The world is growing at a very fast pace in terms of modern technology and the effects are easily on our daily lives. Our life style has changed greatly. This technological advancement has brought many advance equipment in the world that we did not imagine about 10 years ago. The core of these equipment is electronics engineering and the nucleus is the Printed Circuit Boards (PCBs).
The PCB is something that is usually green in color and is a rigid body that holds various electronic components on it. These components are soldered upon the PCBs in the process called "PCB Assembly" or PCBA. The PCB is composed of a substrate that is made of fiber glass, the components, the copper layer that makes the traces, holes in which components are fitted and layers that can be inner layer and outer layers. At RayPCB we can deliver up to 1-36 layers for multilayer PCB prototypes and 1-10 layers for multilayer PCBs for mass production. For single sided PCBs and double sided PCBs, the outer layers present but no inner layer.
The substrate and components are insulated with solder mask and held together with epoxy resin. This solder mask can be green, blue or red in color as commonly found in PCB colors. The solder mask will allow the components to avoid short circuit with tracks or other components.
The copper traces are used to carry electronic signals from one point to the other on the PCB. These signals can be high speed digital signals or discrete analog signals. These traces can be made thick to carry power / electricity to power up components.
In most of the PCBs which are made to supply high voltage or current, there is a separate plane of ground connection. The components on top layer are connected to inner GND plane or inner signals layer by means of "Vias".
The components are assembled on the PCB to allow the PCB to function as it is designed. The most important thing is the PCB functionality. The PCB may not work even if a tiny SMT resistor is not properly placed or even if a small track is cut from the PCB. So it is very important the components are assembled in proper way. The PCB when components are assembled is called PCBA or Assembled PCB.
The functionality of PCBs can be complex or simple depending on the specifications described by the client or user. The PCB sizes are also different depends upon requirements.
The PCB assembly process has automated and manual process which we will discuss.
PCB Layers and Design
As we described above there are multiple signal layers between the outer layers. Now we will discuss on the types of outer layers and functionality.
1- Substrate: It is the rigid board made of FR-4 material onto which the components are "stuffed" or soldered. This provides the rigidity to PCB.
2- Copper Layer: The thin copper foil is applied on top and bottom of the PCB to create top layer and bottom layer copper traces.
3- Solder Mask: It is the layer that is applied on top and bottom of PCB. This is used to create non-conductive region of PCB and it isolates the copper traces from each other to protect short circuit. The solder mask also avoids soldering on unwanted parts and assures that solder goes on that area that is meant for soldering like holes and pads. The holes will attach the THT components on PCB while the PADs are used to hold SMT components.
4- Silkscreen: The white labeling which we see on PCBs for components designator, like R1, C1 or some sort of description on PCB or company logo it is all made of silk screen layer. This silk screen layer provides the vital information about that PCB.
There are 3 types of PCBs classified on basis of substrate material
1- Rigid PCB:
The PCBs are the majority of PCBs we see all around us in various types of devices. These are hard, rigid and solid PCBs with various thickness. The main material is the fiber glass or simple "FR4". FR4 means "Fire Retardant - 4". The self extinguishing property of FR-4 makes it favorable for use in many hard core industrial electronic devices. The two sides of FR-4 is laminated with thin layer of copper foil also known as copper clad laminates. Main applications where FR-4 copper clad laminates are sued in are power amplifier, switch mode power supplies, servo motor drives etc. On the other hand another type of rigid PCB substrate commonly used in home appliances and IT products is known as Paper Phenolic PCB. They are light weight, low density, cheap and easy to punch process. Calculators, keyboards, and mouse are some of its applications.
2- Flexible PCBs:
The flexible PCBs are made of the substrate material like Kapton that can withstand very high temperatures while the thickness is as low as 0.005 inch. The can bend easily and are used in wearable electronics, connectors of LCD displays or laptops, connectors of keyboard and camera etc.
3- Metal Core PCBs:
Alternatively another PCB base material can be used like Aluminum that is very good at dispersing heat effectively. These types of PCBs can be used in applications requiring heat sensitive components like high power LEDs, laser diodes etc.
Types of Mounting Technology:
SMT: SMT stands for "Surface Mount Technology". The SMT components are very small sizes and comes in various packages like 0402, 0603, 1608 packages for resistors and capacitors. Similarly for Integrated circuits ICs we have SOIC, TSSOP, QFP and BGA.
The SMT components assembly is very difficult for human hands and can be time taking process so it is mostly done by automated pick and place robots.
THT: THT stands for "Through hole Technology". The components with leads and wires, like resistors, capacitors, inductors, PDIP ICs, transformers, transistors, IGBTs, MOSFETS are example.
The component has to be inserted on one side of PCB and pulled by leg on other side and cut the leg and solder it. The THT components assembly is usually done by hand soldering and is relatively easy.
Assembly Process Prerequisites:
Before going to the actual PCB fabrication and PCB Assembly process, the manufacturer checks the PCB for any flaws or errors in the PCB that can result in malfunction. This process is called Design for Manufacturing (DFM) process. The Manufacturer must carry out these basic DFM steps to ensure flawless PCB.
1- Component Layout Consideration: Through hole components with polarity must be checked. Like Electrolytic capacitor polarity must be checked, diode anode and cathode polarity check, SMT tantalum capacitors polarity check. ICs notch/head direction must be checked.
The components requiring heat sinks should have enough space for other components so that heat sink do not get touch.
2- Hole and Vias Spacing:
Spacing between holes and spacing between hole and trace should be checked. Solder pad and via hole must not overlap.
3- Copper pads, Thickness, Trace width should be considered.
After performing the DFM check, the manufacturer can easily reduce the cost of manufacturing by cutting down the number of scrapped boards. This will help in quick turn around by avoiding faults at DFM level. At RayPCB we provide DFM and DFT check at circuit assembly and prototyping. At RayPCB we offer PCB OEM services, wave soldering, PCB Card testing and SMT assembly using state of the art OEM equipment.
PCB Assembly (PCBA) Step By Step Process:
Step 1: Applying Solder Paste Using Stencil
First of all we apply the solder paste on the areas of the PCB board where the components will fit. This is done by applying solder paste on the stainless steel stencil. The stencil and the PCB are hold together by a mechanical fixture and then the solder paste is applied by the applicator evenly to all opening in the board. The applicator spreads the solder paste equally. So a right amount of solder paste must be used in applicator. When the applicator is removed the paste will remain in the desired areas of PCB. The grey color solder paste is 96.5% made of tin and contains 3% of silver and 0.5% of copper and it is lead free. This solder paste will melt and creates a strong joint upon application of heat in step 3.
Step 2: Automated Placement of Components:
The second step in PCBA is the automated placement of SMT components on PCB board. This is done by using pick and place robot. At the design level the designer creates a file that will be fed to the automated robot. This file has the preprogrammed X,Y coordinates of each and every components used in PCB and it identifies the location of all components. Using this information the robot will simply place the SMD devices on board accurately. The pick and place robots will pick the components from its vacuum grip and place exactly on top of solder paste.
Before the advent of robotic pick and place machines, the technician will pick the components using tweezers and place it on PCB by carefully looking at the location and avoiding any jittering hands. This resulted in high level of fatigue and eyesight weakness in technicians and resulted in slowed process of PCB assembly of SMT components. Hence the chances of mistake were high.
As the technology matured, automated robots for pick and place components eased the technicians work and resulted in fast and accurate components placement. These robots can work 24/7 without fatigue.
Step 3: Reflow Soldering
The third step after the components are set and solder paste applied is reflow soldering. The reflow soldering is the process where the PCBs along with the components are put on the conveyer belt. This conveyer belt then moves the PCBs and components in a big oven, which creates a temperature of 250o C. This temperature is enough for the solder to melt. The melted solder will then fix the components upon the PCB and create joints. After the PCB is treated with high temperature, it then goes in to coolers. These coolers then solidifies the solder joints in controlled fashion. This will create a permanent joint between SMT component and PCB. In the case of two sided PCBs, the PCB side which has fewer or smaller components will be treated first from step 1 to 3 as mentioned above and then comes the other side.
Step 4: QC and Inspection
After the reflow soldering, there is a chance that due to some erroneous movement in PCB holding tray, the components got misaligned and may result in short circuit or open connection. These flaws are need to be identified and this identification process is called inspection. Inspection can be manual and automated.
a. Manual Inspection:
As the PCB has the small SMT components, so visually checking the board for any misalignment or faults can result in fatigue and eye strains for technicians. So this method is not feasible for advance SMT boards due to inaccurate results. However this method is feasible for boards having THT components and lesser components density.
b. Optical Inspection:
For the large batches of PCB, this method is feasible. This method uses the automated machine that has the high powered and high resolution cameras installed at various angles to view the solder joints from various directions. The light will reflect the solder joints in different angles according to the quality of solder joints. This automated Optical Inspection (AOI) machine is very high speed and take very short time to process large batches of PCBs.
The X-Ray machine allows the technician to look through the PCB to see the inner layer defects. This is not a common inspection method and is only used in complex and advance PCBs. These inspection methods if not properly applied may cause rework or scrap PCB. The inspection need be done regular basis to avoid delays, labor and material cost.
Step 5: THT Component Fixation and Soldering
The through-hole components are commonly found on many PCB boards. These components are also known as Plated through Hole (PTH). These components have leads that will pass through the hole in the PCB. These holes connect to other holes and vias by means of copper traces. When these THT components are inserted and soldered in these holes, then they are electrically connected to other hole in the same PCB as the circuit designed. These PCBs may contain some THT components and many SMD components so the soldering method as discussed above in case of SMT components like reflow soldering will not work on THT components. So the two main types of THT components soldering or assembly are
a. Manual Soldering:
The manual soldering method is the common and typically takes more time than compared to automated setup for SMT. Usually one technician is designated to insert one component at a time and the board is passed on to other technician who inserts another component on the same board. So the board will move all around the assembly line to get the PTH components stuffed upon it. This makes the process lengthy and so many PCB design and manufacturing companies avoid using PTH components in their circuit design. But still the PTH components are the most favorite and common components for most of the circuit designers.
b. Wave Soldering:
The automated version of manual soldering is wave soldering. In this method, once the PTH components are placed on the PCB, the PCB is put on the conveyer belt and is moved to specialized oven. Here a wave of molten solder is splashed on the PCB bottom layer where the components leads are present. This will solder all the pins at once. However this method is only for single sided PCBs and not for double sided because this molten solder while soldering one side of PCB can damage components on other side. After this, the PCB is moved for final inspection.
Step 6: Final Inspection and Functional Test
Now the PCB is ready for testing and inspection. This is the functionality test, where electrical signals and power supply is given to the PCB at the specified pins and output is checked at the specified test points or output connectors. This test requires common lab instruments like oscilloscope, DMM, function generator
This test is to check the functionality and electrical characteristics of PCB and to verify current, voltage, analog and digital signals as described in the requirements of PCB and circuit design
If any of the parameters of the PCB shows unacceptable results, then the PCB is discarded or scrapped as per the company standard procedures. Testing phase is very important because it determines the success or failure of the entire process of PCBA.
Step 7: Final Cleaning, Finishing and Shipment:
Now that the PCB is tested and declared OK from all aspects, it is time now to clean the unwanted residual flux, finger dirt and oils stains. A stainless steel based high pressure washing tool using deionized water is sufficient to clean all types of dirt. The deionized water will not damage the PCB circuit. After washing the PCB is dried by compressed air. Now the final PCB is ready for pack up and shipment.
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