PCB Blog - Talking about some small knowledge of PCB board

1. Manual soldering and rework tools

For PCB board, hand soldering and rework are process steps that require outstanding operator skill and good tooling; surface mount hand soldering is sometimes more challenging than through-hole soldering because of the smaller pin pitch and higher pin count. During the rework process, care must be taken not to overheat the printed circuit board; otherwise, plated through holes and pads are susceptible to damage. Review contact welding and heated gas welding, two common types of hand welding. Contact soldering is done when the heated tip or ring is in direct contact with the solder joint. The tip or ring is attached to the soldering tool. The welding tip is used to heat a single welding point, while the welding ring is used to heat multiple welding points simultaneously. There are various design structures for single nozzle welding tools and welding nozzles. There are also various designs for soldering tips in the form of soldering iron rings. Discrete rings with two or four sides are mainly used for component removal. Rings are designed primarily for multi-leg components such as integrated circuits; however, they can also be used to disassemble rectangular and cylindrical components. Soldering iron rings are very useful for removing components that have been glued. After the solder has melted, the ring of the soldering iron can twist the component, breaking the glue connection. Four-sided components, such as plastic pin chip carriers, create a problem because it is difficult for the soldering iron ring to contact all the pins at the same time. If the soldering iron ring doesn't touch all the pins, no heat conduction occurs, which means some solder joints don't melt. Especially on J-pin components, all the pins may not be on the same reference plane, making it impossible for the soldering iron ring to touch all the pins at the same time. This situation can be catastrophic because the pads that are also soldered to the pins will be pulled from the PCB board when the operator removes the component. Welding Tips and rings require frequent preventive maintenance. They need cleaning and sometimes tinning. A frequent replacement may be required, especially when using a small tip. Contact soldering systems can be categorized from low to high price, usually limiting or controlling temperature. The choice depends on the application. For example, surface mount applications typically require less heat than through-hole applications.

A thermostatic system that provides a continuous, constant output that transfers heat continuously. For surface mount applications, these systems should operate in the 335~365°C temperature range. A temperature-limiting system that has a temperature-limiting capability that helps keep the temperature of the system within a range. These systems deliver heat discontinuously, which prevents overheating, but heat recovery can be slow. This may cause the operator to set a higher temperature than desired, speeding up the welding. The operating temperature range for surface mount applications is 285~315°C. Controls the temperature system and provides high output capability. These systems, like temperature limiting systems, deliver heat discontinuously. Response timing and temperature control are superior to limiting temperature systems. The operating temperature range for surface mount applications is 285~315°C. These systems also offer better deflection capability, typically 10°C. Features associated with contact welding systems include: In most cases, contact welding is an easy and cost-effective method of repair welding and component removal and replacement. Elements attached with glue can be easily removed with welding rings. Contact welding equipment is relatively inexpensive and readily available. Problems associated with contact soldering systems include Systems that do not confine the tip or ring and are prone to temperature shocks, raising the temperature of the tip or ring above the desired range. The soldering iron ring must be in direct contact with the solder joints and pins for efficiency. Thermal shocks can damage ceramic components, especially multilayer capacitors.

Heated Gas (Hot Air) Soldering: Hot air soldering is accomplished by directing heated air or an inert gas, such as nitrogen, at the solder joints and pins with a nozzle. Hot air equipment options range from simple hand-held units to heat a single location, to complex automated unit designs to heat multiple locations. The handheld system removes and replaces rectangular, cylindrical, and other small components. The automated system removes and replaces complex components, such as fine-footed and area-aligned components. The hot air system avoids the localized thermal stresses that can occur with contact welding systems, making it useful in applications where uniform heating is critical. The temperature range of hot air is generally 300~400°C. The time required to melt the solder depends on the amount of hot air. Larger elements may require more than 60 seconds of heating before they can be removed or replaced. The nozzle design is important; the nozzle must direct the hot air towards the solder joint and sometimes away from the component body. Nozzles can be complex and expensive. Adequate preventive maintenance is necessary; nozzles must be regularly cleaned and properly stored to prevent damage. Characteristics associated with hot air systems include The inefficiency of hot air as a heat transfer medium and the reduction of thermal shock due to slow heating rates. This is an advantage for some components, such as ceramic capacitors. Use hot air as the heat transfer medium, eliminating the need for direct tip contact. Temperature and heating rates are controllable, repeatable, and predictable. Issues related to hot air systems include: The price range of hot air welding equipment is medium to high. Automatic systems are quite complex and require a high technical level of operation. Flux and solder, flux can be dropped in vials, and sealed or refillable flux pens can be used. Often, operators use too much flux. I would rather use flux pens as they limit the amount of flux used. I'd also rather use flux-cored solder, which contains flux and a solder alloy. When using flux-cored solder and liquid fluxes, ensure that the fluxes are compatible with each other. Surface mount soldering usually requires smaller diameter tin wires, typically in the 0.50~0.75mm range. Through-hole soldering usually requires larger diameter tin wires, ranging from 1.20mm to 1.50mm. Solder paste can also be dispensed with a syringe, although many manual soldering methods heat the paste too quickly, causing spatter and solder balls. Flux glue, not solder paste, is very useful for replacing area-aligned components.

2. Etching related sayings

Side etching: The etching of the sidewall of the wire under the resist pattern is called side etching. The degree of the side etch is represented by the width of the side etch.

Side etching is related to the type of etching solution, its composition, and the etching process and equipment used.

Etching coefficient: The ratio of the thickness of the wire (excluding the thickness of the coating) to the amount of side etching is called the etching coefficient. Etching coefficient = V/X

The amount of side etching is measured by the level of the etching coefficient. The higher the etch factor, the less side etching. In the etching operation of printed boards, it is desirable to have a higher etching coefficient, especially for printed boards with a high density of fine wires.

Coating widening: During pattern electroplating, since the thickness of the electroplated metal layer exceeds the thickness of the electroplated resist layer, the width of the wire increases, which is called coating widening.

The widening of the coating is directly related to the thickness of the electroplated resist and the total thickness of the electroplated layer. In actual production, the widening of the coating should be avoided as much as possible.

Coating edge: The sum of the widening of the metal anti-corrosion coating and the amount of side erosion is called the coating edge. If there is no coating widening, the coating protrusion is equal to the amount of side erosion.

Etching rate Etching solution dissolves the depth of metal in unit time (usually expressed in μm/min) or the time required to dissolve a certain thickness of metal.

Dissolved copper amount: Under a certain allowable etching rate, the amount of copper dissolved in the etching solution. It is often expressed by how many grams of copper are dissolved in each liter of etching solution. For a specific etching solution, its copper-dissolving ability is certain.

3. Analysis of the reasons why the electroplated nickel-gold plate does not have tin

1) Pre-plating treatment: acid degreasing, due to the low temperature near, there may be some boards or surface solder resist film/unclean, you can adjust the concentration/temperature of degreasing agent, and also pay attention to the depth of micro-etching and board color uniformity.

2) Nickel plating problem: The nickel cylinder is polluted by oil or metal pollution, so low-current electrolysis or carbon core filtration is recommended; if the pH value is abnormal, it can be adjusted with dilute sulfuric acid or nickel carbonate; the thickness of nickel plating is not enough, and the porosity is too high , check the current density of nickel plating, use a current card meter to check the consistency of the current of the conductive rod and the current displayed by the meter, the time of nickel electroplating, and if necessary, do metallographic sectioning to observe the thickness of the nickel layer and the surface condition between the layers; the nickel plating tank additive is low / This kind of situation may occur if it is too high, but it may be larger with low additives; in addition, the content of nickel chloride also has some influence on the solderability of the nickel layer, pay attention to adjust to the value, too high stress is large, too low layer High porosity.

3) The gold layer is falsely plated, and the nickel layer is washed for too long or oxidized and passivated, pay attention to strengthen the control of the washing time, and use hot pure water here.

4) Poor post-processing; it should be dried in time after washing, and placed in a well-ventilated place, not in the electroplating workshop.

5) Others should pay attention to all chemical treatments, and the quality requirements of cleaning water are higher than those of general electroplating. It is recommended not to use city water/tap water, recycle water/well water, and lake water, because the water has high hardness/contains other complex organics for PCB board.