In the PCB industry, there is a specific term called “Cross-out boards” or “X-out boards” to indicate that a panel of PCBs contains defective boards. The term “Cross” refers to marking these defective boards with an “X” symbol.

For example, a 4-in-1 PCB panel means it contains four individual boards. During the manufacturing process, if one, two, or even three of these individual boards are found to have quality issues, the PCB manufacturer will mark each defective board with a large “X” to clearly identify which ones are bad, while the remaining boards are good. This is where the term “X-out boards” comes from. Of course, if all boards in the panel are defective, the entire panel would be scrapped.

But here comes the question: when a panel contains X-out boards, should the entire PCB panel be scrapped, or can we ask the customer to accept it? After all, there are still some good boards within the panel. Scrapping the entire PCB panel seems wasteful, and it’s understandable if someone raises this concern.

In reality, there is no absolute right or wrong answer to this question. Depending on the perspective you take, you may come to different conclusions.

PCB Manufacturer’s Perspective on X-out Boards

Of course, PCB manufacturers hope customers will accept X-out boards because these are still costs—money, really! It’s like a guava with a small blemish from an insect bite. If you cut out the bad spot, even if the panel has some defective boards, there are still good boards left. Selling it at a slightly lower price to the customer means they can recover some of their production costs.

SMT Engineer’s Perspective on X-out Boards

X-out boards actually impact the efficiency of the entire Surface Mount Assembly (SMA) production line. For example, if you have a 4-in-1 panel and one board is defective, then it effectively becomes a 3-in-1 panel.

Additionally, how do you identify which board is defective? The SMT machine program would need to be reconfigured to skip the defective individual board during placement. You can’t knowingly place components on a bad PCB and then scrap the PCBA after SMT is done. So far, SMT machines aren’t capable of automatically recognizing the “X” mark on a defective board. This still requires manual inspection or, alternatively, drilling a specific hole on the board that can be optically recognized by the machine.

X-out boards don’t only impact SMT production output. Some factories also use panelization in pre-assembly stages to save time in loading and unloading boards. For instance, during ICT (in-circuit testing), a 4-in-1 panel allows for a single load/unload action, whereas if the boards were separated/de-panel, it would require four individual actions.

Compromises for Handling X-out Boards

Due to the differing needs of manufacturers and clients, the decision on whether to accept X-out boards often comes down to overall production cost. If a PCB panel is relatively low-cost, it’s easier to justify scrapping the whole panel rather than negotiating. However, if a panel’s total value is above around $US20 (each company may have its own threshold), then it’s worth discussing discounted production.

When X-out boards are accepted by the customer, the SMT production line typically requires the PCB manufacturer to follow these guidelines:

• Marking the Defects: Every X-out board should be clearly marked with an “X” on both the top and bottom of the defective boards, or with another agreed-upon symbol. Some companies also request that a specific pattern hole be drilled in a fixed location on the defective board. This allows the SMT machine to automatically identify the bad boards optically, though it does add some extra detection time.

• Organizing Panels with Defects: PCB manufacturers are generally asked to stack and package panels with defective boards in the same position within the panel matrix, and to clearly label and separate these packages. This allows the SMT production line to quickly adjust its placement program to skip defective individual board in panelization.

• Limits on X-out Boards: For 3-in-1 or 4-in-1 panels, generally only one defective board is allowed. For 6-in-1 or 8-in-1 panels, up to two defective boards might be acceptable, depending on each company’s standards.

On another note, if a batch of PCBs has an unusually high number of X-out boards, it could indicate a potential quality issue with that batch. This should raise concerns about other possible defects that may not have been caught. Unless the PCB design is highly complex with very fine traces or close vias that push the manufacturing limits, a reputable PCB manufacturer will typically monitor and maintain quality to minimize these types of production losses. However, from the manufacturer’s perspective, a certain level of defects is unavoidable, especially with high production volumes. Over time, this can lead to a cumulative amount of X-out boards.

In conclusion, X-out boards remain a challenging issue to fully eliminate. Workingbear recommends keeping track of the serial numbers for panels with X-out boards and including them in a quality monitoring list. If you notice an unusual increase in defect rates during the PCBA production process, it’s essential to discuss this with the PCB manufacturer. Otherwise, there’s a risk of significant quality issues emerging on the customer side, which could lead to even bigger problems.

Related posts:

• The Benefits of Using “Mirrored Panelization” for PCBs
• The Limitations of Using “Mirrored Panels” or “Flipped Panels” in SMT
• Why is Panelization and Break-away Tab Necessary in PCB Manufacturing, then De-paneling after PCBA is Completed?