What life could have been if we were stuck in the old ways of sending out letters as way of communicating to a long-distance friend or riding a horse for hours to get to a nearby town or waiting for the newspapers at your doorstep to know the latest events? Humankind and technology have progressed immensely that life is now much easier. Name an electronic device that helps you go about your day more efficiently. Everyone at least has a mobile , personal computer, TV or even a car integrated with electronic system in it. Inside these devices are functioning circuit boards otherwise known as Printed Circuit Boards or PCBs. These boards resemble a miniature city with streets and buildings, which are called &#;traces&#; and surface mounted &#;components&#;. Before delving into how a PCB is made, it is important to comprehend first what composes a PCB. More specifically, we will respond to below fundamental questions in understanding PCB assembly process:
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What is a PCB Assembly?
What are the different assembly methods in PCB Assembly Process?
What are the key processes in PCB assembly?
What are the different defects in PCB Assembly Process?
How to ensure quality in PCB Assembly Process?
What is a PCB Assembly?
A PCB is a physical structure that is made of a laminate material(usually FR-4) used to structurally support and connect electronic components using copper traces and conductive pathways. The PCB is made by laminating copper sheets into a dielectric substrate and mounting chips and other components. A common substrate material is an epoxy glass composite called FR4 which is short for &#;Flame Retardant&#; with designated number &#;4&#; which means woven glass reinforced resin.
The types of components can be broadly classified into passive and active components. A passive component does not have contribution to amplification of a circuit. It does not need an input except a signal to function, such as resistors, capacitors,inductors, transformers, etc. On the other hand, active components can amplify a signal and controlling an input voltage or currents. Active components include integrated circuits (ICs), MOSFETs, diodes, transistors, etc.
PCB assembly is the process of soldering electronic components on the PCB bare board. Then, fully functional circuit boards are ready to be used for electronic devices.
What are the different assembly methods in PCB Assembly Process?
What is PCB assembly process? It is the PCB assembly process steps from beginning to the end. usually starts from solder paste printing to component placements and reflow soldering.
There are two assembly methods by which components are added into the printed circuit boards: Through-hole Process and Surface Mount Process. In Through-hole Process, component leads are inserted into the vias protruding until the other side of the PCB. It is a preferred method in assembling connectors if mechanical strength is a primary requirement. Surface Mount Process is a more dominant method of soldering components into the PCB copper pads especially for higher density and lower cost options.
What are the key processes in PCB assembly?
Below are the general processes in the PCB assembly. Take note, however, that the process steps can be modified depending on process design systems of a manufacturer.
Solder Printing
Solder printing is a method of stencil printing the solder material into the PCB by using a metal squeegee to make a consistent solder paste volume. Several lead-free alloys are being used as solder paste such as SAC which is a short term for SnAgCu (Tin, Silver, Copper). This composition has melting point of around 217 to 220 degrees Celsius. The size of the solder material deposited on the PCB is dictated by the size of the stencil aperture. Defects that can be encountered during solder printing are solder bridging and excessive solder.
Component Placement
Pick-and-Place or chip mounter is used to mount the components into the PCB and fixed in place by the wet solder material. Placement accuracy is getting more critical with the increasing density and decreased size of components. An SMT machine is composed of component feeders which can be in tape or as tray form. The placement arm, which may consist of several nozzles, picks and places the components into the PCB. It is critical to place the components correctly and precisely in PCB assembly process to ensure good quality.
Reflow Process
After component placement, the PCBs undergo reflow process to melt and cure the solder material. The solder material must be heated above the specified melting point to allow the alloy to collapse and bond in the interface. The quality of the solder joints is affected by the soak time and temperature which can be monitored using a reflow profile. Extreme temperatures can cause damage to the boards hence, engineering evaluations should be performed to determine the optimum temperature and time based on the solder material properties and board thermal sensitivity.
Reflow soldering in PCB assembly process takes place in an inert environment, usually with a nitrogen gas, to minimize occurrence of pad oxidation. A reflow equipment with several heating zones has better temperature control and is beneficial for complex and large boards.
What are the different defects in PCB Assembly Process?
PCB assembly process is also not immune to defects may greatly affect board quality. Listed below are some of the common defects that can be encountered during assembly process. Knowing these potential defects help engineers to improve and control the PCB quality.
Solder balling is characterized by tiny solder balls found in the periphery of the flux residue after reflow process. This can be caused by oxidized solder paste and incorrect thermal profile.
Solder bridging is a common PCB defect encountered when solder from a component comes in contact with a solder from another component. This phenomenon can result to short circuit. Possible cause is too much squeegee pressure or incorrect alignment of components.
Insufficient solder results to an incomplete bond between the component lead and the pads. Solder paste viscosity and voiding can lead to poor wetting of solder material into the pads.
Tombstoning is a term used to describe a small component standing at one end after reflow. This can happen if there is too much, or too viscous solder paste and there is uneven heat distribution when one pad melts before the other.
Missing components are encountered when a component is missing in a location where it is intended.
How to ensure quality in PCB Assembly Process?
Some failure preventive controls are implemented in PCB manufacturing line to ensure quality of PCBs. These quality checks include solder paste inspection, x-ray inspection, automatic optical inspection (AOI) and testing. Let&#;s go through each type of control below:
Solder Paste Inspection
Solder Paste Inspection is being done after solder printing to check for the completeness of the paste pattern on the board. An automated solder paste inspection machine checks for the solder paste volume and alignment. Accurate solder paste printing is critical in PCB assembly process.
X-ray Inspection
X-ray inspection is a non-destructive type of inspection that checks the solder joints and various interfaces of a PCB. Screening of internal defects is made possible by allowing x-ray photons to pass through materials with different material properties like atomic weight and density. An x-ray image is produced based on the different behavior of materials when hit by x-ray photons. X-ray helps in detecting solder quality issues like voids that are usually not detectable by human eye. X-ray only needed in PCB assembly process with BGA assembly.
AOI Inspection
Automatic Optical Inspection is an inspection step after reflow to check for any visual anomaly on the panel. AOI machine performance is assessed based on its capability to screen out the defects accurately. Just like other inspection processes, false alarms and missed defects are the attributes that must be measured. PCB manufacturers should put emphasis on PCB quality by complying to quality standards that are internationally recognized in the electronics manufacturing industry. For instance, PCB inspection process should abide IPCA-610 which discusses the acceptability requirements for PCB. AOI inspection is very common in PCB board assembly process.
PCB Assembly Testing
Since inspection processes will not completely detect issues of the PCB due to line-of-sight limitations, testing still needs to be performed to ensure PCB quality. Flying Probe and In-Circuit Testing are the two test methods to measure PCB performance. In Flying Probe Testing, robotic test probes are used to access the test points and measure resistance capacitance and other electrical parameters. In-Circuit testers have fixtures, also known as &#;Bed-of Nails&#;, to contact the test points which make it a preferred option for high-volume manufacturing. Before shipment, manufactured boards undergo functional testing to validate its operational performance by validating the correct voltages and communication on the device. Testing is very important in PCB assembly process.
Through the years, printed circuit board design has developed into higher density, thinner conductor tracks, laminates and via hole. Quality controls and improved reliability must be the key metrics in PCB assembly processes. A single misprocess in PCB assembly can result to field failure, accidents, and malfunctions.
Jump To: PCBA Terminology | Through-Hole Assembly | Surface Mount Technology | Mixed Technology | PCB Manufacturing at Millennium Circuits Limited
The printed circuit board (PCB) assembly process consists of various steps and guidelines that must be performed in the proper sequence for the finished product to function as designed. To ensure that this happens, PCB manufacturers use screen templates and controlled heating and cooling mechanisms to regulate how components are applied and fastened into place.
When you assemble a printed circuit board, you must choose the right technology for the type of components at hand. All of the parts and pieces must be correctly aligned in their designated spot, as specified in the PCB design. Any deviation, ever so slight, can have huge ramifications on the functions of the finished board.
PCBA Terminology
To understand the PCB assembly process, you need to know the meaning of several terms:
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- Substrate: The foundational material of the printed circuit board, the substrate makes each board firm and rigid.
- Copper: Each working side of a PCB contains a thin layer of copper, for conductive purposes. On single-layer boards, copper is placed on the active side. On double-sided PCBs, copper appears on both sides.
- Solder mask: This is the layer at the surface (typically green) of every printed circuit board. Solder mask provides insulation between copper and other materials, preventing shorts that could otherwise occur if different conductive materials come into contact. Solder mask provides structure for the layout of the PCB by keeping everything in its place. Each board contains holes that go through the solder mask. Solder is placed within each hole, which offers a foundation for each component that gets added to the board.
- Silkscreen: The final touch on every printed circuit board is the silkscreen, a transparent layer that displays numbers and letters next to the different parts of a given board. This allows manufacturers to identify the specific components of each board.
- Manual soldering: This is a process where a technician manually inserts a single component onto its designated spot on a series of printed circuit boards. Once completed, each board is sent to the next technician, who adds another part and passes the board onward.
- Wave soldering: Wave soldering involves correctly soldering where the board is placed on a conveyor and run through a heating chamber. Here, a wave of solder is applied to the bottom, fastening all the underpins of the board into place in a single process.
It is also important to understand the differences between through-hole, surface mount and mixed technology assemblies.
Through-Hole PCB Assembly
Through-hole technology is the ideal choice for printed circuit boards that feature leads or wires that are threaded through holes in the board and then secured on the other side with solder. PCBs with large components are especially suited to through-hole technology, especially capacitors.
The basic steps of through-hole PCB technology can be summarized as follows:
- The technician manually assembles parts onto specific areas on a printed circuit board, according to the design specifications of the PCB. Each component must be set in an exact position, as specified, for the PCB to function properly.
- The board is examined to ensure that all the parts have been properly assembled and that each component is set in its exact place. If any of the PCB parts are misplaced, now is the time to correct those imperfections.
- The components are now soldered into place on the circuit board. This is typically done with wave soldering, where the board moves above a wave of hot solder liquid which solidifies the PCB assembly. This can also be done by hand or by using selective solder. Selective solder is similar to wave soldering, however, the operator can solder areas selectively and this helps when you don&#;t want solder on certain areas.
Through-hole boards typically contain components with wire leads, either axial or radial. Compared to surface mount technology, through-hole boards generally feature stronger bonds. However, more work is required to produce a through-hole assembly because of the additional drilling involved.
If a through-hole board consists of multiple layers, signal traces have limited routing on the internal layers because the holes cut through between the top and bottom surfaces. Therefore, through-hole technology is often restricted to some of the bulkier PCB components like electrolytic capacitors and semiconductors. Boards that require additional firmness and support, such as electromechanical relays and plug connectors, are also made with through-hole technology.
During the prototyping stage, technicians often favor the bigger through-hole to the surface mount because the former works more readily with breadboard sockets. However, if a board is intended for high-speed or high-frequency purposes, the design might require surface mount technology to reduce stray reluctance. Otherwise, the function of the circuits would be degraded due to inductance or capacitance in the leads.
During the application of solder paste, a solder stencil is placed on top of the printed circuit board to ensure that the solder stays within the limits specified in the design. The stencil is a thin replica of the original design with cutouts for the areas where the components are placed.
Once the components have been set into place and the board has undergone its first inspection, the solder paste is heated over a hot liquid until the tiny metal balls within the paste solidify with the bonding chemical, flux. This bonds the components to the board permanently. After the heating and bonding have finished, the board is cooled in a controlled setting. This brings the board back to a normal state and prevents shock.
Now complete, the printed circuit board must have its components examined for any possible misalignments. If the components are relatively large, this can often be done with a visual inspection. These days, however, optical and x-ray inspectors can examine PCBs with much higher accuracy. If design flaws are detected, the problem must be fixed before more boards go through the process.
Surface Mount Technology
Surface mount technology is the practical option for printed circuit boards that contain tiny and sensitive components that could otherwise be difficult to set into place without damage. Examples of the kind of components that undergo this process include diodes and resistors.
The basic steps of surface-mount technology can be summarized as follows:
- The first step involves a printer designed for the application of solder paste. A solder screen template is used to ensure that the tiny components are put into their proper places. Before the components are actually placed, the stencil is inspected to make sure that the template is aligned correctly.
- The board is now taken to a machine where the components are put into place according to the design of the solder screen template. Reels in the machine ensure that the parts are aligned and attached to their corresponding pads.
- With the components properly set on the board, a heating process is employed to solidify everything into place. At this stage, the PCB is sent through a heating chamber that liquefies the solder, fastening the parts in the process.
Before the components are placed onto a PCB, select areas of the board itself must first be coated with solder paste, which functions as the adhesive for the unique parts of the board. The areas that require solder are primarily the spots where pads will exist for the corresponding components.
Solder paste is made of tiny grains and flux. The process of placing the solder paste onto a printed circuit board is much like that of a printing application. A solder screen is set over the board to an exact, specified alignment. A roller is then run over the screen to press the solder paste onto the board.
The solder screen is printed in advance according to the design of the PCB. Therefore, the aperture in the screen are aligned to the component pads on the board. This ensures that the solder mask distributes exclusively on those areas. The quantity of solder distributed during this process must be regulated to ensure that each joint is neither over-covered or under-covered.
Once the paste has been applied, the board is sent through a pick-and-place machine where specified components are applied to the soldered areas. As long as external forces do not jolt the board during this process, the tension of the solder should be sufficient to hold the components in place.
On some pick-and-place machines, tiny amounts of adhesive are added to the designated areas to secure the components. This is mainly intended for printed circuit boards that undergo the process of wave soldering. The drawback of this added adhesive is that it can make corrections difficult on boards where the components have been misaligned to the specs of the original design.
With the components secured to their proper spots, the PCB is sent through the soldering machine. On older boards, manufacturers often employed wave soldering, though this has largely been taken out of practice. In productions where the method is employed, solder paste is not used because solder is applied by the wave soldering machine. Today, however, reflow soldering is the more commonly used method among PCB manufacturers.
Once the printed circuit board has come out of the soldering machine, technicians inspect the board&#;s composition for any imperfections. If the board contains more than 100 different components, the board is sent through an automatic optical inspector (AOI), which can detect even the slightest flaws, such as misaligned joints, off-place components and incorrect placements.
Each printed circuit board must undergo a series of tests before it leaves the assembly. The board endures several tests to verify that it can function as intended in the original design.
While the operation is in progress, the equipment must be inspected to ensure that everything works as intended. The first places to check are the outputs, as these will help verify the success of a given production. The output of the soldering machine should be inspected at the start of each production, along with the initial boards that come through the output. This way, any imperfections can be caught early, before a misprinting error becomes large and costly.
Mixed Technology
Due to the rapid evolution of computerized technology, an increasing number of printed circuit boards are being made with smaller and smaller parts. This means many PCBs are made today with a combination of methods, commonly referred to as mixed technology. An assembly that involves mixed technology will encompass one of the following approaches:
- A single-sided mixed assembly, where a printed circuit board undergoes surface mount technology as well as through-hole technology, both on the same side of the board.
- A split assembly, where one side of the printed circuit board is assembled with surface mount technology and the other side of the board is assembled with through-hole technology. PCBs like these have normal-size components on one side and tinier components on the other side.
- A double-sided mixed assembly, where both sides of the printed circuit board are assembled with a combination of through-hole technology and surface mount-technology. A board of this type features regular components as well as tiny and delicate ones on both sides.
Before a PCB design goes into production, the boards intended for use must be inspected for quality purposes. On the components, foot oxidation or oily residue could serve as red flags. If stored in a cool environment, the solder paste can only be applied once it has been thawed and stirred. The PCB must be dry before any paste is applied to the surface.
On printed circuit boards with mixed technology, a more complex set of processes are required within the pick-and-place machine. Here, a mixture of different component sizes will need to be accounted for, either on one or both sides of the board.
If the printed circuit board consists of a double-sided assembly, the soldering process will need to be applied to both sides. Basically, everything that happens on one side also happens on the other. The only difference is the specific components and their placements, as one side might contain smaller components than the other. Wave soldering is not possible for two-sided PCBs because resubmitting the board for the second side would ruin the delicate parts on the first side.
Any printed circuit board comprised of mixed technology should be submitted to an automatic optical inspector. This way, technicians can be ensured a foolproof inspection that will detect even the tiniest imperfections, such as minor misplacements of microscopic parts.
To account for the complexities of a two-sided mixed technology board, a more thorough set of tests are required for the initial PCBs produced in such lines. Since there are more components to account for, there are more potential problems if even one part is out of alignment.
PCB Manufacturing at Millennium Circuits Limited
In today&#;s high-tech world, printed circuit boards are becoming increasingly complex as technicians find ways to load more data and energy onto tinier and tinier chips. As computer devices and electronics become smaller, so too will the PCBs that power these devices and connect them to the wireless grid. For PCB manufacturers, this means that the production of printed circuit boards will require advanced levels of engineering.
To make PCBs efficient by current standards, you must have the proper specified bare PCB to carry out each step more efficiently in the assembly process. At Millennium Circuits, we use the latest technology at our facilities to fulfill each bare PCB order according to customer specifications. Contact Millennium Circuits for more information about PCB manufacturing.
Do you have questions about our services or your project? Contact us today to speak with MCL&#;s award-winning support team!
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