When a fabricator is supplying bare PCBs, you have the option of receiving the boards as individual PCBs or in the form of an array. Manufacturers combine a single PCB multiple times, a process they call step and repeat, to form a large array of mechanically connected boards. Later, they may separate individual PCBs from the array by breaking them away at the points where they join with adjacent boards. Depending on requirement, you may opt to receive panels rather than individual boards, and separate them after assembly. Other names for describing the array of PCBs are: stepped out, panelized, rout and retain, and palletized. PCB arrays offer several advantages to both, the fabricator and the assembler.
Advantage of Arrays in Fabrication
PCB manufacturers start with large sheets of prefabricated laminated boards, cutting them to the size of the intended PCB. Alternately, they carve out several PCBs from one large board, adding spacing between each PCB and tooling strips on the edges. The manufacturer can use tabs, Vgrooves or scoring in between the boards for separating them later.
Therefore, if the individual PCB is fairly small, manufacturers prefer to step and repeat the design in both X and Y directions, until they have formed a panel that their machines can handle comfortably. They must execute the step and repeat process for all the Gerber files, including all films.
Rather than handle several small boards, the fabricator can now operate on a single large panel. This makes the manufacturing process much simpler, as the fabricator can make several boards with one setting. Although there is additional expense involved with the step and repeat process, and the addition of space between PCBs and tooling strips on the edges, it amalgamates over the total number of boards in the panel. The net effect is a lowering of the fabricating cost per PCB when in a panel, rather than when fabricating them as individual PCBs. In addition, manufacturing time per PCB reduces considerably when fabricated as a panel.
Advantage of Arrays in Assembly
Most PCBs use SMT or Surface Mount Technology, where the assembler mounts SMDs on the board using pick and place machinery. These machines need prior programming for picking up each SMD from reels, tapes, or trays, and placing them at specific location on the board. As the machine completes mounting components on one board, a conveyor mechanism brings a fresh bare board under the machine, pushing the completed board forward. During the time the completed board moves forward, a new board comes in, and the operator completes the process of registration, the pick and place machine remains idle.
The number of times the machine idles increases if each PCB is small and has only a few SMDs on it, as the machine must wait frequently for the next PCB to arrive. On the other hand, if the small PCBs are available in a large panel or array, the machine takes more time for mounting components. Although the idling time remains the same, the idling frequency reduces drastically. This results in a better utilization of the machine, and increases its throughput.
All processes preceding and following the process of SMT mounting also requires changing accordingly. To start with, the PCBs must be available in the form of an array. The solder paste deposition for the array requires a larger matching stencil. The entire array, along with the mounted components, goes into the reflow machine for soldering, and this requires adequate mechanical support to prevent the panel from deforming due to the heat. However, depending on board complexity, testing an array after assembly may or may not be simpler than testing individual board assemblies.
Disadvantages of Paneling
Although manufacturing and assembly of small boards benefit from arranging them in panels, the method has its disadvantages. The major disadvantage with panels is the small increase in the size of individual PCBs. Although the increase is smaller when using scoring between the PCBs, designers must factor in the larger size when designing the rest of the gadget around the PCB.
Manufacturers may also use tab routing to connect the PCBs together in a panel. In this process, they remove much of the spacing between the PCBs, leaving them connected only by small tabs. Separating the PCBs requires breaking the tabs. However, this leaves a small protrusion on the edge of the PCB, which may need a separate process of removal by grinding if it must fit in a tight space.
Depanelizing a PCB from an array induces physical stress in the board. Designers must take adequate care of spacing critical areas of the circuit away from the breaking zone. Not doing so may result in fine traces developing hair-line cracks after the process of separation, leading to intermittent electrical operation.
Separating PCBs from a panel may also affect vias placed too close to the breaking zone. The physical stress may cause cracks to develop in the barrel, leading to quality and reliability problems.
The panel size must conform not only to the manufacturing capabilities, but also to the automation assembly equipment. This includes paste printers, SMT machines, reflow ovens, and AOI equipment.
Which is Right — Single PCB or Array?
Considering the above, you may need to work out the ease and cost of assembling the PCB when it is within a panel versus assembling an individual board, to decide which is right for you. For instance, an array may be better in the following cases:
- Large production quantity of small PCBs with low number of SMDs on each board
- PCBs with irregular outlines that do not conform to manufacturing equipment capabilities
- A large assortment of multiple small PCB combinations with or without irregular outlines
On the other hand, individual PCBs are preferable in the following cases:
- The PCB has BGA or other fine-pitch components on board
- The PCB is very thin
- It is not possible to arrange the PCBs in even numbers within a panel—odd numbers will generate more scrap
- The PCB has through-hole components requiring auto-insertion
Determining the necessity for a single PCB or an array depends on whether they increase the manufacturing efficiency. For an array, the designer must consider the difficulty and cost of depanelization methods as well. Reinforcements during assembly may be necessary as reflow temperatures can cause the array to deform. Thin PCBs may not be able to withstand the stress of depanelization.