DVD-18C Soldering Terminals - IPC

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DVD-18C Transcript Another method of forming a solder heat bridge is to first place the soldering iron tip in contact with the wire and the terminal – and then add a small amount of solder to the contact point. Again, additional solder is applied on the opposite side of the contact point to form the necessary solder fillet. At this point, we'll remove the flux to inspect the joint. The cleaning requirements are the same as any other hand solder connections on a circuit board. Some companies use low residue or noclean fluxes which may not require cleaning. Other fluxes can cause corrosion or contribute to dendritic growth. These residues will need to be cleaned off. It is important to adhere to the manufacturer's specification for solder temperature and cleaning requirements. Your company will supply you with a compatible cleaning solvent and instructions for its use. They will make the decision whether to clean or not based on customer requirements for reliability or cosmetic quality. Once the flux is cleaned, we’re ready to inspect the solder joint. Do you know how to determine the correct amount of solder? What exactly do you look for when examining a solder joint? IPC-A-610, J-STD-001 and IPC-A-620 all provide acceptance criteria for soldering terminals. Check that the solder is feathered out smoothly – showing good wetting action over the wire and the terminal. Wetting is the adhesion of a smooth, unbroken film of solder onto a base metal. The outline of the wire should be visible under a thin film of solder. This connection location is called a fillet. On a proper solder joint, the solder fillet is slightly concave, or curved inward. A concave fillet that tapers to a feathered edge not only looks good, but also reassures us that the solder bonded to the two surfaces we were trying to join. When there is insufficient solder, the joint may not have acceptable mechanical strength. When there is excessive solder, the fillet will be convex and the outline of the wire will be buried in the mound of solder. This is undesirable because the excess solder can hide defects in the connection. It is important to solder so that the joint can be visually inspected. Soldering for inspectability means you're always able to see the outline of the wire in the joint. When excess solder is evident, the outline of the wire will not be visible. Cup Terminals The next type of terminal we’ll discuss is the cup terminal. The cup part is a hollow cylinder. The wire will be inserted into the cup and then soldered. During any soldering process nothing except the technician’s hands should move. This will eliminate the risk of disturbed solder. The terminal needs to be mounted securely – and the non-soldered wire end should be held by a third- hand tool, or equivalent. The wire size must be matched to the terminal. Never modify the wire or the terminal to make the wire fit. Several smaller wires may be placed into a single cup – as long as the wires do not exceed the diameter of the cup. It is not acceptable to twist multiple wires together before inserting them. The twisted wires may trap flux inside and may also change the acceptable diameter of the wires that are inserted. 4
DVD-18C Transcript Now let’s examine the steps to properly solder wires in cup terminals. First, insert the pre-tinned wire into the cup to measure for correct wire length and insulation clearance. For minimum insulation clearance, the insulation may contact the top of the cup, but not interfere with the solder wetting to the inside of the cup terminal. Maximum insulation clearance should be less than two wire insulation diameters, or 1.5 millimeters whichever is larger. As you can see, the insulation clearance in this example is excessive and should be corrected before we solder this connection. Now, we’ll place the properly stripped and pre-tinned wire into the cup – and secure the wire using a third hand tool, or equivalent. The conductor should be fully inserted into the cup – touching the back wall for its full length. Next, we’ll place a large, clean chisel tip against the back of the cup to transfer heat into the terminal. Then we’ll insert the solder wire into the cup opening – against the back wall to form a small fillet between the back wall and the wire. In the final step, we’ll move the solder wire to the center to avoid touching the left or right edges –minimizing the chance of spilling over as the cup fills with solder. The solder wire and soldering iron tip are then removed from the cup at the same time. It’s important to leave everything still until the solder solidifies. If excessive solder is used, it will spill down the sides of the cup. If insufficient solder is used, the solder connection won’t have the required mechanical strength. At this point, we’ll clean off the flux – if required. Here is what the acceptable solder joint will look like. The wire is fully in the cup and there is an insulation clearance from the top of the cup to the insulation of the wire. The solder contains no dips or recesses within it, and hasn't spilled over and down the sides of the cup. Also, check that there is no buildup of solder on the cup where the soldering iron made contact. If there is a small quantity of external solder, it should be a smooth film with no bumps. When the solder volume is correct, the fillets are slightly concave and show good wetting on all the surfaces. Unlike the other terminals, the outline of the conductor is not required to be visible inside the cup. When there is insufficient solder, the connection may not have acceptable mechanical strength. When excessive solder is used, solder buildup on the outside of the cup may negatively affect the form, fit, function or reliability. In situations where the cup terminal is inside a connector, the soldering operation is much more difficult. The area you'll be working in is considerably smaller and there is the danger of possibly burning or damaging adjacent wires. In this case, a resistance soldering tool can be useful. This tool contains a pair of electrodes that can grip the cup and send electrical current through it. The current will heat up the metal and melt the solder. The manual for the resistance soldering tool will specify the correct setting based upon the type of terminal you're soldering. The advantage of this tool is that the electrodes can be positioned before the heat is turned on. That means you won’t accidentally heat the wrong area and possibly cause damage. The cup containing the solder is gripped with firm pressure and a foot switch is depressed to start the heat. Insert the wire – press the foot pedal – and feed in the solder. Then release the foot switch. After solder solidification, remove the electrodes. The foot switch should always be released before the electrodes are removed to prevent arcing. Electrical arcing could damage the plating on the terminal. 5
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DVD-18C Soldering Terminals - IPC

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