Introduction To PCB Assembly

Electronics is the integral part of our day to day life . Everything from a small LED Bulb, Smart Phone to automatic cars , robots,satellites etc. are impossible without electronics and  the PCB which stands for printed circuit board is the heart of all these electronics.

A printed circuit board (PCB) mechanically supports and electrically connects electrical or electronic components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate.

We know that , An advanced circuit board unable to provide its functionality until components are mounted on it. A PCB with components mounted on is called an assembled PCB and the manufacturing process is called PCB assembly.The  copper lines on bare PCB which we called traces are electrically connected and provide signal to perform the function for which the PCB is designed.

Basic of PCB Design

Printed circuit board is the basic unit of the PCB Assembly which consists  of several layers. Different PCBs have different numbers of layers depending upon the requirement.

Substrate: The base material which we called as substrate, is usually fiberglass. this fiberglass is designated with "FR4". This solid core gives the PCB its rigidity and thickness. There are also flexible PCBs built on flexible high-temperature plastic (Kapton or the equivalent).

Copper Layer: The next layer is a thin copper foil, which is laminated to the board with heat and adhesive. For a double sided PCBs, copper is applied to both sides of the substrate. 

Soldermask Layer: The layer on top of the copper foil is called the soldermask layer. This layer gives the PCB its green color. It is overlaid onto the copper layer to insulate the copper traces from accidental contact with other metal, solder, or conductive bits. This layer helps the user to solder to the correct places.

SilkScreen Layer: The silkscreen layer is applied on top of the soldermask layer. The silkscreen adds letters, numbers, and symbols to the PCB that allow for easier assembly and indicators for humans to better understand the board.

Solderpaste Layer: Solder paste layers are the layers having pads for SMD components , solder paste is applied on this layers to solder the components.

PCB Assembly Technology:

There are two types of mounting technologies are prevailing in the modern PCBA industry one is Surface Mount Technology and other is thru-Hole Technology . PCBs are assembled with the help of these two technology.

Thru-Hole Technology:

This technology works well on components with leads or wires that have to be mounted on board by plugging them through holes on board. The extra lead part has to be soldered on the other side of the board. This technology is applied on PCB assemblies containing large components such as capacitors, coils , Transformers, Relays  etc to be assembled.

Before assembly process DFM check is performed and required technical documents like BOM file ,drill file, gerber files containing information about each layer and x-y data are provided to the production team.

In thru-hole assembly process components are mounted manually and hand soldering Is used to solder the components.

In large production requirement semi automatic manual insertion line is setup and wave soldering is used for soldering purpose .

Surface Mount Technology:

Surface mount technology is the method in which the electronics components are mounted directly onto the surface of printed surface boards (PCBs). These electronics components are called surface mount devices(SMD).

Before proceeding to production stencil are designed by the help of gerber data for taking paste on PCBs , stencil basically contain the foot print of PCBs , laser technology is used to cut aperture for smd pads .

The diagram below shows all the process which are performed during production of PCBs that contains SMD mounting.

Loader: This machine is used to load the PCB to the solder paste printer machine , PCBs are kept in magazine stacks and automatically PCBs move from magazine to printer one by one with the help of loader.

Printer: Solder paste printing is a key part in the SMT process. Since the solder paste is a mixture of powder and flux, the solder paste is fluid during the printing process. Solder paste printing is process of printing solder paste onto a substrate using a printer and a stencil.

SPI: Solder paste inspection is one of the important process  in the SMT because approx 60-70 % percent of defects are due to paste printing .SPI machine is used to inspect solder height, volume, area of solder on each pad on PCBs after paste printing.

Pick & Place : SMD components placement systems , commonly called Pick and Place machines are robotic machines which are used to place surface mount devices onto a printed circuit board.The  P&P machine has a head on an arm which can reach all reels, sticks, tray etc and picks up and place components with the help of vacuum and places onto a board.

Reflow: Reflow soldering is a process in which a solder paste is used to temporarily attach one or thousands of tiny SMD components to their contact pads, The aim of the process is to form acceptable solder joints by passing the board after P&P machine first pre-heating and then melting the solder without causing damage by overheating.after which the entire assembly is subjected to controlled heat. Separate reflow profile is created for each new product.

AOI: AOI stands for Automatic Optical Inspection ,This is one of the critical process in the SMT after reflow the final product is process through AOI Inspection to Inspect the PCB and find the defect like Dry solder, Less solder, Shifting, Tomb stoning, Billboard wrong value found on PCB etc.

After inspection the PCBs are cleaned in the cleaning solution, after cleaning the Assembled PCB goes through QC stage and after passing through QC stage Assembled PCB is hand over to Testing department for Functional Testing After passing the Function Testing Assembled board is process through final assembly for system level integration.