Arc Season and Board Design Observations
John Maxwell, Director of Product Development, Johanson Dielectrics Inc.
Enrique Lemus, Quality Engineer, Johanson Dielectrics Inc.
Presented at CARTS 2006 |
| This years arcing season is just now fading into bad memories as another year is past but
knowing that a new season is just a Monsoon away. ARC season is well known to high
voltage capacitor vendors and users alike and with the drive to smaller high voltage (HV)
components like ceramic capacitors we know that the next season will arrive sooner and
last longer. From a vendor’s standpoint parts are shipped that meet the published
specifications but when the customer mounts those same capacitors on an assembly, high
voltage problems can arise. There may be interactions between the board and flux
residue, humidity absorption of both the PWB (Printed Wiring Board) and flux residue,
what type of flux is used in the solder paste and the high voltage capability of the board
itself. These are the areas that need to be investigated. |
Field Problems
Problems include test equipment correlation problems, test methods used for HV testing,
but the big source of defects appear to be from a combination of the board layout,
assembly process, choice of solder paste flux and cleaning where required. A large
number of contract manufacturers around the world use water soluble flux solder paste
because it is much more aggressive that No-Clean flux allowing for wider reflow process
windows and minimizes component solderability problems. Unfortunately organic acids
used in water soluble solder paste are very active and if the assembly is not adequately
cleaned those flux residues start their damage and are difficult to remove. Now we are
faced with high leakage currents, low arc voltages and assembly failures. Add humidity
and we have a new ARC season. An interesting ARC season failure is shown in Figure 1.
The board was reflow soldered using water soluble solder paste but not completely
cleaned. The board design used a 2mm slot beneath 1808 case size high voltage safety
capacitors. The use of slots are a typical high voltage board design technique. Complete
and thorough cleaning of all flux residues is mandatory when this design technique is
used with water soluble flux. There were droplets of flux residue on the assembly bottom
and visible flux residues on the bottom of all 1808 size safety capacitors. The burned arc
path shown in Figure 1 is along the slot edge in the PWB following flux residues. |
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The Experiment
High voltage 1206 3kV 120pF X7R capacitors were mounted on boards with 4 variations
of pad/solder mask/slots to evaluate the impact common board layout techniques on AC
breakdown voltage. A 5 mil thick stencil was used to replicate the customer environment.
Stencil apertures were 95% of the pad area. No-Clean and water soluble solder paste
formulations were evaluated to determine the interaction between flux and board layout
after reflow soldering and after humidity exposure. An exposure time of 48 hours at
85oC/85%RH was chosen to drive moisture into the board and flux residues trapped
beneath the chips. The capacitors were not biased consistent with most high voltage
isolation applications typical for smaller safety capacitors.
These capacitors chosen for this experiment are 1206 case size X7R 120pF and are
designed for 1500VAC withstanding voltage commonly found in larger safety capacitors.
A 107 piece sample of these parts was tested to AC breakdown with the results shown in
Figure 1. |
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Board layout variations included solder mask between pads, no solder mask between
pads, solder mask with a 0.040” (1mm) wide slot between pads and no solder mask with
a 0.040” slot between pads. A larger slot was not practical for 1206 sized capacitors
because of pad to pad spacing of 0.080” (2mm) was used. Figure 3 shows each layout
variation for reference. Removing solder mask from between pads allows solvent access
during cleaning when water soluble flux is used and minimizes flux flow between the
capacitor pads due to capillary forces during reflow soldering.
Square pads were used as they are most commonly encountered on end customer’s
boards. Additionally square pads are the easiest to draw in board layout programs. Each
evaluation board had 80 positions for each pad/solder mask/slot variation for increased
confidence levels in the results. Bare board arcs were from pad corner to opposing pad
corner as expected as square pad corners will have the highest electric field gradients. |
 |
| Two different solder paste and flux chemistries were used in these experiments and are
listed in Table 1. These were chosen as they were readily available and are typical of
solder pastes commonly used during board assembly. |
Table 1. Solder Paste Used in Experiments
| Paste Description: |
Type of Paste: |
| ROL0 ANSI/J-STD-004 |
No-Clean |
| ORH0 ANSI/J-STD-004 |
Water Soluble |
|
The experimental procedure consisted of exposing both bare boards and assemblies to
AC breakdown. A bare board as received was tested for AC breakdown voltage
distribution. A second evaluation board had Sn63 No-Clean solder paste stenciled on the
pads and reflowed. Those results are listed in Appendix 1 and are used as a baseline for
comparisons with capacitors mounted to boards with different fluxes and humidity
exposure. What is interesting is that the board AC breakdown capability significantly
increased after exposure to reflow soldering temperatures. After components were
soldered to the boards and exposed to 48 hours of 85oC/85%RH those breakdown
voltages returned to values consistent with the bare board in an as received condition.
Real world assemblies will absorb moisture after soldering and depending on where the
assemblies are manufactured, stored and the 48 hours of humidity exposure the results
inline with the board in its as received condition. The data for the bare board and those
with capacitors solder to it is consistent with reflow soldering drying out the assembly.
Appendix 2 lists AC breakdown results for capacitors reflow soldered to an evaluation
board with No-Clean solder paste, tested approximately 18 hours after reflow soldering
and an assembly that was exposed to 85oC at 85% RH for 48 hours and then tested 1 hour
after the board was removed from humidity exposure. Appendix 3 has assembled board
breakdown results approximately after two hours after cleaning. This assembly used
water soluble solder paste. The other breakdown data is for an assembly with capacitors
and water soluble solder paste, reflow soldered, cleaned and exposed to 85oC/85% RH
for 48 hours. This assembly was tested to breakdown within 2 hours of humidity
exposure. Table 2 is a summary all of the AC breakdown results shown in the
appendices. |
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Observations
Examining both the mean of the breakdown voltages and the actual distributions shown
in Appendices 2 and 3 are revealing. Solder mask between pads was consistently the
poorest performer in AC breakdown testing in these experiments. Slots between pads
were consistently the best performers as expected. No solder mask between pads was
comparable with the layout option of solder mask and a slot between the capacitor pads.
Figures 4 and 5 are the distributions for No-Clean solder paste with no solder mask
between pads and water soluble solder paste with solder mask between pads with a
0.040” (1mm) slot. Both assemblies underwent 48 hours exposure at 85oC/85%RH. |
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PWB Layout Suggestions
The data are consistent with eliminating high voltage slots where allowed and removal of
solder mask between pads. Figure 6 represents a chip soldered to pads with and without
solder mask showing the added gap between chip bottom and PWB surface. |
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| Flux residues are trapped between capacitor bodies and solder mask during reflow
soldering when the flux is liquid. This is due to capillary forces wicking the flux between
the solder mask and capacitor body. Figure 7 shows three parts removed from a board
that suffered ARC season defects. The flux is clearly visible in this photo. The parts were
removed by placing the assembly on a hot plate and were plucked off using tweezers
when the solder reflowed. These particular boards had a 2mm slot between pads but
failed 1500VAC during HiPot testing. Bottom side of capacitors showing flux residues
trapped between capacitor bodies and solder mask. The flux flow stopped at the PWB
slot and has arrows pointing to the flux gaps. |
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| In addition to eliminating solder mask between pads, the pads themselves should have
radiuses instead of rectangular corners. All arcs emanated from pad corners due to those
locations having the highest electric field gradients when exposed to high voltage fields.
Adding a radius at each corner will reduce field gradients aid in high voltage
performance. Suggested pad radiuses are shown in Figure 8. |
 |
Summary
The data are consistent with cost savings by eliminating slots where possible by
eliminating solder mask between pads. There is the additional benefit of creating cleaning
access if water soluble solder paste is used in board assembly. And finally spreading out
field gradients by adding a radius to pads can aid in improving AC breakdown
performance. Each layout suggestion needs to be evaluated in new designs as each design
will have unknown quirks.
As a side note high voltage components should never have traces beneath the component
body and voltage creepage requirements to eliminate arcing to adjacent components and
traces. |
| Presented at CARTS 2006 |
| Notice: Specifications are subject to change without notice. Contact your nearest Johanson Dielectrics
Sales Office for the latest specifications. All statements, information and data given herein are believed
to be accurate and reliable, but are presented without guarantee, warranty, or responsibility of any kind,
expressed or implied. Statements or suggestions concerning possible use of our products are made
without representation or warranty that any such use is free of patent infringement and are not
recommendations to infringe any patents. The user should not assume that all safety measures are
indicated or that other measures may not be required. Specifications are typical and may not apply to all
applications. |
| |
| Appendix 1. Bare Board and Sn63 NC Solder Paste AC Breakdown Results |
| Appendix 2. AC Breakdown Results for Capacitors Reflow Soldered w/Sn63 |
| Appendix 3. AC Breakdown Results for Capacitors Reflow Soldered w/Sn63 Water |
