Challenges of Backpanel Plating

Introduction

Hopefully, this will be the first in a series of articles relating to challenges faced when plating backpanels, and other more technically advanced, multi-featured pwbs. For purposes of this discussion, we’re assuming a minimal backpanel thickness of .250” (6.35 mm); however, as being noted in later articles, plating a .350”-.400” (8.89-10 mm) board can present some
additional challenges.

When medium technology pcb firms succeed in plating (to spec) pane ls in the .125” (3.17 mm)
range with an 8:1 aspect ratio, they often assume that progressing from this level to backpanels will involve a series of logical steps, but with few obstacles. Unfortunately, this is NOT the case, and speaking from our experiences as designers/manufacturers of automated plating and pulse equipment, we have had the luxury of viewing these situations from a perspective as “total systems’ suppliers” where we have seen firsthand the large number of important variables that MUST to be considered when plating backpanels with DC, (even more so with pulse). Presumably, this and subsequent articles will provide the reader with a reasonable overview so that many of these matters will be at least considered in advance of moving ahead.

Backpanels Plating

Often, the first realization that “things are truly different when plating backpanels” comes when one attempts to physically lift one of these larger panels by hand, perhaps doing so in a manual, prototyping plating tank, as the size, weight, and even the particular circuit pattern likely is not conducive to be run concurrently with other panels on an existing automated line which is dedicated to more “average,” thinner boards. Additionally, the average board, rack, anode dimensional “windows” will typically be inadequate to accommodate these much larger panels. It is also likely, if processed on a manual tank, that vertical spline racks will be a necessity on both sides of the panel, due to the physical challenges of carrying the weight, and adequately bringing current to the board. It is interesting to note, also, that most recently designed/built automated platers utilize top loading racks only. This will likely be a difficulty requiring inherent flexibility in the control systems, where different workbars with different racks can be brought to potentially different plating stations, and allow the work to be plated for widely varying plating times.

There are various sorts of issues that need to be discussed (chemical, physical, scheduling, etc.,), however, in this first article, we will limit our discussion primarily to SOME of the many “physical” issues unique to plating backpanels. The number and kinds of physical issues should NOT be minimized, as these can be far reaching. For example:

• A pwb manufacturer considering the purchase of a new automated line which MAY need to run
  some backpanels, should allow for sufficient depth to accommodate a potentially deeper board. It may or may not make sense to provide this added depth and elongated anodes in every cell, which goes back to the matter of the inherent intelligence of the main control system.
  
• Similarly, one must determine if the plating line will be equipped in whole (or part) with
  vibration, as the more robust and flexible of these can be very expensive. Therefore, one may wish to provide only specific workbars (or specific plating stations) that may be used for backpanels or boards with very small holes, thus saving money. Additionally, one must decide whether to use exclusively vertical spline racks rather than top-loading types, recognizing that the latter DO have an advantage in terms of surface distribution due to their “continuous” surface (no interruption). Our experience has been that top-oading racks will work for backpanels but only when there is either a bottom support member, or tooling provisions are made to actually “engage” the panels with registration holes at the top, insuring the boards cannot be vibrated “off”
  the rack. There is a debate (even with bottom support and tooling) about whether vibration using top-loading racks can be as effective as when using top quality vertical racks.
  
• There are many designs for vibration, some on the workbar and others on the tank rim or support structure. There are even patented designs requiring expensive frequency drives for each station allowing the workbar (via the main control system) to cycle through a wide range of frequencies, thereby optimizing the results).
  
• Depending on whether one elects to panel or pattern plate the boards, one should re-check the assumptions relative to maximum panel circuit density so as to determine if existing rectifiers (and/or pulse equipment) is adequately sized. Two things to consider here:
  
• Backpanels (vs. normal pattern plated work) often have large groundplanes on one side which require more amperage.
  
• Backpanels often have significant numbers of holes. This can be very deceptive, as the surface area of a large number of holes in a very thick panel can, in some cases, exceed the surface area of the circuitry on the panel, and often does. Unless thought about in advance, this might mean that rectifiers on an existing line need to be replaced to accommodate backpanels.
  
• If pulse plating IS utilized (beware that this is, in and of itself, a VERY involved topic that will only be minimally touched upon in this first article), then one will, by definition, be plating at a much higher current densities than with DC plating, another instance where existing rectifiers may be very much undersized!
  
• The design and quality of racks is also a potential concern, especially if pulse plating, as one needs to be certain there is adequate current carrying capacity, and that the integrity of the contacts are sufficiently robust to both transmit the current and vibration.
  
• Another advantage to using vertical racks with backpanels is the ability to design these with the ability to readily affix the required “shielding” or “thieving.” Unlike simpler panels that are normally “mass-produced” and even automatically loaded and unloaded, one should anticipate that the “payload” on a workbar of backpanels will likely be less. The value added on these types of panels dictates the care that needs to be taken when processing (vs. simpler panels).
  
• The additional amperage loading on the system required by these higher amperage panels
  (especially if using pulse) also requires investigation of existing tank saddles, on-tank bussing (and if pulse is utilized the potential of inductive loads), cooling, etc.,
  
• Consideration of methods of both mechanical and solution agitation are also much more critical with backpanels as the need to transport and avoid depletion of the metal ions within the deeper holes is more challenging. Hopefully, this first article which discusses the more obvious “physical-only” issues relating to plating backpanels (vs. thin, less complicated boards), provides the reader with a preview of some of the more involved technical challenges that can be anticipated with these kinds of products. In the next article we will attempt to visit some chemical issues, and touch on IST testing and the
  physical properties of the copper electroplating deposit, as well as the advantages (vs. additional challenges) of utilizing pulse when plating backpanels vs. DC plating. While the results with pulse are significantly better than with DC (see the cross section this page), one MUST view this additional challenge within the context of truly a “total systems’ approach,” as ALL of the physical issues discussed here are still in effect, but further complicated with when utilizing pulse.

By
Neil Baker
President
Baker Technology Associates Incorporated
Respectfully Submitted:
Jacket Wu
Executive Vice President
U-TAH Industrial Co., Ltd.