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Reliability of Copper PTH

For High Current Applications

10.02.14

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Agenda

Reliability Concerns Involving Through Hole Plating

Advantages of Thicker Copper Plating in Through Holes

Test Procedures and Shared Results

Current Capacity

Other Process Choices and Layout Tips For Heavy Copper Designs

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Overview / Reliability Concerns

Most commercially available PCBs are manufactured for low-voltage, low power applications, and use copper trace and plane weights from ½ to 3 ounces. Copper plating in holes is typically between ½ to 1 ounce.

A heavy copper circuit is manufactured with weights ranging from 4 to 20 ounces or more. The holes on these boards will be under a lot of stress. It’s necessary to think about them a little differently.

Because of the extra current handling requirements, and the heat that is generated in these high-powered designs, adding enough extra copper into the plated through holes is important for long term reliability.

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Real World Advantages of Thicker Copper Plating in Through Holes

Extra copper in the barrel offers several advantages, including:– Increased current carrying capacity.

– Increased reliability underthermal strain.

– Improved heat transfer toheatsinks.

– Increased mechanical strengthin Z-axis at connector sites andat other PTHs which are usedas mechanical attachmentpoints within the final assembly.

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What Thermal Cycle Testing Shows

Analysis of 1-oz plated holes on FR4 using Thermal Cycle Testing (TCT) shows that around 32% will fail after 8 thermal cycles. Failure is defined as an increase in resistance of 20% or more.

Increased resistance through a PTH indicates the presence of thermal cracks. The most common defect is corner cracking, caused by the difference in CTE between the copper and the laminate.

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Increase Copper, Increase Reliability

Use of exotic materials with CTE ratings more closely matched to that of copper can reduce the failure rate (to as low as 3% for Cyanate Ester) but the cost of such materials can range from 5x to 10x the cost of FR4. Exotics are also more difficult to process in general, making them a less than ideal solution in cost-sensitive applications.

A much less costly approach is to insteadincrease the PTH copper plating thicknessto 2-ounce nominal (.0025 minimum.)Published TCT results indicate that, withall other factors remaining the same, thisstep alone will result in a reduction infailures to approximately one half of onepercent after 8 cycles.

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For heavier copper PCBs, the current requirements will generally be much greater than for conventional PCBs. It may be necessary to plate considerably more than 2 ounces of copper in order to meet the requirements.

IPC-2152 is the most up-to-date industry specification for determining current carrying capacity. It includesrecommendations and formulaefor calculating the appropriatetrace widths and hole diametersfor your design.

PCB CAD tools include thermalmodeling capability, as do somestandalone calculators availableonline. Try to use a tool basedon IPC-2152, as opposed toone based on the earliergeneral design specification,IPC-2221.

Current Capacity

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Process Suggestions Based On IST Testing

IST (Interconnect Stress Testing) suggests that using the following guidelines will further improve results:– A small hole’s aspect ratio will increase as the hole closes down during the

plating cycle, so use larger vias for heavy copper plating. This will result in more even plating through the hole wall. Ideally, the FHS aspect ratio should remain below 8:1 throughout the cycle.

– Place multiple vias rather than one large hole when possible. The multiple interconnections will contribute to the Z-axis stability.

– Calculate the quantity to provideequivalent current handling and cooling.

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Finish Choice

HASL is not recommended for heavy copper PCBs, primarily because the circuit features stand higher than normal above the substrate. The circuitry can reduce the air knives’ effectiveness as they blow the excess solder from the PCB.

ENIG is the preferred surface finish for heavy copper. The CTE is similar to that of the other elements of the via structure. The PCB below uses 7-ounce copper in the holes.

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Understanding Fabrication Adjustments

Fabricators adjust your design based on known process behavior. Holes are typically drilled about .005” over FHS to allow for plating. Traces and pads are adjusted for anticipated etching loss by

approximately .001 per ounce of copper foil. Below are adjustments typical for a typical (light copper) PCB.

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Heavy Copper Hole &Trace Fabrication Adjustments

For heavy copper, the adjustments are more extreme. The example is 8-oz. It is good practice to design-in slightly more annular ring, as etching can be

less precise than on standard, light copper PCBs. The drill diameter is adjusted by (+.005 over FHS, + .001 per ounce extra. ) Holes may still close down below spec. If so, they can be re-drilled to remove

excess copper before the surface finish is applied.

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Heavy Copper Thermal PadUnder-Designed

A common layout oversight that can lead to processing problems is failure to adjust padstacks and keep-out values to account for some of the unusually large adjustments the fabricator must make. Here the air gap of thermal pad will not process as intended, and could create a soldering problem later.

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Heavy Copper Thermal PadDesigned For Fabrication

When designing thermal pads, add at least .001 per ounce to the size of your keep-out / air gap. This will prevent the air gap from closing down excessively when the fabricator adds the etch allowance, and will ensure that your pad will not behave as a heat sink during assembly soldering.

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Summary

Additional copper in the hole wall increases reliability, and is necessary for high-current PTHs.

Use IPC-2152 to determine the amount of copper required for a given hole diameter.

Use multiple vias to increase electrical and mechanical reliability.

Understand your manufacturer’s capabilities, and observe published minimum design rules.

Whenever possible, use values larger than the minimums, to increase processing window and yield.

Discuss your design with your fabrication house ahead of time. Heavy copper PCBs present unique challenges. The fabrication house will often have ideas for improving the process sequence for a more repeatable result.

Contact Epec.

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Our Products

Battery Packs Flex & Rigid-Flex PCB’s User Interfaces

Fans & Motors Cable Assemblies Printed Circuit Boards

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Design Centers & Technical Support

Battery Pack & Power Management – Denver, CO User Interfaces – Largo, FL Fans & Motors – Wales, UK PCB’s – New Bedford, MA & Shenzhen, China Flex & Rigid Flex – Toronto, Canada Cable Assemblies – New Bedford, MA

Our Engineering and Design teams are ready to help our customers create world class and cost effective product solutions.

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Q&A

Questions?– Enter any questions you may have

in the Control Panel.

– If we don’t have time to get to it, wewill reply via email.

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