Take a look into the reliability of plated through-holes and compare the impact of various amounts of copper plating in the through holes. Results of the thermal stress testing will be presented. Learn about the critical factors of Heavy Copper Printed Circuit Boards for determining the appropriate Heavy Copper elements such as acceptable heat rise, copper thickness, hole-size, and if capable of support vias.

1. Reliability of Copper PTH
For High Current
Applications
10.02.14
 DELIVERING QUALITY SINCE 1952.

2. 2
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

3. 3
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.

4. 4
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 under
thermal strain.
– Improved heat transfer to
heatsinks.
– Increased mechanical strength
in Z-axis at connector sites and
at other PTHs which are used
as mechanical attachment
points within the final assembly.

5. 5
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.

6. 6
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 instead
increase the PTH copper plating thickness
to 2-ounce nominal (.0025 minimum.)
Published TCT results indicate that, with
all other factors remaining the same, this
step alone will result in a reduction in
failures to approximately one half of one
percent after 8 cycles.

7. 7
 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 includes
recommendations and formulae
for calculating the appropriate
trace widths and hole diameters
for your design.
 PCB CAD tools include thermal
modeling capability, as do some
standalone calculators available
online. Try to use a tool based
on IPC-2152, as opposed to
one based on the earlier
general design specification,
IPC-2221.
Current Capacity

8. 8
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 provide
equivalent current handling and cooling.

9. 9
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.

10. 10
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.

11. 11
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.

12. 12
Heavy Copper Thermal Pad
Under-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.

13. 13
Heavy Copper Thermal Pad
Designed 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.

14. 14
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.

15. 15
Our Products
Battery Packs Flex & Rigid-Flex PCB’s User Interfaces
Fans & Motors Cable Assemblies Printed Circuit Boards

16. 16
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.

17. 17
Q&A
 Questions?
– Enter any questions you may have
in the Control Panel.
– If we don’t have time to get to it, we
will reply via email.

18. 18
Thank You
Check out our previous webinars at www.epectec.com.
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