When it comes to Printed Circuit Board (PCB) design and fabrication, soldering is of great importance. This is because, soldering is the process through which all components are placed on to board and if it is not of high quality, then PCB performance is also compromised. Soldering can be considered in a similar way to welding. This is process of bonding components on to board. Solder is basically an alloy which is placed among different components and modules over a board and are bonded together to achieve a specific purpose. Solder is being melted through a gun or soldering iron and when solder cools down it joins together copper track with components. There are various types of soldering depending on method and quality. This article is dealing with information related to soldering processes.
Soft Soldering: The process of soft soldering is very popular and widely adopted in electrical and electronic industry. This method is creating an electrical connection among the components over circuit boards. The connection is also sometimes joined together through connectors or copper pipe. The solder used for this method is made of combination of tin with some other metals. For better bonding of the components on to the circuit board through soft soldering, an acidic substance known as flux is added for ensuring tight soldering connects. A gas or electric powered soldering iron is used for soft soldering process and the bond created as a result is often weaker than other types of soldering.
Hard Soldering: The hard soldering is known for the creation of a stronger bond than soft soldering as it incorporates use of higher temperature for melting of soldering materials. The soldering material used is usually silver or brass and blowtorch is utilized for melting purposes. The metal which is bonded as result is known as the base metal and is heated till a point in which silver or brass solder is melted for creation of stronger joints. Sometimes when soldering material used is silver, it is referred as silver soldering. Silver is much expensive when compared to brass even it melts at lower temperatures.
Brazing: Brazing is actually involving higher temperatures than that of both soft and hard soldering. The process of brazing is very similar to that of hard soldering process as the metal pieces or components are bonded with each other through heating. In this process by the time when you have heated the base metal, then the soldering material is placed on to it which is then known as the brazing filler metal among different surfaces. The material is instantly melting and the molten filler is joining components or base metals with help of capillary action.
The Reflow Soldering: The process of reflow soldering is one in which the soldering paste is heated. After heating now soldering paste is turned to molten state for connecting pins and pads of different components with each other. The bond created through reflow soldering is permanent.
There are total of 4 steps in this process which are also known as zones. The zones are named as cooling, reflow, soaking, and preheating. All of the four zones are explained below in detail.
- The Preheating Zone: The preheating zone is usually referred to the increase in temperature from normal rang till 150-180 degree Celsius. The jump of temperature from normal to that of the 150 degree Celsius is in mere 5 degrees per second. Therefore, the total time required for going from a normal range to 150 degree Celsius is around 220 seconds or 3 and half minutes. The advantage of this slower warmup is that the solvent and water both are allowed to come out from the paste vapor in no time. This is also allowing the bigger components to be heated up constantly along with smaller components.
- The Soaking Zone: The soaking zone is ranging from the period of preheating from 150 degree Celsius to the time when alloy is molten. This means that the flux is getting active whereas the oxidized substitute is removed over the metallic surface and is ready for making a very good solder joint among pins of components and the printed circuit board pads.
- The Reflow Zone: The reflow zone is also known as the time above liquidus. The time above liquidus is abbreviated as TAL and is considered as the part of the process where highest ever temperature is reaching. A very common peak temperature adopted is in range of 20 to 40 degree Celsius above the liquidus state.
- The Cooling Zone: The temperature is slowly and gradually decreasing in the cooling zone till the time when solder becomes solid on the joints. It is to be noted that maximum tolerable cooling slope is required to be considered for avoiding defects to occur in the process. Therefore, a standard cooling rate of about 4 degree Celsius is recommended to be adopted.
Profiles for Reflow Process
There are two basic profiles which are involved in the reflow process i.e. the slumping type and soaking type. The soaking type is very alike to the trapezoidal in shape whereas the slumping type is similar to the delta shape. In case if the board is very simple and there is use of no intricate components e.g. BGAs or components of bigger size, then it will be best to choose the slumping profile else soaking profile is optimum to choose.
The following image is illustrating the difference of curves among the trapezoidal type and the delta type. It can be seen that trapezoidal type is having more curves while the delta type is growing in linear way till it reaches its maximum point and then drops down. Curve for both delta and trapezoidal is divided in to four parts. Part A is achieving 150 degree Celsius. Part B is till temperature elevates to 180 degree Celsius. Part C is when temperature goes beyond 230 degree Celsius and part D is when temperature drops down again from 230 degree Celsius. It is to be noted that peak temperatures for both types are same i.e. 230 degree Celsius.