A board’s level of technology should dictate how often to expect imperfections.
One of the most common questions I get from PCB buyers is, “How many X-outs are acceptable?” Some might say receipt of a PCB manufacturing panel or array with any X-outs indicates the supplier cannot maintain a high level of quality.
This is not necessarily the case.
An X-out occurs when a defective board in an array or manufacturing panel of like PCBs has been shipped. The board is literally marked with an X in permanent marker to signify it is flawed.
While a panel or array with zero X-outs is ideal, the board’s level of technology should dictate how often to expect this kind of perfection. If the board is a single-, double- or easy four-layer item, then a PCB buyer should expect – in fact, should demand – the manufacturer deliver panels free of defective boards.
However, if it is a higher technology – such as an HDI design – scrap will happen in every manufacturing lot. To expect otherwise is not realistic.
Any experienced PCB supplier knows this custom-made item – requiring more than 100 different manufacturing processes – will have a board (or boards) with some sort of rejectable imperfection in every manufacturing lot released to the floor. To address this issue, before the board order hits the production floor, the fabricator will release an overage, depending on the board’s technology, to ensure its manufacturing processes yield the number of boards required to fulfill an order.
The more high-tech the board is, the more overage may be necessary to account for any fallout.
For example, let’s say 1,050 pieces are released to meet an order requiring 1,000 individual boards. In this case, the manufacturer decides a board’s technology will require only an additional 5% in materials overage.
At final QC, the vendor then finds 32 pieces (or about 3%) did not make it through the manufacturing process for various reasons. Those bad boards are scrapped.
Because a 5% overage was produced, 1,018 pieces are good. So, the 1,000-piece order is shipped as promised, and the additional 18 pieces are put into finished spares. Everyone is happy, with most customers not paying any attention to the fallout that occurred within that overage amount.
Sometimes, though, having PCBs delivered in an array or panel format might highlight manufacturing challenges, especially when a customer has a “No X-out” policy.
If you put that same order in a four-up array, then 250 arrays would need to be perfect to meet the 1,000-piece requirement. Based on those numbers for that technology, the manufacturer could expect the same percentage of fallout. But boards found at final QC that don’t pass muster are “connected” to three pieces that passed with flying colors. This means 32 arrays with an X-out or 128 pieces total (32 x four-up) are not allowed to ship, regardless of their quality.
The vendor must release more overage (about 15%, or 152 pieces, because it’s a four-up array) to accommodate the normal fallout that occurs during the manufacturing process for arrays that can’t have any X-outs.
Whether your company accepts X-outs or not should be detailed in your firm’s PCB fabrication specifications. This information will guide your board suppliers on how much material (overage) they must release – based on their technical capabilities – to fulfill an order.
Here is an example of an EMS company’s X-out policy that is clearly spelled out:
“X-outs are allowed. However, not more than 20% of the PCBs in the array can be X’d-out, and no more than 10% of the arrays to be shipped may contain an X-out.”
This means if you have, say, a 2,000-piece order manufactured in a 10-up panel requiring 200 arrays to be received, the most you should receive is 20 panels that contain no more than two X-outs each.
Your fabrication specs should also state how X’d-out panels are to be received to avoid causing headaches for both receiving and production departments. A statement like this works:
“X’d-out arrays are to be segregated and identified accordingly at time of shipment.”
PCB buyers should keep in mind the amount of real estate needed for perfect arrays (no X-outs) means the overall cost of the board will be higher. The adage about not allowing the perfect to become the enemy of the good is applicable here.
Before your company sets its X-out policy, sit down with your manufacturing department. There are ways for an assembler to handle manufacturing panels with X-outs, but the department responsible for shipping quality, finished assemblies should have final say on X-outs.
A rigid “No X-out” policy will likely cost you more without improving the PCB manufacturing process. In most cases, a more flexible approach is warranted.