Soldering of Metals: Meaning and Adhesion | Industries | Metallurgy

In this article we will discuss about:- 1. Meaning of Soldering 2. Adhesion 3. Wave Soldering for Manufacture of Printed Circuit Boards (PCB) 4. Simulation of Welding Process.

Meaning of Soldering:

It is a method of joining two or more pieces of metal- sheets by means of a fusible alloy or metal, called solder, applied in the molten state. The melting point of the filler metal is below 420°C. Generally lead and tin alloys are used in various compositions depending upon the use of the joint.

Composition of some of the solders is listed below:

(a) Soft solder — lead 37% and tin 63%.

(b) Medium solder — lead and tin each 50%.

(c) Electrician solder — lead 58% and tin 42%.

(d) Plumber’s solder — lead 70% and tin 30%.

The above mentioned alloys have melting point between 150°C and 400°C. The strength of soldering joint depends upon strength of the alloys and its adhesive qualities. Although any conventional heating process can be employed in soldering, yet much of soldering work is done with a common soldering iron which is specially suitable for small parts and light gauge metals.

Depending on method of heating, soldering processes may be classified as dip, iron, resistance, torch, induction, furnace, infrared and ultrasonic type.

Soldering is divided into two classes, viz. soft and hard soldering. Soft soldering is extensively used in sheet metal work for joining parts that are not exposed to the action of high temperature and excessive loads or forces. Hard soldering is employed when a stronger joint is required than that is obtainable by the soft solder.

Silver alloyed with tin is used as a hard solder, the melting point of which is 300 to 450°C. The process of cleaning the surfaces and applying the flux is same as an in case of welding and brazing.

In soldering process, butt joints are avoided and lap joints are preferred. Clearance is usually 0.075 to 0.25 mm. For greater strength, interlocking seams, crimping, edge reinforcement and reverting techniques are used.

Electric connections, wire-terminals and similar small parts are the examples of this class. Soldering of lead-pipe is known as wiping.

Adhesion of Slodering:

Industrial adhesives are used to joint plastics and other engineering materials. They can also join metals and non-metals.

There is no metallic bond in case of adhesives but bond is created either by chemical action between the adhesive and metal being joined, or by homopolar bond between the adhesive and an oxide film on metal surface, or by secondary bonding caused due to forces of molecular attraction between the adhesive and the metal.

These bonds are of comparatively low strength. Sufficient adhesive must be used to fill the voids and irregularities of the surfaces to be joined and to permit for shrinkage during solidification. Usually synthetic thermosetting resins are used as adhesives which polymerise on solidification.

The surface, before applying adhesive, is degrease and suitably roughened. Usually the adhesive is applied in a liquid solvent and light pressure applied and joint heated to about 150°C for half an hour for curing. Sometimes curing may be achieved without heating by chemical reaction using a suitable activator. Since curing temperature is low; the metal to be joined is not at all affected.

It is possible to join two dissimilar materials and two widely dissimilar sections. A wide range of electrical conductivities can be achieved by choosing a suitable adhesive. Adhesive joints can be made to resist water or gas pressure and adhesive bonds have high damping capacity and thus high fatigue resistance.

Wave Soldering for Manufacture of Printed Circuit Boards (PCB):

The reliability of an electronic equipment mostly depends on the quality of soldered joints more than the quality of components used. Until sixties the industry relied on the skills of the operators. With the advancement of electronics, it had become a necessity to automate this operation so that the soldering operation is mechanised with set parameters to give a zero defect in soldering.

With the introduction of wave soldering, dependence on the individual’s skill for a good solder joint has been eliminated with an assured quality of soldering, thereby enhancing the reliability of the sophisticated electronic equipment.

Wave soldering is a highly automated method of soldering in which a Printed Circuit Board with components passes over a wave of molten solder. All components are bonded in one quick operation.

This is achieved by pumping molten solder vertically upwards through a narrow slot to form a steady waterfall or wave. The dynamic movement of the solder across the work surface decreases soldering time, improves wetting, and minimises heat distortion resulting in a clean, oxide-free, bright surface of solder.

After the assembly of printed circuit boards with the required components, they are loaded on a linear conveyor that carries it through the successive stages in the soldering operation.

The board can be mounted into an individual pallet or held by a continuous series of finger grips on an endless belt. It is also possible to integrate the soldering into a complete conveyorised production system which might include component lead cutting and flux removal units.

The first stage is the fluxing station which may be of several designs; a common type is a foam fluxer. In the foam fluxer a wave of flux is foamed by continuously aerating it by using a porous element within the bath. Non-foamed waves of flux may also be employed. Application of flux is to assist the solder to wet the surfaces.

In the second stage, the board passes over a set of radiant heaters, sometimes combined with hot air blowers. This stage is called preheating stage which serves to evaporate most of the flux solvent and also minimises the thermal shock to the components on printed circuit board during soldering.

Finally the board is ready for soldering and passes through the crest of a standing solder wave so that contact is made with the entire lower surface (but none of the upper surface) of the PCB. The path of travel is often inclined upwards at a small angle upto 8° to faciliate drainage of excess solder from the board and prevents bridge formation.

For boards having plated through holes, the wave promotes capillary rise of the solder into the holes, thus connecting with the inserted component terminations.

One type of wave soldering machine employs a universal wave system called the ‘Lambda wave’ which is claimed to carry out high performance soldering of a wide range of board layouts at improved production speeds.

The Lambda wave is basically a supported inclined wave with an adjustable back plate used with an optimum conveyor inclination of 6°. The back plate is adjusted so that there is no relative velocity between the solder and board as the board leaves the wave, which is stated to prevent the formation of bridges.

In another machine having wave width of 300 mm which can accommodate a maximum of 300 mm width PCB’s, the conveyor is a fixed incline (6°) pallat type with idle areas at each end, the speed is variable from 0 to 3 m/min. The flux is applied by a moulded PVC foam fluxer with integrated air pump and the preheater is a solid steel panel.

This machine has been fitted with jet air blower before the preheat panel, a diaphragm air pump for the fluxing station which has an exit brush to remove excess flux, a programmable time clock, which starts the machine heater, at a preselected time so that the molten bath of solder is ready at the start of the shift.

The solder pot is made of stainless steel and all moving parts operate outside the solder with the exception of the impeller and shafts. The heating elements are or the cartridge type enclosed in stainless steel tubes extending to the entire length of the pot and operating temperature of the molten bath is variable upto 345°C. The wave height can be adjusted with a finger wheel to ensure proper soldering of the board.

The wave soldering process gives the following advantages:

1. Heat is applied in a controlled manner with a fairly uniform gentle preheating followed by very efficient rapid heating by the solder wave to a specified and precisely controlled temperature.

2. Uniform cooling of whole assembly after soldering.

3. Thermal stress and damage to the board and components is kept to a minimum. Hence the service life of circuitry is enhanced.

4. It is usually immediately apparent if a joint is defective.

5. True metallurgical bond essential for long term reliability can be achieved.

Simulation of Welding Process:

Non-linear process like welding can be simulated due to development of computers and computational algorithms. Finite Element Method and other techniques based of physical principles are used for this purpose. Simulation can be used as a means for improving the welding process because the influence of process parameters on the properties of the material can be studied, and microstructure, material properties and residual strains and stresses can be predicted.

Simulation also enables to study the influence of welding process on the in-service behaviour of the welded product. It’s aim is to predict the properties of the fabricated component due to thermo-mechanical process, i.e. the mechanical and thermal properties, and residual stresses and strains which may affect subsequent use of the material.

 

Fig. 9.62 shows the integration of product design and simulation of material processing. Simulation of material processing is a thermo-mechanical simulation, and microstructure evolution. Fig. 9.63 shows the process of typical thermo-mechanical analyses.

Welding inspite of several advantages and most common process of joining steels has inherent problems due to the thermal cycle, viz. welding deformations, residual stresses and deformations, defects and cracking. Simulation of welding enables prediction regarding fitness for service of a welded structure.

In simulation of a welding process, first the residual stresses due to cold forming of components is calculated and then the effect of welding is simulated. The residual effective stresses after cold forming and subsequent welding are calculated.

Simulation model calculates the accumulated plastic strains and residual stresses which form the basis for calculation of creep, damage and voids that the welded structure will strand for long time, thus providing the quantitative prediction about the integrity of the component.

In welding, the modeling of weld heat input is important as one has to be sure of quantum of heat getting into the material and its distribution in time and space. Heat input can be determined by means of combination of calculations and experimental evidence.

Transient temperatures near the joint, size of the molten zone and the final acquired microstructure are measured, and the net heat input is estimated by running simulations with varying net heat input till a good match with experiments is obtained.

Some preliminary information like gross heat input, focus of beam or size of weld pool, etc., serve as clues for how the heat input should be modelled and thus appropriate boundary conditions are known.

Simple elasto-plastic model with Von-Mises yield criteria and the associated flow rules are used for estimating properties of materials at high temperatures. Hardening rule could be either simple linear and isotropic, or kinematic hardening, or non-linear hardening. The calculated stresses from model need to be tallied with the measured stresses from a pilot experiment to certify validity of model.