Panels and de-panelling
Rationale for using panels
We have seen that board fabricators often make multiple circuits at a time, in order to minimise cost of handling and processing, and make best use of their “process blank”. Similarly, arrays are frequently handled in multiples at the assembly stage, in order to optimise the processes. For example, having a two-up panel will reduce the time taken by general board handling, and halve the process time for printing, reflow and wave soldering.
Figure 1: An example of a board assembled on a two-up panel
Source: Ultra-CEMS
As shown in Figure 2, having a single panel that is larger than the finished assembly:
- makes it possible to mount components right up to the edges, whilst providing a handling margin
- makes it possible to have circuits of irregular shapes
- allows heat-sensitive components to be screened against wave soldering using parts of the board that are removed after assembly.
Figure 2: An assembly that makes effective use of a handling margin
Source: Ultra-CEMS
The term panelisation refers not only to the multiple panels shown in Figure 1, but can also refer to a single board to which break-off strips have been added, for example, to provide a straight processing edge for handling during placement and soldering.
Figure 3: Added break-off strip
If designed properly, the panel can become the work-holder and tooling fixture necessary for the assembly production machines. If needed, tooling holes and fiducial marks can be added to the waste rails.
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Panelisation considerations
Whilst many arrays of circuits on a panel will be a matrix of rectangular parts, in some cases better use can be made of the panel by laying out boards as in Figure 4.
Figure 4: Panelised board
Note that, in this case, the direction of travel for each of the circuits is reversed. As you will appreciate, the direction of travel of the board in respect to the process is important, particularly for wave soldering, so any panelisation of this type needs to be decided upon before components are placed on the board. Ideally, panelisation is one of the first things to consider when starting a new design.
Whilst in theory it would be possible to perform the final board routing stage on the assembled panel, in practice, panels are partially processed by the fabricator, to make it easy to remove individual boards. The most common ways of achieving this are by pre-scoring the board, creating a weak point, or by carrying out most of the routing but leaving break-off tabs. On occasions, as in Figure 1, both methods may be seen on the single board.
Break-off tabs
Where routing is used, the board profile is completely routed, with the exception of break-off tabs placed in strategic locations to support each circuit within the panel. A typical slot width is 2.4mm, and the tab is generally rather narrower than this. However, even such a small tab is quite robust and it is not possible to guarantee where it will break. The usual way around the problem is to pre-drill holes close to the break-off point, and placed slightly in-board, as shown in Figure 5. The holes define the weakest link, and create a break line through the centre of the holes. This helps to ensure that no burrs protrude beyond the edge of the board after it has been removed from the panel.
Figure 5: Close-up of routed panel showing break-off tab with pre-drilled holes
Source: Nortel Networks
V-scoring
A considerably cheaper alternative is “v-scoring”. Here the edge of each circuit is defined using a 60° or 90° cutter on both sides of the board, to produce a v-shaped groove. This reduces the material thickness, allowing circuits to be separated from the panel, either by snapping by hand or using a v-score cutter, often referred to as a “pizza cutter”. Using a cutter is to be preferred, as this puts less stress on the assembly, and in particular the components nearest the board edge. As you will recall, ceramic capacitors near the edge have a tendency to crack!
Figure 6: Schematic and photograph of a v-score
Source: Ultra-CEMS
V-scoring allows circuits to be closely butted against each other, so there is little waste material, but is only possible with a regular matrix of circuits. Whilst it is possible to use scoring to define handling strips at the edges, these are sometimes quite difficult to hold sufficiently securely to effect a clean break.
Figure 7: A typical specification for a v-score
Figure 7 gives a typical specification for a v-score. V-scoring can be carried out on 0.8mm to 2.4mm thick PCBs (Table 1). If the remaining material is too thick, it is difficult to cut; if too thin, the v-scores will not provide adequate support during assembly.
| board thickness | residual material thickness |
|---|---|
| <0.8mm | not recommended |
| ≥0.8mm <1.6mm | 0.3mm ±0.1mm |
| ≥1.6mm <2.4mm | 0.4mm ±0.1mm |
| =2.4mm | 0.45mm ±0.1mm |
| >2.4mm | not recommended |
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De-panelling methods
As well as arranging circuits within a panel, the designer must give thought as to how to remove individual assemblies. As has been mentioned, using operator finger pressure may snap components as well as the board, but this approach is regrettably common. For complex small shapes, and volume production, a safe and quick solution is to break all tabs in a single pass using a custom punch and die.
Break-off tabs
Where volumes are smaller, break-off tabs can be removed by using the same type of routing equipment employed during board fabrication, as shown in Figure 8. Compared with board fabrication, the normal practice is to have the router approach from underneath the board, as the lower board surface is usually not fitted with high-profile components. Clearly a serious piece of equipment, and not applicable to every assembly house!
Figure 8: Router used for panel separation
Source: Nortel Networks
For smaller quantities, the individual circuits can be removed from the panel using a “nibbling machine”. This has a metal blade that is inserted through the routed slot in the panel, moved over the break-off tab, and then pulled down past the level of the panel, cutting through the break-off tab, as shown in Figure 9.
Figure 9: Schematic of nibbler operation
The machine operator will locate the panel over the blade, which is pneumatically operated and controlled by a foot pedal.
Two types of blade are in common use, as shown in Figure 10. The benefit of the T-shaped blade is that it can approach the break-off tab from either direction, which may reduce the need to manoeuvre the board. However, the hook type needs less clearance between tabs in order to fit the head through the gap.
Figure 10: Common blade types
V-scoring
When v-scoring, the score on the underside is located on a ridge on the machine bed, and a rotating disk used to break through the score from the top side. In most cases the board is pushed along the ridge manually until it comes into contact with the rotating disc, which pulls the board through the machine.
Figure 11: V-scoring machine
In other variants, the ridge on the underside is replaced by a second disc (Figure 12). In both cases, the separation between disc and opposing ridge or disc is small, so the board is effectively cut as with a pair of scissors.
Figure 12: Typical “pizza cutter” head
Source: Ultra-CEMS
Such machines are simple to use, though there are some safety issues, and it is necessary to adjust the clearance between disk and opposing surface to give correct results, depending on the residual thickness of board to be cut.
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Design considerations
When panelising, we must consider:
- Any board size limitations from the assembly plant; each machine will have a maximum and minimum board/panel size.
- Any break-off rails that may be required to square off irregular shaped boards or to provide component clearances from the transfer edges.
- The number of circuits in a panel needed to optimise the assembly process. If a board has to go through solder paste application and then SMT population, the best panel array would be one where the time taken to screen print the paste onto the board is matched with the time to populate all the components on that side of the board.
- The material utilisation of the board fabricator’s panel. The assembly panel will be stepped and repeated into a standard process blank. The size of this will depend on the material the PCB fabricator buys in as standard.
- The board thickness. A board whose thickness is less than 1.6mm should have a reduced assembly panel size to prevent the panel from bowing during reflow/wave soldering.
- Panelised boards may require two test beds to be incorporated into the test fixture, one bed for the panel and one for the single PCB. After the panel test, the boards are de-panelised. The good boards continue along the assembly process and the other units are reworked and re-tested on the single unit test bed.
- The process for field returns once the boards are removed from the panel.
- The aspect ratio of the panel (the longer edge should be parallel to the direction of travel)
It is also necessary to consider the method of de-panelisation, leaving sufficient clearance for router, nibbler or cutter. For nibbling, for example, the length of a typical T-shaped head is 18mm, whereas the L-shaped hook is 12mm. To give sufficient clearance to prevent the tool from jamming in the routed slot, its thickness is usually 0.15mm less than the diameter of the router bit used to create the slots in the panel, for example, a 2.25mm blade is selected when a (typical) 2.4mm router has been used.
Figure 13: Schematic side view of nibbler and blade
Physical clearance is also needed on the underside of the assembly, because the board edge will be pressed into contact with a 1.5mm thick metal anvil (Figure 13), and any copper tracks within this area would be stressed or fractured. Sufficient component clearance is also needed to prevent contact with the blade anvil (Figure 14).
Figure 14: Dimensions of breakout blade shown in relation to its anvil
For break-out of a v-scored panel, remember the board edge is where the cut will be made, that is at the tip of the blade. Care needs to be taken when placing tall components near the board periphery, because the blade can interfere with the component, as shown in Figure 15.
Figure 15: Component too close to PCB edge
The clearance will depend on the machine parameters, such as the blade thickness, but typical values are given in Table 2.
| Position from board edge (mm) |
Acceptable height of component (mm) |
|---|---|
| 1.7 |
4 |
| 2.4 |
8 |
| 3.8 |
16 |
| 5.2 |
24 |
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Author: Martin Tarr
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