Thick printed circuit boards (PCBs) over 4 mm thick present both a
challenge and an opportunity for the copper plating industry. The
opportunity comes in the form of a niche but growing market, with an
estimated growth rate of 3-4 percent per year, and a market size of $1
billion for thick PCBs. The primary end application is in
communications, with growth of 5G infrastructure set to take off
dramatically by 2020. Huawei released the industry’s first 5G slicing
router at MWC 2017 in Barcelona in February, and Qualcomm has
announced plans to release commercial chips for 5G services in 2018,
and competitors are likely to follow suit. This growth, along with
opportunities in medical, military, and aerospace applications, will
accelerate the demand for thick PCBs.
The technical challenges involved in plating these boards center around the high aspect ratio (AR) for through holes. Direct current (DC) plating is the industry-standard single-plating process to conformally plate both through holes and blind vias, and create fine-line imaged patterns. It is relatively low cost, but it doesn’t work for AR higher than about 14. Back panels and high end line cards are typically 4 to 6 mm thick and may have 0.25 mm diameter vias, which translates to an AR in the range of 15 to 25. For the thickest PCBs, the AR is even higher.
Clearly, the industry needs an alternative to DC plating. Periodic pulse reverse (PPR) plating, a process which the board electric potential is switched between forward(catholic) and reverse(anodic), provides a possible solution. Multiple suppliers offer PPR plating. The problem is that most existing PPR processes suffer from insufficient throwing power. The chemical capability is not able to meet the stringent plating requirements that high AR holes present. Throwing power tends to degrade as the bath ages, further compounding the problem.
These technical challenges provide an opportunity for suppliers that can help overcome them. Dow is leading the way with the introduction of improved bath chemistries for PPR, known as PPR-II. Dow’s Copper Gleam™ PPR-II acid copper is especially designed to provide a one-step plating process for thick PCBs with high AR through holes. It can reliably deposit copper for through holes, blind vias, and fine-line patterned plating, and is compatible with both hoist and vertical continuous plating (VCP) lines.
Figure 1: Plating of through holes, blind vias, and fine-line surface patterns with PPR-II copper
This plating chemistry allows PCB manufacturers to take advantage of the improved throwing power and high throughput of PPR without the downsides of unstable baths and low bath life. Throwing power (TP), expressed as a ratio between the plating thickness in the hole and on the surface, is a critical metric that determines the performance and cost of through hole plating. High TP enhances reliability by ensuring that the minimum amount of copper required in the hole, usually 20-25μm, can be achieved. Higher TP means less copper is plated on the surface, reducing plating time and cost, and allowing for finer feature formation on the surface.
The PPR-II process has demonstrated high TP for PCBs up to 8 mm thick, the thickest boards currently available for evaluation. PPR-II baths have demonstrated an impressive bath life of over 300 ampere-hours per liter with no degradation in performance when plating from an aged bath compared to a newly made bath. Depending on total product volume, manufacturers may need to treat the bath only one or two times a year, compared to much more frequent maintenance required with most existing PPR processes. This provides a substantial advantage in cost and time, greatly improving process efficiency.
Figure 2: Bath life stability of PPR-II, showing consistent throwing power for a 3.2-mm-thick board with 0.25 µm diameter through vias
Although some manufacturers, especially in Asia, run plating lines 24 hours a day, 7 days a week, this schedule is not the norm everywhere. Facilities that are closed on weekends and holidays are forced to waste time when restarting the plating process after idling for a day or more. Unlike many existing PPR processes, the PPR-II process does not suffer from reduced throwing power when the system is restarted after idling, even after idling for as long as two weeks. This limits the need to run a dummy process on a restarted bath, saving time and materials.
Figure 3: Idling of PPR-II bath @200 AH/L, demonstrating consistent throwing power after 2 and 13 days idling
Several factors contribute to cost savings for PPR-II baths compared to typical PPR chemistries:
The PPR-II process has demonstrated consistently reliable performance and high TP on thick PCBs with AR above 16, solving the challenges that such boards pose for consistent, single-source copper plating.
With the technical challenges addressed, PCB manufacturers can take full advantage of the opportunities for providing thick boards with reliable copper vias to support the growing 5G telecommunications market.
Learn about the recent recognition from PCD&F for Dow's Copper Gleam™ PPR-II acid copper.