FLAT-WRAP: A Novel Approach to Copper Wrap Plate

Estimated reading time: 13 minutes

Abstract

Copper Wrap Plate, as specified in IPC 6012B Table 3-2, is a requirement developed to enhance reliability for PCBs designed with via structures that require planarization and surface capping. PCBs built without wrap plating are more prone to failures associated with separation between the interconnection of the barrel copper to the surface copper. The improvement in reliability is a function of the copper wrap thickness, which supports the difference in IPC requirements for Class II and Class III programs. The general rule is "the thicker the wrap plating the better the reliability." The increase in copper thickness, associated with wrap plating, however competes with the ability for PCB fabricators to manufacture products with high density and fine features. The general rule for manufacturing fine features is "the thinner the copper the better the manufacturability."

The technology developed by DDI Corporation, called FLAT-WRAP™, offers a copper wrap solution that does not require build-up of copper on the external surface of a filled plated hole. This allows the improvement in reliability without sacrificing the ability to manufacture designs with high density and/or fine features. This technology also facilitates, in process non-destructive copper thickness measurements and ensures consistency of copper wrap thickness across the entire board surface. In this technology, the external surface copper thickness of filled plated holes will control the copper wrap thickness. In PCB designs requiring multiple copper wraps, the benefits of this technology are even more evident.

This article examines the current process problems with copper wrap plate and discusses the benefits provided by the new technology with respect to manufacturing and reliability.

Introduction

Multilayer PCB production is a constantly evolving, increasingly complex interplay of, processing techniques, customer demands, design rules and product specifications. Many times new processes will be added to meet certain demands, but are not easily and fully integrated into the existing process web. There is always a search for better ways to improve and simplify manufacturing processes. IPC added Copper Wrap Plate requirement to IPC 6012B specification, requiring copper plating from the filled plated hole to continue around the knee of the hole and onto the surface. This requirement was introduced to improve reliability for failures due to separation between surface features/caps and the plated hole-wall. The increased surface copper thickness due to copper wrap plate posed additional challenges for manufacturers to manufacture and designers to design the PCBs. This article highlights the current issues for dealing with the copper wrap requirements specified in IPC 6012B and the benefits of the new technology called FLAT-WRAP™.

IPC 6012 Wrap Plating Specification

IPC-6012B specifies that Copper Wrap Plating shall be continuous from the filled plated hole onto the external surface of the plated structure and extend by a minimum of 25 microns (984 micro-inches) where an annular ring is required. Figure 1 shows this requirement. Figure 2, shows that any reduction of wrap plating by processing (sanding, etching, planarization, etc.) resulting in insufficient wrap plating is not allowed. IPC-6012B Table 3-2 gives the requirements for copper wrap thickness. The continuous minimum wrap requirement for class 2 designs is 0.000197" and for class 3 designs is 0.000472".

Figure 3 shows a sketch of a PCB design with three different sets of blind vias sharing a common layer and having terminations on different layers built with conventional copper wrap process. Three sequential copper wrap plates are required for this type of design.

Reliability vs. Copper Wrap Plate

Figures 4 and 5 are cross-sectional views of filled plated holes showing varying degrees of separation between the copper barrel and their corresponding surface copper caps. The thermally induced separations are the result of insufficient or no copper wrap plating.

Figures 6 and 7 are cross sectional views of filled plated holes with sufficient Copper Wrap Plate. The holes in figures 6 and 7 were thermally stressed similar to the failed holes shown in Figure 4 and 5. These holes show no sign of separation between the copper barrel and their corresponding surface copper caps validating that copper wrap plate improves reliability.

Figures 8 and 9 are cross sectional views of filled plated holes with visible copper wrap plate but exhibiting separation between hole wall and the plated copper caps over the filled plated holes. The holes in figures 8 and 9 were thermally stressed similar to the failed vias shown in Figure 4 and 5. In these examples, copper wrap is evident, however the thickness is not sufficient to prevent separation. The hole size, type of fill material and board material, construction & thickness are functions of how much copper wrap plate may be sufficient to insure reliability.

PCB Industry Issues

PCBs requiring filled plated holes, with copper wrap plate are less prone to thermally induced separations between surface features and the plated hole-wall. However, conventional wrap plating increases surface copper thickness due to copper wrap build up over surface copper. This increased thickness makes it difficult for fabricators to produce PCBs with higher density and fine lines. IPC specification for Class 2 requires a minimum of 0.000197" continuous copper wrap from hole-wall onto the external surface of a plated structure and IPC Class 3 requires a minimum of 0.00047" continuous copper wrap. The problem here is, how to build high reliability PCBs without compromising the design rules that are required to fabricate HDI designs.

Table 1 illustrates the effect of copper wrap on fabrication design rules for line width and space. For example, starting with 3/8 oz copper foil, this particular fabricator shows a space requirement of 0.0035" to produce 0.003" features for designs that do not require wrap plating. In the same example, if two copper wraps are needed the space requirement increases from 0.0035" to 0.007" for Class 2 and from 0.0035" to 0.0085" for Class 3 (standard builds).

Fabrication processes such as lamination; plating and planarization have an effect on the consistency of copper wrap plating thickness across a PCB panel. Variation from these processes force fabricators to plate more copper on external surface of filled plated holes than required by IPC specification 6012B. This is done to insure minimum copper wrap is maintained across the entire panel which further reduces the capability to produce fine lines and spaces. Figures 10 shows copper wrap plating for a filled plated hole with three Class 2 wraps. The total surface copper thickness of three copper wrap plates and initial copper foil is 0.0014". This copper thickness is etched to create the surface copper features. Figure 11 shows the top of the BGA pad features illustrated by the cross-section view in figure 10. Starting with an imaged pad size of 0.022", the resulting finished diameter of the pad is 0.0176" as measured from the top. Likewise, figure 12 shows copper wrap plating for a filled plated hole with three Class 3 wraps. The total surface copper thickness of three copper wrap plates and initial copper foil is 0.0029". This copper thickness is etched to create the surface copper features. Figure 13 shows the top of the BGA pad features illustrated by the cross-section view in Figure 12. Starting with an imaged pad size of 0.022", the resulting finished diameter of the pad is 0.0137" as measured from the top.

Conventional Process

There are two commonly used conventional processes for processing PCBs with copper wrap requirements.

The less common technique is to image and plate a dot image around via fill holes and the barrel at the same time. After via fill, the bumps are partially sanded in order to leave an annular ring of copper wrap plate on the surface. The only way to measure the amount of Copper Wrap Plate is with destructive cross-sectioning technique. The complete lot is generally accepted or rejected based on one or two correlations between test coupons and the board area. Since each panel is planarized individually, cross sections from one panel may not represent the copper wrap plate thickness on other panels within the lot. With this technique, if the through-holes with wrap requirements are in a tight BGA area, it becomes very difficult for the photo resist to conform around pads/bumps for subsequent processing. This could ultimately cause shorts between BGA pads.

The most common Copper Wrap Plate process is to panel plate the hole-wall and the surface simultaneously. Plating on the surface must exceed the minimum required thickness of copper wrap plate to facilitate the planarization process. A selective barrel plate process is used to further plate the hole-wall to the customer specification. The holes after selective barrel plate are via filled and planarized to remove the excess via fill material and plated bumps around the filled holes to obtain a flat copper surface. This is done with care to leave sufficient copper wrap plate thickness on the panel surface in order to meet the minimum specified wrap thickness in IPC 6012B, table 3.2. Destructive cross sections are required to verify wrap thickness. Using this wrap process, additional wrap plating cycles will add additional thickness to the surface copper. Table 2 shows the variation of one Class 2 copper wrap with 3/8 oz. foil. The table illustrates the variation across one panel and from panel-to-panel within a 5-panel lot.

A Novel Solution to the Wrap Plate Issues

Flat-Wrap™ Technology offers a novel solution to the design and manufacturing issues related to copper wrap requirements. This technology offers reliability without limiting fabrication capability. The new technology also offers the benefit of verifying the wrap thickness without coupon correlation and destructive cross-sectioning analysis. With this technology, one can control the copper wrap thickness by changing the starting surface copper thickness. Figure 14 shows a sketch of a similar PCB design as shown in Figure 3 with three different sets of blind vias sharing a common layer and having terminations on different layers built with Flat-Wrap™ Technology. This design also requires three sequential wrap plate cycles.

Table 3 shows theoretical surface copper buildup for multiple wraps, comparing conventional Class 2, Class 3 and Class 3 with FLAT-WRAP™ Technology. With conventional wrap process, the surface copper thickness increases with every wrap plate cycle. With the FLAT-WRAP™ Technology, the surface copper thickness remains the same regardless of the number of wrap plate cycles.

Table 4 shows a comparison of surface copper thickness and finished pad size starting with half-ounce foil and an imaged pad size of 0.022" for Conventional Class 2 & Class 3 and Class 3 with the FLAT-WRAP™ Technology. The surface copper thickness is critical as this thickness of copper is etched to create copper surface features.

Table 5 illustrates the benefit of the FLAT-WRAP™ Technology on fabrication design rules with copper wrap requirements. Starting with 3/8 oz. copper foil, this particular fabricator shows a space requirement of 0.0035" to produce 0.003" features for designs that do not require wrap plating. Starting with the same surface copper thickness, the space requirement remains the same with the FLAT-WRAP™ Technology regardless of the number of wrap plate cycles.

Figures 15-17 show the advantage of controlled feature size produced by using the FLAT-WRAP™ Technology. Figure 15 shows the image photo-tool with a pad size (0.022") used to produce the pads shown in Figures 16 and 17 and previous Figures 10,11,12 and 13. Figure 16 shows copper wrap plating with the FLAT-WRAP™ Technology for a filled plated hole with three Class 3 wraps. The total surface copper thickness of three copper wrap plates and initial surface copper is 0.0007". This copper thickness is etched to create the surface copper features. Figure 17 shows the top of the BGA pad features illustrated by the cross-section view in figure 16. Starting with an imaged pad size of 0.022", the resulting finished pad diameter is 0.0190" as measured from the top.

Description of the New Process

In FLAT-WRAP™ Technology process, the copper wrap plate is created with an innovative approach that allows verification of the wrap plate thickness without any destructive cross sectioning. The wrap thickness can be measured by using a surface copper probe. This process also enables fabricators to verify wrap thickness across the entire panel and for each panel within a lot. Figure 18 shows that the wrap plate thickness is essentially the same as the initial starting thickness of the surface copper. Figure 19 shows a cross section of the entire filled plated hole shown in Figure 18.

Table 6 shows the variation of one Class 2 copper wrap starting with 3/8 oz. foil. The table illustrates the variation across one panel and from panel-to-panel within a 5-panel lot.

Figure 20 and 21 shows a schematic sketch comparison of a PCB design with three different sets of blind holes sharing a common layer that terminate on different layers. For this type of design, three copper wraps are required on the common layer. Figure 20 represents the design built with conventional copper wrap plate. Figure 21 shows the same design built with FLAT-WRAP™ Technology. Figure 21 compared to Figure 20, illustrates the benefit of creating multiple copper wrap plates without adding additional copper to the surface copper thickness.

Reliability Test Results of Flat Wrap Technology

The reliability of FLAT-WRAP™ Technology was demonstrated through a series of vigorous thermal testing methods including lead free assembly simulation. Sample coupons were tested using PCB industry recognized test techniques as follows:

1. Solder Float test (3X & 6X) at 500°F a

2. Lead free reflow assembly simu

3. Interconnect Stress Test (IST)

4. Highly Accelerated Thermal Shock (HATS)

The test matrix and test results are listed in Table 5, 6, 7, 8 and 9.

Summary of Benefits

• Flat-Wrap™ Technology allows the measurement of copper wrap thickness easily with a surface probe. This non-destructive method allows measurements to be taken anywhere on the board and does not require coupon correlation.
• Thickness of initial external surface copper determines the thickness of the copper wrap plate. Overall, surface copper thickness is independent of number of copper wrap plate cycles on any common layer for different filled plated hole structures. This benefit allows fabricators to build HDI designs that are difficult to produce with the increased surface copper resulting from conventional copper wrap plate.
• Flat-Wrap™ Technology provides IPC Class 3 copper wrap thickness starting with half-ounce copper foil.
• Copper wrap plate thickness has improved consistency within the panel and from panel-to-panel within a lot. This reduces the overall variation in etched feature dimensions.
• Flat-Wrap™ Technology provides thinner overall copper on plated layers, which allows more consistent and predictable dielectrics. This leads to improved impedance control and reduced thickness variation.
• Flat-Wrap™ Technology requires less additive plating and subtractive copper etching. This reduces generation of hazardous waste making it a more environmentally responsible process.
• Flat-Wrap™ Technology reduces the overall weight of the PCBs, which can be a benefit in some applications.

Conclusion

There is an increasing need to produce HDI-PCBs with smaller form factors that have higher functionality, better signal integrity and improved thermal management. Designers have adopted the use of blind, buried and through-hole vias as a method to satisfy this growing need. The advent of PCBs designed with via structures coupled with a variety of laminate materials, via fill materials and assembly thermal profiles led to new requirements for ensuring reliability. To address the reliability concern, IPC revised 6012 to Rev B with clarification in amendment 1 to include requirements for copper wrap plating.

The IPC amendment for copper wrap plating improves reliability of via structures. However, conventional techniques for copper wrap plate increase the surface copper thickness restricting fabricators from producing HDI designs with fine line features. The restrictions increase with the number of copper wraps required on common layers for different via-hole structures. In some cases, designs fabricated before the wrap plate amendment was put in place, can no longer be fabricated with the new IPC guidelines for copper wrap plate.

Flat-Wrap™ Technology provides a solution to the Copper Wrap Plate problems. Flat-Wrap™ allows for copper wrap plate without the build up of surface copper thickness. This aspect of this technology provides the benefit of producing highly reliable PCBs without sacrificing fabrication capability.

Acknowledgements

The author wants to thank the members of Technology Team, Operations and Management Groups at DDI, Corporation who all played an integral part to develop this new technology. The author also wants to thank the IPC 6012 committee for the opportunity to present this technology at the 2007 IPC Midwest conference in Chicago and IPC technical publications committee for the opportunity to present this paper at IPC/APEX Expo 2009 conference.