PCB Assembly

PCB Assembly Process: Methods, Processes, and Defects Explained

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 phone, 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:

  • 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.

PCB assembly component placement

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.

Also read: Understanding PCB Assembly: Components, Process, and Best Practices

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.

PCB assembly reflow process

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.

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