Most electronic equipment have at least one PCB or printed circuit board inside. The PCB offers a convenient method of physically mounting and electrically interconnecting various electronic components. According to PCB Trace Technologies Inc, the PCB inside an electronic equipment many be simple or advanced depending on its functionality being simple or elaborate.
What are Advanced PCBs?
Simple electronic equipment with limited functionality typically have a simple PCB assembly inside. This is generally a single-sided rigid board with only a few surface-mount or through-hole components.
More complex electronic equipment offering elaborate functionality need a more advanced PCB assembly. These advanced boards may be:
- Multi-Layer Boards, made of rigid FR4, glass epoxy
- Multi-Layer Boards, made of flexible material like Polyimide
- Multi-Layer, High-Density, HDI Boards, made of Special Material
- Multi-Layer, High-Frequency Boards, made of PTFE or Ceramic-filled PTFE
In addition to the material and structure of advanced boards being different from that of simple ones, the advanced boards may also have complex electronic components mounted on them. Although the process of mounting and soldering electronic components on a PCB are essentially the same, complex electronic components require more sophisticated technology for handling and placement.
Why Advanced PCBs?
A major advantage of using advanced PCBs is the substantial amount of time they save. Interconnecting electrical components using traditional methods can take up a huge amount of time, especially if the components are tiny, surface mount type. Advanced PCBs offer a very convenient way of assembling surface mount components in a short period.
The conventional way would be to use wires to interconnect the components. This can easily lead to wrong, loose, and or short connections. Advanced PCBs have copper tracks on each layer that take a predefined path fixed at the time of design. Advanced PCBs use these copper tracks to make interconnections between components. As these tracks are bonded to the substrate material of the board, they cannot move, and therefore, there are no chances for loose connections or short circuits to occur. Moreover, problems in a board assembly are easy to locate and repair.
Furthermore, interconnections between layers in multi-layer boards typically use vias. These are short metallic cylinders passing through the layer and interconnecting tracks on either side.
The basic construction of advanced PCBs also makes them highly reliable. Almost all electronic equipment in various industries now use advanced PCBs because of their higher reliability, capability to tolerate extreme weather conditions, and their ability to withstand shocks and vibrations without suffering damage.
Why Special Assembly Techniques for Advanced PCBs?
Advanced PCBs typically must provide enhanced functionality. They do this by using complex electronic components like BGAs and QFNs. These are very large scale integration components with substantial functionalities. They also have numerous connections, most of which are under the body of the component. Hence, it is not possible to solder them using conventional methods of soldering. Successful assembly of these complex components requires proper steps taken right from the design stage.
Proper Breakout for BGAs
BGAs typically have multiple close-pitch connections under the body of the component, requiring careful track layout to successfully interconnect all the connections. The technique of routing connections for a high-density array of pins under a BGA is called breaking out the BGA.
The designer must plan ahead how they want to breakout or route a high-density BGA. For this, they must consider its overall size and the pin out of the part. Typically, a high-density BGA may not allow simple geometrical calculations for routing traces between the pins, as the gaps and traces are just too small to pass in between them.
Designers must, therefore, resort to using blind and buried vias for properly routing complex high-density BGA components. One of the methods designers use is:
- Routing the pins on the outside row of the BGA on layer one
- Routing the pins on the next row of the BGA on layer two
- And so on, using blind and buried vias along with a multi-lamination fabrication
Different high-density components will require various breakout strategies.
Soldering High-Density Components
Most high-density components have very small pins closely spaced. Consequently, the pads for these pins are also closely packed and very small. Soldering the component requires depositing controlled amounts of solder paste on these pads.
The most efficient way of depositing solder paste in the correct amount requires using a stencil. Stencils are thin metallic sheets with highly accurate holes in them corresponding to the position of the pads on the PCB.
The operator typically mounts the stencil on a bare PCB and aligns it to match its holes with the pads on the board. They apply a band of solder paste on the stencil and use a squeegee to drag it across. The solder paste drops through the holes in the stencil and is deposited on the board. The holes in the stencil are highly controlled to allow just the correct amount of solder paste on the board.
Positioning High-Density Components
Once the solder paste has been deposited properly on the board, the operator must mount the high-density component accurately in its position. They typically use a pick-and-place machine for the job.
The pick-and-place machine is computer controlled and has a highly accurate camera for vision. The camera first tracks the fiducial marks on the board near the component to be mounted, and the computer calculates the precise position of the component based on these readings. The machine them picks up the component and places it with great precision on the board.
Soldering High-Density Components
Once the pick-and-place machine has mounted the BGA in its proper position, the entire assembly must be brought to a certain temperature to get the solder underneath the BGA component to melt. As the BGA heats up, the solder melts in the soldering process. Surface tension holds the component properly aligned with the circuit board as the solder cools and solidifies.
The soldering temperature and the alloy composition of the solder must be selected carefully to ensure the solder does not fully melt but remains in a a semi-liquid state, allowing each ball to stay isolated from its neighbors. The main issue is that heat application must be uniform and adequate to melt all the balls in the grid satisfactorily.
Inspecting BGA Solder Joints
It is impossible to use regular optical techniques to inspect BGA solder joints since they are underneath the component and hence not visible. Testing the electrical continuity of the solder joints is also not possible for providing a complete picture of their performance. An insufficiently formed joint can fail over time. Only by using x-rays, is it possible to test each solder joint.
Using X-rays, BGA inspection is made simple and fast. X-rays allow inspection and testing of the solder joints. The technology, known as Automated X-Ray Inspection is a popular technology.
Conclusion
Using advanced PCB assembly techniques for assembling complex printed circuit boards is a common process in many industries. At PCB Trace Technologies Inc, we design very complex HDI circuit boards and assemble them using advanced techniques.
