A good soldering technician observes the following stages of preparation for each job.
Ask an experienced soldering technician, "What is the most important task to perform before soldering?" Many technicians, even those who have been soldering for years, will probably answer, "heat," "iron tip," "solder." They usually miss the most critical task of all: cleaning.
Clean the soldering iron tip, component lead or wire, the item that the component is being soldered to (board or terminal), tools being used to form the wires/leads, and even the solder itself.
Cleaning the Soldering Tip
The soldering iron tip should be bright silver with no flux residue or solder on it. Any major buildup of oxide on the tip is removed by wiping the tip on a damp sponge before applying it to the area to be soldered. This shocking action steams off the oxide and leaves the tip pristine and in the proper condition for soldering. To start, you need the correct soldering iron for the job. For the majority of electronics work, this means a 25-to-30-watt pencil type iron with variable heat control. This makes is easy to ensure that the proper temperature is used for the work at hand. Soldering guns or irons with magnetically controlled heaters can possibly damage the very tiny and sensitive integrated circuits or ICs because of the electromagnetic fields radiating from these types of irons.
If a soldering iron does not have a thin consistent layer or solder over the entire surface, the tip has not been properly tinned. If the iron tips is not properly tinned, start with a cold iron, turn the heat on, and hold the flux cored wire solder against the tip as it heats up. Wipe off the excess solder, then shock the tip on the sponge before soldering. Do not wipe the excess solder, burned flux residue, and other contaminants onto the sponge. The purpose of the sponge is to shock the iron. If you keep dumping your excess solder, burned flux, and other residue onto your sponge, the sponge quickly becomes useless. Every time you then touch the sponge, you pick up the dirt you put there earlier. This adds contamination to the solder connection. The sponge should remove the thin layer or oxide that builds up when the iron is heated. Find another means and another place to remove the dirt from your iron. One method is to use a paper or cotton wipe, ones that will not be shred and leave particles behind, and very gently wipe the dirt from your iron. Then shock the iron tip to touching the dampened sponge. Some solder stations now have sponges with openings in the middle that allow you to wipe the excess solder off into the opening leaving the sponge free of contaminants that would otherwise end up on the tip of the iron. (Practical Hint: When you are not using your iron, make sure you leave a large lump of solder on the tip. This maintains the tinning on the tip, and the tip will last much longer. Many technicians mistakenly clean the tip before they put the iron into the holder. Leave the solder on the tip to protect it.)
In a manufacturing facility, a relatively clean board is generally available, but this should not be taken for granted. If boards have been stored without protection against oxides and other airborne contaminants, cleaning may be required before you do any soldering. Wire terminals may need to be pre-tinned to remove oxides before a wire is installed. Dirt films on metals may consist not only of oxides, but sulfides, carbonates, and other corrosive materials from the environment. These will hinder solder flow or wetting of the solder onto the surfaces being soldered.
Component Leads and Wire Cleaning
Component leads should be tested periodically for solderability. Take items from stock at random and test them to ensure that problems will not be encountered when components are installed onto the board. If necessary, re-tin the leads, then clean them off. Wire, tinned by hand or solder pot, should have the burned flux residue removed. If this residue is not removed, this contaminating material will be included in an unreliable connection. Clean the wire with a liquid cleaner. Items such as a pink eraser, steel wool or similar types of cleaning tools are not a good idea. The eraser leaves a gum residue which you now have to remove and the steel wool could actually remove the tinning, ect. Some technicians feel that the heat of the soldering iron cleans off the area to be soldered. This is a very common misconception. Some of the techniques used actually increase the oxidation rate. Make sure everything you use or solder is clean.
A second very important item in preparing to solder is the flux. Flux has a very definite purpose: It prevents oxidation and removes the thin layer of oxide and the atmosphere gas layer from the area to be soldered. When the flux is applied to the area, it permits the solder to flow, or wet, smoothly and evenly over the surface of the lead, wire, or pad being soldered. It also improves the flow of heat, resulting in faster heating of the items are area being soldered.
Types of Flux
There are various types of fluxes available. (Caution: Some types of flux should never be used on a circuit board because they corrode the board and the lead parts if the flux is not removed immediately. Acid- or zinc-based fluxes should not be used on a circuit board. Fully activated rosin flux, known as RA, also is not recommended for use on a circuit board.) The acceptable types of rosin flux include the pure rosin and the mildly activated rosin (R or RMA). This later flux is in common use today, with some inroads being made by so-called low residue and no clean fluxes. It has been found that some residues left behind from flux becomes water absorbent and should be removed within a maximum or thirty minutes after the connection has been made. RA flux is acceptable for use in tinning bus wire or component leads, but should not be used on a circuit board or even kept in the same room, in case it gets picked up and used by mistake. Activators will degrade the board and cause problems that would otherwise not occur. Most boards operate in an enclosed environment where there is considerable heat, moisture (relative humidity), airborne bits and pieces or dirt. The environment softens the flux left on the board, turning it into a gooey, particle-attracting, water-absorbing blob or useless material. This mess will become conductive as it absorbs moisture, resulting in leakage paths that cause problems in the operation of the equipment. In late 1992, a new water-soluble flux, developed originally by Huges Aircraft, received acceptance: final approval for its use was made in early 1993. The flux is made from lemons. This makes it very easy to clean, but the cleaning must be very thorough, or residue may cause corrosion.
If the correct flux is properly used, it will greatly assist all aspects of soldering and desoldering. It improves the intermetallic bonding and consequently the solder flow, which is one of the important areas of inspection. Poor wetting is usually the result of poor cleaning procedures or lack of sufficient heat. De-wetting problems relate to the material that is being soldered as a result of the intermetallic compound reaching the surface of the tinned area. The feathering out of the solder on a connection indicates that wetting has occurred.
Heat, Time, Mass
The third item in preparing to solder includes three very important factors to be considered. These are the heat to be used, time on the connection, and the mass of the joint. Because not all connections are the same, consideration must be given to the differences in the mass of the joints and adjusting the heat and/or time accordingly. You should not use the same heat and length of time to solder a diode to a small pad as would be needed for soldering a wire onto a terminal. The diode would be would be damaged, the pad area where the lead is being soldered could be damaged, and the solder will be overheated. An iron that is too cold will result in a mush type of melt and poor wetting action. The maximum time from when a soldering iron comes into contact with the parts that are to be connected until the joint is finished should not exceed two to five seconds. In some cases just one second is the maximum allowable time. One other thing to keep in mind as far as heat is concerned is the oxidation rate of the soldering iron tip. At a normal temperature of 600 degrees, there is a certain amount of oxidation produced, depending on the time it is left unused and without any solder on the tip. At 700 degrees the rate is nearly ten times the level of oxidation and at 800, approximately hundred times. This oxidation acts as a barrier to the transfer of heat and therefore the proper flow of solder. Because we are not robots and because people work and react differently to what is happening, it is beneficial for personnel to be able to easily regulate the amount of heat being applied. Changing the heat of the tip of the iron should be the simplest task possible; for example, turning a dial. People should be able to recognize what is actually occurring as compared to what they feel is going on. Experienced solders have a genuine knowledge of what happened to a particular joint. They know by observing what has happened and can judge whether a joint will be reliable or will break down in a short period of time. To inspect solder connections, a 10X stereo microscope should be made available for managers and supervisors. They should also be trained to know what they should see during inspection.
The fourth point in preparing to solder is to consider the type of solder to be used. Most companies and technicians use 60/40 solder. There is nothing wrong with 60/40 solder, but there is a better one- 63/37 or eutectic solder. Note that for the 60/40 solder, there is a time when the solder is neither liquid nor solid. It is in a plastic state during this period. It is very important that there be absolutely no vibration or movement of the connection when the solder is going through this plastic region, otherwise a disturbed joint will be the result. The 63/37 solder has no plastic period and reduces the possibility of a disturbed connection.
Heat sinking is a method used to prevent the overheating of components, wires, or circuit boards. It usually is a small metal clip or clamp which is attached to the area between where the solder connection will be made and the item to be protected. The use of a heat sink for soldering some components is not required if the proper technique for soldering is followed to the letter. However, if the proper soldering technique is not followed, heat sinking becomes an alternative, but not a good one. Heat sinking is used mostly by persons who are unaware of the proper procedures. Compensation has to be made for the additional mass of the heat sink by increasing the heat and possibly the length of time. If this concerns a diode in a double-sided board in a plated through hole will a small pad area on each side, the chances of lifting a pad becomes greater by the millisecond. The heat sink would have to be placed on the top of the circuit board attached to one of the leads. Because the solder and iron are on the bottom of the board, it will be difficult to get the solder to flow throughout the hole and wet onto the component side pad - which is what should happen for the lead to be soldered correctly.
Cleaners and/or Flux Removers
Item five is preparing to solder involves the selection of a good chemical cleaner. When it comes to the cleaner to be used for removing flux and cleaning in general, there is a wide variety of cleaners from which to choose. The cleaner must be able to remove ionic and non-ionic residue from anything that is being, or has been soldered. Check the contents of the cleaner, then check the Material Safety Data Sheet (MSDS) for information on the various chemicals involved. See if anything in the cleaner is carcinogenic (cancer causing). Even if carcinogens are present in only small quantities, you should try something else. Isopropyl alcohol (IPA), also known as isopropanol, is a decent cleaner; but there are others that contain blends of alcohol that are even better. The key is to find a cleaner that will not harm your work or - more importantly - yourself.
Various techniques have been tried ever since soldering was first used in electronics. The old saying "the bigger the blob, the better the job" can no longer be accepted. What was considered too meticulous and fussy is now the standard. Soldering can no longer be taken for granted. It is an art, and there are very few gifted painters. In 1989 a person who received two weeks of formal soldering training in 1981 said, "Soldering sure has changed in eight years!" This comment underscores the need for training from knowledgeable instructors who keep up-to-date with soldering techniques. The usual soldering technique is as follows: First apply the solder to the tip of the iron, then apply the iron to the area to be soldered. If the flux is not put onto the lead and pad first, the purpose of the flux in the wire solder is defeated. The flux dissipates over the iron tip and turns into carbon pieces rather than going onto the lead and pad to remove the oxides. So much for a clean, oxide-fee surface; so much for the wetting action; and so much for a good, reliable, problem-free connection.
There are a few exceptions, but the following is a tried and proven technique. Believe it or not, it has been known for decades.
By applying the solder before the iron, you make proper use of the available flux and form a heat or solder bridge. This technique heats up the surface faster and allows you to complete the job properly in the shortest possible time. As you reduce the amount of time needed to do the job, you also reduce the probability of board damage due to excessive heat and time.
Amount of Solder
When soldering a joint, it is not how much solder is added but the technique used to make the joint. Very little solder is needed in most cases. Usually about one-half to one-third of what is usually considered necessary is all the solder needed. The larger the blob of solder, the more difficult it is to determine if proper wetting of the soldered surfaces has taken place.
A second method of soldering is referred to as reflow soldering. This is normally used where plated through holes are not involved, such as the installation of surface mount items or repairing circuit board traces. The technique is relatively simple.
Unreliable Solder Joints
Some examples of poor and therefore unreliable connections to watch out for are:
Characteristics Of A Good Connection
A good solder joint has very few things to look for compared to a poor one. A good solder joint shows the following characteristics: