What Is PCB Trace? Simple Ways and Stages of Work

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PCB tracing is one of the design steps, which is the process of determining the location and implementation of the conductive pattern of the board.

PCB tracing is the development of the topology of electrical connections between the footprints of electronic components installed on a printed circuit board.

The tracing of printed circuit boards is carried out after the product circuitry has been developed, the equipment has been selected, and the design for the installation of the PCB has been selected.

When wiring boards, conductors are laid that connect certain components to each other. This process takes place after the final placement of the components on the PCB surface, but it should be understood that PCB layout may necessitate design optimization by repositioning the components.

There are currently three ways to implement PCB tracing:

Manual tracing for PCB: It requires the developer to independently apply conductors to the circuit board. This method is quite labor-intensive and requires the most time from the developer;

Automatic tracing: CAD, without the intervention of the developer, applies conductors to the board drawing, using only specified restrictions (such as, for example, the minimum gap and conductor size). With this approach, the designer controls the process and, if necessary, corrects it by making adjustments to the initial parameters, such as the location of components on the printed circuit board;

Interactive tracing: It combines the work of CAD and humans. The program draws the circuit, as well as controls the PCB layout rules, and the developer, in turn, indicates the sequence of actions in those sections of the tracing that are most difficult to implement. This method of wiring can be used to finalize the printed circuit board in both manual and automatic wiring.

Our technical specialists will perform the correct tracing of your printed circuit board based on the developed circuit diagram of the future product.

The following input data is required to carry out work on tracing a printed circuit board:

1. Schematic diagram. The scheme can be provided in one of the following types:

  • preferably in an automated design system (PCAD, ORCAD, Protel)
  • electronic drawing
  • drawing on paper

2. Specification - a list of ECs for mounting on a printed circuit board, BOM. The specification can be provided in one of the following forms:

  • preferably in electronic form (Word, Excel)
  • in paper form

3. Dimensional drawing of the printed circuit board indicating the dimensions of the PCB outline, as well as indicating the mounting holes, connectors, heatsinks, and other elements, the location of which must be fixed. The drawing can be provided in one of the following types:

  • preferably in an automated design system (PCAD, ORCAD, Protel)
  • in paper form
  • in the form of a drawing in electronic form

4. The terms of reference should contain information about the requirements and wishes for the location of the EC on the board, the tracing of circuits, the width of the conductors, the wave impedance, if necessary, etc.

Stages of work on tracing a printed circuit board:

  • Create a library of CIs to be used in this product. The library of components is selected taking into account the technological features of the further installation.
  • Creating a netlist
  • Packing the components on the board
  • Layout (pre-positioning) of components on the PCB
  • Board layout approval
  • Creation and approval of the wiring project for all circuits on the printed circuit board
  • Creation and approval of the tracing project
  • Making adjustments, if necessary
  • Final route approval
  • Transfer to the customer of all technical documentation for the project

BGA component sizes continue to get smaller and thinner as they are increasingly used in portable devices. Requirements for end devices include the need to maintain all connections even during a crash, which necessitates the use of an underfill.

Increasing demand for increased board density creates additional rework challenges for adjacent or mirrored parts. Higher solder temperatures for lead-free processes put more stress on the shielding of adjacent components. These changes create certain difficulties in finalizing the BGA.

The most problematic aspects of BGA revision and repair and how to solve them

As the density of components on the PCB increases, the need to protect adjacent thermally sensitive components near the BGA, such as ceramic capacitors, dies, and plastic-encased components such as connectors, increases. These devices must be protected from heat during the soldering process. If this is not done, they may be damaged in an explicit or "hidden" form.

 In addition, the long-term reliability of such components can also suffer from exposure to heat, even if it is not visible externally.

Such components can withstand a peak temperature of 260 0C (as defined in J-STD-002), but their reliability may suffer in the medium to long term if the intermetallic compound layer is too thick. Adjacent underfill material may "bleed out" because its melt temperature is lower than the liquidus temperature of the lead-free solder.

There are many different ways to shield in and around the BGA rework zone. Increasing the liquidus temperature of lead-free solder has increased operating temperatures in areas where there are sensitive components with severe package temperature and exposure time limitations.

The thickness of the layer in PCB Tracing

The thickness of the intermetallic layer, which to a certain extent reflects the mechanical strength of soldered joints, can become too large when exposed to temperatures above the liquidus for a long time. If it becomes too large, the solder joints will become brittle. And this, in turn, affects the reliability of solder joints.

To prevent these negative effects on neighboring devices when finalizing the BGA, high-quality shielding is necessary. The study showed that the shielding effectiveness of "historical" materials such as Kapton TM film and stainless steel, is not as high as with modern materials such as clay-based shielding gel and ceramic nonwovens.

 The study confirms that to fully protect the device from thermal damage, clay-based gel material as a heat shield is twice as effective as Kapton film at close range, and three to four times more effective at long distances. Nonwoven fiber ceramic material is almost as effective as the gel in terms of heat protection properties, but it does not need to be cleaned after use.

The Connection of BGA Package with PCB Tracing

Modern devices such as smartphones and tablets use an underfill to ensure that the BGA package can withstand drop test requirements without damaging the solder joints. For the reworker, the problem is the malleability of the underfill.

The viscosity properties, even if they are above the melting point, create a mess under the BGA package. Even if the underfilled BGA can be separated from the board, the applied mechanical force can potentially damage the device or the board.

The clay-based gel protects and shields devices from thermal damage.

UNDERFILL DEVELOPMENT

The underfill softening temperature is lower than the PCB soldering temperature. This means that under the BGA, as well as under any other unprotected device, the underfill softens and expands before the solder melts.

This leads to both stickiness and problems with removing the device. During extraction, the underfill will flow out as local pressure pushes the solder out when it reaches its melting temperature. As a result, this all leads to chaos on the surface of the device. When removing the device from the printed circuit board, the mechanical force from a special nozzle or lever can cause significant damage to the board.

In addition to the fact that the board can be damaged when the device is removed, damage can also occur around and under the device area when the underfill is removed from the circuit board. This damage can be caused by the desoldering nozzle, which can scratch or damage the solder mask pads while removing underfill residue.

Moreover, the underfill adhesion can be so strong that the pads simply come off the board. A similar phenomenon occurs more often with pads that do not have connections under the BGA.

Results of mechanical cleaning of BGA with underfill

One way to avoid serious damage when preparing the BGA site after removing the device is to use a high-speed milling system. This solution does not use direct heat to soften the underfill, but the hardened underfill is polished off by high-speed milling, as well as the remaining solder balls. When performing this mechanical operation, high precision is required.

If the sanding is not sufficient, there will be too much underfill at the location of the BGA device, making the pads unsuitable for soldering. This reworking method must be carried out very carefully and accurately so that the mechanical vibration and stress during reworking do not lead to a decrease in the reliability of the printed circuit board.

4. Solder mask damage

Soldermask damage under the BGA device occurs for several reasons. This may be damage in the form of a missing mask or a violation of the adhesion of the mask to the printed circuit board. This can be caused by the use of a solder braid during site preparation, an uncontrolled heat source when removing the BGA, an abnormally high PCB heat cycle, or poor initial soldermask adhesion.

As a result, there is a problem in the form of solder running off in a dumbbell shape, which creates a lack of it in solder joints. A poorly bonded solder mask can cause short circuits and other problems.

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