e.g., two years for cell phones, fifteen years for the James Webb Space Telescope, five years for appliances). Reliability tests are meaningful when the test samples are identical, and the sample set does not contain manufacturing mistakes. Early failures of samples that contain manufacturing mistakes are called “infant mortality failures” and are not usually counted in the end-of-life prediction.
It is not unusual that a solder joint with a particular type of macro defect will last a very long time. Researchers have found it extremely difficult to simulate identical macro defects in sufficiently large sample sizes to generate reliability results for particular types of macro defects. However, common knowledge about the relationship between macro defects and circuit failure has developed over time from the examination of reliability test failures and hardware returned from use for repair (“field returns”). The manufacturing parameter settings which create these macro defects are also known. Both the appearance of common macro defects (it can be viewed externally or by X-ray) and the conditions are identified to create them are the subject of requirements in industry quality standards.
Solder joints which do not contain manufacturing mistakes are considered high-quality. Manufacturing mistakes are known to be caused either by poor planning for or poor control of the variables of the manufacturing process. The work to design the soldering process, including a selection of the materials to be used and the process conditions including temperatures, times, and surface preparation, is called process engineering. Successful soldering process engineering will prevent manufacturing mistakes which create macro defects in solder joints which cause early life failures. It is impossible to prevent micro defects in solder joints, however they can be minimized by understanding the effects of soldering temperature conditions and trace metals that are present in the solder selected and in the alloy that is formed when the melted solder combines with the leads or surfaces being soldered.
Visual inspection is used to check finished solder joints for macro defects. Many solder joints with externally observable defects can be fixed by reheating the joint (though solder joint heating can be damaging to a printed circuit board). However, many of the macro defects that cause early life failure cannot be seen externally, and the user must rely on the process of engineering to know that the solder joints are high quality. Some visual inspection criteria defined in industry quality standards use the appearance of solder joints made under well-controlled conditions as an inference for the absence of macro defects inside of the joint; if the outside looks good, the inside is probably also good. Solder joint quality assurance consists of encouraging the manufacturer/maker to employ good process engineering while visually checking the appearance of the finished joints for macro defects or evidence that macro defects may exist internal to the joint.
Flux is a chemical compound used to chemically strip oxygen from a metal surface, when heated, to enable the metal of the surface to readily combine with molten solder. Metal surfaces which have corroded by absorbing oxygen from the air and creating an oxide surface layer, will not readily attach to melted solder. During soldering, flux is made present, either by applying just before soldering or by combining the flux with the solder itself, to provide an oxide-free surface at the moment when the molten solder is contacting the surface. If the surface is not sufficiently oxide-free the solder will not attach or “wet”. A non-wetted surface has a distinct appearance, is considered a macro-defect, and is associated with early life solder joint failure. In some solder joints, due to their size or geometry, the flux can strip off the oxygen layer. However, that oxygen, and other volatile materials that are part of the flux, get trapped inside the solder joint when it solidifies, creating voids. High concentrations of voids can be macro defects that cause early life failure. Voiding cannot be viewed externally but only with X-ray inspection which is an expensive and time-consuming quality assurance inspection method for solder joints. Solderability requirements found in industry soldering quality standards describe how to ensure surfaces are protected or prepared adequately for soldering to prevent non-wetting and voiding. A process called “tinning” which consists of dipping a part lead or a wire into molten solder can be used to ensure surface solderability. Some flux compositions are the cause of excess voiding even when the surfaces meet quality requirements for solderability.
Flux formulations vary, and in particular in the number of ionic compounds that are added to increase their “activity” (or ability to strip off oxygen from the surface and dislodge other residues). When the flux is fully activated, these compounds are neutralized in the process. There is no guarantee however that all of the flux will be evenly heated during soldering and neutralized. Remnant flux can be corrosive to the board or parts and can provide the main ingredient of metallic dendrites which can grow in electrically small components. Residues that are present on parts, boards, and in cleaning solutions have been associated with permanent damage to boards and early life failures. This is why cleaning the components is an essential part of soldering before and after the actual solder joint is created.
Types of Soldering
Soldering methods fall into three main categories: hand soldering, reflow soldering, and wave soldering.
Hand soldering forms each joint individually by simultaneously applying a hot soldering iron and a wire made of solder to the two surfaces to be joined. Some solder wire is built around a core of flux (i.e., flux-cored solder) and some solder wire is metal only, and a separate flux paste or liquid must be used in addition to the solder wire. Hand-soldering is most often used for assembling connectors onto cables, for installing jumper wires onto parts or printed circuit boards, soldering wires from transformer coils, and for repairs.
Reflow soldering uses a solder paste which is comprised of miniature solder balls suspended in liquid flux. The solder paste is screen-printed onto a printed circuit board, the parts are placed on top of the printed paste, and the assembly is slowly passed through an oven which exposes it to a temperature up and down-ramp which activates the flux, melts the solder, and provides smooth temperature transitions which prevent thermal shock damage and drive down internal defects during solder solidification. Reflow soldering is the most common type of soldering process and used for surface mounted assemblies on printed circuit boards.
Wave soldering uses a rolling bath of flux and molten solder that is passed across the areas to be joined to fill cavities such as holes through a printed circuit board to create a solder joint. Wave soldering is most commonly used to install through-hole connectors to boards after all of the surfaces mounted parts have been installed using a reflow process.
Some soldering operations require a multistep process where one joint is made and then another is made for a different interconnect close by. Soldering near another solder joint can partially reheat the first solder joint and damage it. When this manufacturing sequence is necessary, different alloys of solder are selected, and the solder with the higher melt temperature is used first so that the second soldering operation temperature will not affect the first joint. The higher temperature solder will often be in the form of a preformed “piece” of solder that is positioned and then heated with a soldering iron or by passing the assembly through a reflow oven or multi-stage conveyor belt heat plate. This type of soldering is common for parts whose underside acts as both a heat sink and an electrical contact where a relatively large area, all of which is hidden from view, is required to be covered with solder.
Electrostatic Discharge Control
Many electronic components that are interconnected by soldering are sensitive to electrostatic discharges (ESD). Before handling the parts or boards their sensitivity must be known, and ESD controls must be used to ensure they are not exposed to discharges of energies that could damage them. An ESD Control Program should be created and implemented for this purpose. (See the AAQ ESD course for more information on ESD.) An ESD Control Program will define the electrical grounding systems that will be used to ensure static charges are not accumulated, and that those that do accumulate are drained to ground through current-limited connections. It also ensures that electric fields do not build up around sensitive parts from charged insulators and that people working with ESD-sensitive items follow the required practices that ensure that the ESD control methods are utilized as intended. For soldering, it is vital to use industry standard techniques and limits to ensure that soldering equipment that touches the hardware (the soldering iron when hand-soldering and pick-and-place machines automated surface mount soldering) are adequately grounded and, for soldering irons, that voltage differentials that exist at the tip do not exceed part ratings over relevant frequency ranges.
The minimum steps to protect ESD-sensitive devices are:
- Only work in a grounding configuration which includes the operator and all work surfaces and equipment that ensures equipotential grounding.
- Use work areas and tools with grounded dissipative surfaces. Control charge accumulation with balanced ionizers for essential insulators.
- Always check the performance of the operator grounding method before beginning work (wrist strap system or conductive shoes/floors).
- Use a static-dissipative bag or container to store or carry parts into and out of the ESD controlled area.
Process Engineering Considerations
The following are some of the main parameters that must be considered when preparing a soldering process to ensure the joints will be reliable and to reduce manufacturing defects:
- Solder alloy
- Flux composition
- Pre-heat and controlled cool-down
- De-moisturizing printed circuit boards
- Cleanliness of boards and parts
- Cleaning methods and materials (before and after soldering)
- Surface solderability
Example of a Hand Soldering Procedure for a Wire
1. Prepare, Position, and Clean Connection
Properly prepare the connection to be soldered by stripping the wire insulation to the required length (determined by design and quality rules for the maximum allowed exposed uninsulated conductor). Use a thermal wire stripper or a mechanical wire stripping toolset for the wire gauge size.
- Inspect under 4x-10x magnification to check for nicks or cuts in any of the conductors. Follow the quality rules for rejecting damaged stripped wires.
- Clean both sides of the connection by established work area procedures to remove oils and other residues.
2. Clean Assembly
Heat solder tinning pot to the temperature defined in the procedure which is selected for the solder to be used (the most common is eutectic tin-lead which has a melt temperature of 183 °C or 361.4 °F).
- Add flux to stripped wire.
- Remove dross from the solder surface.
- Ensure that the shape of the wire contains a stress-relief bend as defined by the quality rules. Do not reform lead after it has been tinned.
- Tin the lead by dipping it to within 0.5mm of the wiring insulation (a soldering rod may also be used).
- Clean the flux off the tinned part of the wire, by established work area procedures.
- Inspect again (ideally under 4x-10x magnification).
3. Clean and Position Iron, Solder
Wipe iron with shop wipe.
- Wipe the iron tip to remove old solder and corrosion.
- Tin the iron tip by placing a small amount of solder on the tip.
- Position the iron such that it is in the proper position to perform soldering.
- Position the two sides of the interconnect (wire and substrate or wire and contact) such that neither side will move during melt and solidification of the solder.
- Heat the interconnect with the soldering iron.
- Apply the solder to the heated connection.
4. Clean and Inspect Connection
Clean connection, by established work area procedures.
- Inspect Connection (ideally under 4x--10x magnification) per the applicable quality requirements.
Soldering involves typically the evolution of volatile materials from flux as it is activated. A fume extractor is recommended to reduce the volatiles from the work area during soldering. Scraps of solder at the work area and from solder pot dross removal will contain elemental lead (Pb). This material should be removed from the work area and controlled as a hazardous material. No eating or drinking should be done near soldering workstations both for the safety of the individuals and for protecting the hardware. Eye protection should always be worn.