Processing irregular and fragile PCBs in standard assembly machines is often a challenge. Due to their non-standard dimensions, the oddly shaped, thin, or fragile boards do not fit properly in the reflow, component placement, or other standard processing machines. Lack of adequate support and alignment may strain, bend or damage the fragile boards as they go through the processing system. In addition, this may impact the accuracy of the process.
To overcome these challenges, engineers may use SMT fixtures or the panelization techniques. Each of the two methods provides a carrier with standard dimensions and provisions for fitting, securing and supporting the irregular boards.
SMT Fixtures for Irregular and Fragile PCBs
SMT fixtures are rigid PCB carriers that allow manufacturers to load single or multiple irregular boards, and then take them through the standard processing machines. This makes it possible to carry out tasks such as automatic mounting of electronic components, infra-red reflow soldering, wave soldering, automatic testing, etc.
The fixtures are usually available in a variety of designs, materials, sizes, and thicknesses. Some can carry more than one board hence allowing the processing of multiple PCBs simultaneously. Other designs have adjustments to support both the primary and secondary side processes.
Synthetic stone SMT fixtures for reflow, wave soldering and PCBA processing. Image courtesy of Southern Machinery
Generally, the fixture materials, which range from synthetic plastics and stones to aluminum alloys and epoxy glass, should be electrostatic neutral, and able to withstand repeated thermal cycles and chemicals without deforming or wear. In particular, the thin fixtures provide better thermal management.
Typical fixtures have high-temperature resistance, good mechanical strength for precision machining, and good abrasion and chemical resistance to withstand the pallet cleaners and soluble fluxes.
Where Are Fixtures Best Suited?
Fixtures are more suited for the flexible circuit boards as well as the fragile and thin PCBs. However, each new board design requires its custom fixture which can take between three and four days for a manufacturer to produce and deliver. As such, using fixtures can add up the costs and time to produce the PCBs.
Considerations When Selecting SMT Fixtures
The fixture should support the board and prevent distortion as the board goes through the printing, component placement, and reflow processes. Its construction should ensure adequate protection for the bottom side components during the double-sided reflow process.
The ease of loading and unloading the PCBs, as well as the ability to handle multiple PCBs, are important in high production lines. In addition, fixtures should provide thermal protection to the heat sensitive areas of the PCB.
Other desirable features include:
- No clamps, bars, or other features above the PCB surface height
- Adequate support for the large PCBs (however, there should be consideration for the parts)
- Draw type pins or low profile clamping are required for the thin PCBs
- No material beneath the BGAs, fine pitch, and other critical areas (allows proper thermal transfer)
- Holes in the fixture (to help in reducing the thermal mass, hence improving the reflow)
Selective solder fixture. Image courtesy of MB Manufacturing
Panelization is the arranging of arrays of multiple PCBs onto a single substrate that fits in the standard processing machine. A panel may hold multiple boards with similar or different sizes and shapes. This method is faster and less costly compared to the fixtures, which require a custom carrier for every new PCB design.
There are different panelization and de-panelization methods depending on the application of the PCB, thickness, shape, component layout, type of edges, and other factors.
The two popular methods are the V-groove panelization and the breakaway tab panelization. Each of these uses a different method to separate the boards from the main array and has its advantages and limitations.
V-groove panelization involves cutting a third of the board’s thickness from the top and bottom sides. The remaining part joins the separate boards and is then cut with a machine during the depaneling. This helps to reduce the stress on the PCB. One challenge with the V-groove method is that it is restrictive and cannot be used with PCBs that have overhanging components over the edges.
Tab-Route or Breakaway Tab Panelization
The tab-route or breakaway tab method is suitable for PCBs with either similar or different designs, or where it is not possible or practical to use the V-grooves. The designer leaves a perforated tabs routing space between the different or similar boards, hence allowing for separation after processing.
However, the SMT components and the traces must be at least 3.00 mm from the perforation holes. This prevents the damage to the PCB or components which can occur due to surface stress and splinter when separating the boards. One drawback with this method is that it may leave some unwanted board protrusions on the edges.
Challenges of Panelization
- PCBs with SMT on both sides and those with mixed TH and SMTs may start to flex and bow when in the reflow oven or selective soldering machine. For this reason, the maximum size for this type of board is usually smaller than the single-sided.
- Breaking out the boards manually can stress the board and components near the edges, the solder joints, or leave out rough stubs on the edges.
PCB Depanelization Methods
Depanelization is the process of separating the individual boards from the main array and usually depends on the panelization method applied. The choice depends on the clearance to provide at the edges of the PCB, and the presence of sensitive SMT devices, or hanging connectors near the edges.
There are automatic methods, such as the laser cutting or depaneling router, as well as manual methods that involve breaking the tabs using hands or other tools.
Automatic machines translate to added costs and some such as the depaneling router creates vibrations, noise, and vast amounts of dust. In addition, you need to hold the board firmly. Although laser cutting is precise and has less mechanical stress, it is capital intensive and only applicable to board thicknesses of around 1 mm.
Manual removal of the tabs depends on the design and thickness of the board. Extra care is required since using some tools such as the hook-shaped blade to break the solid tabs between the boards can be challenging and inefficient. For example, if the blade rotates within the small clearance between the boards, it can easily take a bite out of the useful part of the PCB. Also, taping the blade-cutting edge leaves a small part of the tab protruding from the board.
Manual depanelization. Image courtesy of YUSH Electronic Company
Care should be taken when depaneling the perforated-tab PCB array; otherwise, an incorrect method will splinter or tear the solder-mask or active surface layer. The ideal breakout method should not cause any damage to the board or transfer stress from the PCB surface to the components.
Manufacturers can use fixtures or panels to secure or frame fragile, thin, or oddly shaped PCBs. This allows them to easily fit the boards in the standard assembly, reflow, or automatic inspection machines.
In addition to handling the irregular boards, the technologies enable the simultaneous processing of multiple boards as opposed to working on each board individually, hence reducing the production time and costs.