Across consumer electronics and all other industries that rely on well-manufactured printed circuit boards — or PCBs — the ability of vias to conduct signals appropriately is tantamount to a successfully designed PCB. Vias are the conduits that allow electronic signals to pass between the layers of a PCB.
Oftentimes, manufacturers add a layer of a conductive metal — typically copper — to a PCB’s substrate to connect the board’s layers as the necessary layout and holes are made. For some applications, copper plating is sufficient. However, for applications that create high levels of heat and/or strong currents, plated through holes can also be filled with more conductive materials like copper. This setup creates what’s commonly referred to as copper filled vias. To learn more about the benefits of and differences between filled vias and plated through holes, check out What Are Copper-Filled Vias? now.
For organizations whose applications are even more demanding and/or that want the latest in PCB design, electroless nickel immersion gold — or ENIG — is proving to be one of the most popular surface finishes on the market today. Since ENIG finishes contain no lead, they’re also a smart choice for companies that want to be in compliance with the restriction of hazardous substances — or RoHS — directive 2002/95/EC. This directive from the European Union restricts the use of specific hazardous materials in electronic devices and products.
ENIG finishes are prized in the PCB industry for providing the following advantages over other types of surface finishes — such as HASL and Immersion Tin:
- Extremely flat surface
- Long shelf life
- Ideal finish for fine pitch SMT parts
No PCB finish is without its drawbacks. For ENIG finishes, the risk of black-pad problems poses a real threat to the operation of whatever ball grid array — or BGA — components or other items are attached to the PCB. Unfortunately, ENIG finishes can’t be re-worked, so the problem of black pad is a crucial one that needs to be addressed properly. Otherwise, an entire product can be rendered inoperable, resulting in lost business, unhappy customers and the distinct possibility of a costly recall.
WHAT IS ENIG BLACK PAD?
There might not be any topic more controversial in the PCB industry than that of ENIG black pad. In simple terms, black pad refers to an inferior connection that occurs between the solder and nickel interface. While many professionals consider this phenomenon to be extremely rare — occurring by some estimates in less than one or two percent of ENIG PCBs — the fact that a black pad problem isn’t typically discovered until an assembly process commences makes it a costly error to discover and correct in the post-manufacture phase.
WHAT CAUSES BLACK PAD?
Black pad is the result of the immersion gold and electroless nickel process. While it produces a highly solderable finish — ideal for applications where reliability and complex circuitry go hand-in-hand — the process can also create flawed joints that, when stressed, will break their connections. The resulting open circuits expose a layer of dark nickel that’s corroded — which gives this phenomenon its notable name “black pad.”
According to recent research and industry reports, black pad is the result of an excessive level of phosphorous that’s a byproduct of the dissolution of nickel. Ultimately, as the result of consecutive reflow processes, a starting seven percent phosphorous level can easily climb to nine percent — or even higher. And the higher the level of phosphorous, the greater the risk of nickel corrosion during the gold immersion/deposition process.
HOW TO PREVENT BLACK PAD
Industry experts unequivocally agree on one thing when it comes to black pad — it couldn’t happen at a worse place along the development and manufacture pathways of electronic components and goods. It’s rarely discovered during PCB manufacture, but instead is found out only during assembly — or later — when stressed joints and open circuits become a recognizable and potentially catastrophic problem.
What’s more: Unless nondestructive testing is performed specifically on the PCBs in question, black pad is almost impossible to discover until components are attached to the PCB.
To this end, preventing black pad becomes a crucial responsibility of PCB manufacturers and suppliers. Of course, knowing that black pad is caused by high levels of phosphorous is different from actually controlling those levels during the manufacturing process. That’s why choosing Millennium Circuits Limited as your PCB supplier is most likely the smartest way to avoid black pad issues with ENIG finishes. From small batches to high-volume orders, we’re totally committed to providing you with PCBs built to fully meet your expectations.
HOW TO FIX BLACK PAD
Since ENIG finishes can’t be re-worked, the best way to fix a black pad issue is to prevent it from happening in the first place. This goal is most efficiently and reliably accomplished by keeping strict control over the nickel bath, where the exchange between gold and nickel is kept as close to ideal as possible. A key factor in maintaining this control is to carefully monitor the pH level in the nickel bath, as doing so will indicate the level of phosphorous that’s present.