To optimize the entire process of PCB making, the process right from fabrication of printed circuit board to assembly must be done under one place. That particular place should have high Printed circuit board assembly capabilities to provide high quality industry under strong regulation of standards.
The circuit board before assembling of electronic components is knows as printed circuit board. Once these components are added it is known as Printed circuit assembly. Different standard and automation are used in this process of assembly. There is difference between assembly of circuit board and manufacturing of printed circuit boards. Manufacturing of PCB involves Printed circuit board design and creation of prototype. Once the board is ready addition of active passive electronic elements by suitable joining methods, then it can be used in any application. According to application type of electronic components and method of joining could vary. Following parts are the basic need of a assembly process
- 1- Printed circuit board
- 2- Active-Passive electronic components
- 3- Soldering materials ( wire, solder bar and balls for BGA)
- 4- Soldering flux
- 5- Soldering machines including soldering station, SMT machines, inspection and testing facility etc.
Once all the above mentioned things are available one can start the assembly process of circuit board.
Printed circuit board Assembly with thru-hole electronic components
Electronic components that have leads with them, and those leads are suppose to get through from the tiny holes available over the printed circuit board, are called thru-hole electronic components. Wave soldering and hand soldering is usually used with the thru-hole electronic components. In Wave soldering process a solder bar is used under high temperature atmosphere, as it melts and passes through the same tiny holes by which electronic components are being attached with the printed circuit board. The solder bar material could vary according to the requirement but usually the traditional lead-tin alloy is used, because of its low melting point due eutectic nature. After soldering the assembly is cleaned and tested. If any defect such as pin hole or blow hole within soldered material is found, the assembly would be sent back for rework.
Surface Mount Technology (SMT) for printed circuit Board Assembly
Surface Mount Technology is used for assembly of SMD electronic components. SMD electronic components don’t have any sort of leads attached with them. They are directly placed over the printed circuit board. The soldering material used in this process is completely different from the thru-hole electronic components assembly process. SMD electronic components and thru-hole electronic components are no different, as far as the electronic function is concern. SMD components are smaller and provide good electrical performance. There are So many processes of joining involved with joining of electronic components, some of them are explained below:
Solder paste printing
This method of joining was take into account because of its lead free composition, the usual alloy use in this method is made up of tin, silver and copper and abbreviated as SnAgCu. A near eutectic mixture is been prepared in order to achieve the low melting point by mixing these three alloys. Solder ball defects are quite common in this method. The thinning substrate in BGA technology and increased package warpage during the SMT process, in some cases the package may tend to bend away from the printed circuit board. The reverse condition is also possible that the package warps because of compression against the printed circuit board and then smaller stencil is used to minimize the bridging defects.
In reflow soldering the solder paste must be heated enough above its melting point to become completely into molten state, in order to melt the balls of BGA components, which forces them to completely collapse in order to form a reliable joint. If lead is present in the solder paste, the paste usually remains wet.
Another variation of this process is lead free reflow soldering. It requires relatively high temperature, because of the high melting range of typical lead free soldering alloy. Such alloys usually have melting point of order of 200oC. The temperature range of typical lead free soldering alloy must be narrow, in order to get a reliable joint and of course to have protection from damaging of components. Since it requires a higher temperature for operation, there are chances of additional oxidation. In this situation an inert reflow (possibly nitrogen ) atmosphere would be beneficial for lead free soldering.
On the other hand this high temperature processing also came with need of a new printed circuit board materials, which can sustain these parameters and avoid the package warpage which has quite high chances, compare to the previous process.
In both the above mentioned processes it is required to determine the solder flowing profile. This process required sophisticated thermocouples, which can exactly trace the profile of flow using the temperature gradient. All the designer need to exactly locate the thermocouple in order to get the correct temperature. Since we are seeking the gradient in this regard therefore it is appropriate to locate the thermocouple at hottest and coldest point in the printed circuit assembly. Usually small passive electronic components are the peak temperature point over the profile board. Similarly the coldest point is the larger BGA and sockets.
The above mentioned technologies of assembly have their own advantages, but the recent advancements in industry have tried to combine these two processes and extracted most of it. The use of thru-hole electronics like BGA and pin grid array for high end upcoming future.