Backplane X-ray Aanalysis
This is one of those things that drive
people in this business mad. A very expensive and difficult board was manufactured
successfully. All the electronic and environmental testing was
completed and the board went into service. Months later the board began to perform erratically in the field.
Of course the failure was "high
visibility" and everyone was under the gun to find out how and why this
is happening. And fix it!
The customer urgently directed all possible
resources to bear and after a week or so of testing found the source of
this very sneaky problem.
They discovered a dual challenge: a very thick
board and leads that were (on occasion) contaminated - most likely
There was only one component type involved
and this component was placed at about a dozen locations per board and
not every component demonstrated the problem.
|Figure 1: The lead and plated hole center-bottom has insufficient solder fill.|
What happened was, at the
affected holes on the through-hole component, barrels incompletely
filled with solder, which caused the solder connection with the
component lead to occasionally fail after the board had been in the
field working for some time.
When the boards arrived at Circuit
Technology Center our challenge was to first identify the holes that
were inadequately filled and fill them. At the same time the customer
wanted to ensure that all pins on the problem part were reflowed to
ensure they would forevermore demonstrate proper wetting.
Once this was
complete, there had to be proof that the holes were filled to
everyone's satisfaction. To complicate matters these boards were covered
with a thick conformal coat.
The rather large board was placed, solder side up, at about a 30 degree angle to the x-ray emitter head
in order to permit full view of the barrel length. Once the board was under view, each location was photographed and
a marker placed on the photo noting the holes that were clearly
Figure 1 is an example of an x-ray snapshot of a plated hole and lead with insufficient solder.
Now came the hard part. It's easy enough to talk about reflowing
ground planed pins on a .150-inch thick conformal-coated board, but to
actually do it?
In order accomplish this task without damaging non-affected areas,
the board had to be prepared by placing thermal resistant tapes and
heat deflecting material on the surrounding surface areas and components
to protect the unaffected components and the solder side conformal
A vacuum desoldering tool was used to remove the conformal
coating from the solder-side leads to permit flow and fill at those
locations. The interesting thing is that this rework was going to be
performed on a BGA rework machine in order to maintain the board at a
steady state high temperature using the BGA rework machine's bottom
heater plates to warm the whole substrate and the topside air nozzle to
drive heat into the specific rework location.
In order to maintain control of this volume
of heat, thermocouples were placed under the rework locations.
Thermocouples were also placed to monitor the heating of sensitive
devices near the rework area.
The board was slowly heated until the proper base
temperature was reached then a vacuum de-soldering iron was used to
suction out the existing solder ridding the barrel of any suspect
The final step of the soldering phase was to add solder at the
target barrels using the soldering iron and wire solder. This process
was repeated until all affected plated through holes were properly
The board was then cleaned in a de-ionized water washer. Once again the board was x-rayed at every location.
The rework environment was hot and demanded great patience
and skill from the technicians involved. It ain't always
easy, but somebody's got to do it.
Several members of the Circuit Technology Center team contributed to this feature story.