Printed Circuit Board Assembly

PCB Assembly

BGA Pework Process in PCBA

  Since the late 1990s ball grid array (BGA) packages have been gaining as a preferred package style
for several reasons. Their IO density compared with the previous high density ultra fine pith quad flat
packs (QFPs) is such that they have shrunk the necessary footprint on the PCB by a factor of 50%. If
we lump the BGA style of package, a high density IO with solder balls for the interconnection in with
its stacked version counterpart the POP then this increase in density approaches nearly 100%. The
ability to get more done in less space along with shorter trace spacing and length requirements for
board layouts using this package style have allowed boards to be clocked at much higher rates
thereby increasing processing speed. In addition the reliability of the placement of BGAs has been
high as initially the tin lead solder balls “self centered” on the pads during reflow. Lastly the reliability
of BGA packages has been increased with the use of underfills, the use of specialty flex solder balls
and even solder columns with integrated springs.

  The increased use of BGAs and the underlying trend of ever-smaller package sizes, finer pitches and
their placement on to ever denser printed circuit boards has led to greater and greater challenges in
BGA rework. In addition to these challenges there are many others that have made the job of BGA
rework technicians more and more difficult. One of the trends in making BGA rework more difficult is
their ever-increasing usage in handheld device. Due to the drop test requirements of these products
in many cases the BGAs and other higher density devices need to be underfilled. Underfilled BGAs
have the challenge of the tacky material “squirt out” causing solder shorts underneath the BGA. In
addition, the tacky nature of this material tends to lift pads as well as destroying the underlying solder
mask underneath he BGA. This makes the BGA rework even more challenging. The ever-thinning
device packages of BGAs cause the packages to warp. This too makes BGA rework difficult.

  While large packages with large-sized pitches placed on sparsely populated printed circuit boards
made BGAs simple to remove and replace, BGA rework today requires a greater level of machine
sophistication. With pitches down in to the 0.3mm areas and package sizes routinely under 10 x
10mm placement with a vision system using a highly precise and repeatable XY motion system is
required. Placement accuracies for today’s BGA rework need to be in the less than a 1mil tolerance
range. In addition with the lead free packages the rework system now require sophisticated
temperature control with programmable multi zone bottom heaters, nitrogen capability and low flow
rates at the air nozzle. In addition to the rework equipment set the inspection equipment needs to be
of a higher capability. X-ray inspection equipment with very small spot sizes is a requirement for BGA
inspection post BGA rework. The ability to measure the sphericity of the solder balls, the solder ball
diameter, the ability to shoot through RF shields as well as higher density ground planes is now a
necessity for the x-ray equipment requirements for BGA rework. Also the endoscope is a necessity for
checking the ball collapse, the surface of the BGA ball post reflow, the wetting action as well as other
attributes is important in BGA rework.

  Besides the equipment requirements for modern day BGA rework, the skill level, dexterity and
process knowledge of the rework technicians working on BGA rework is even more demanding today.
The BGA rework technician needs to understand reflow profiles in order to deal with printed circuit
boards with a high density of ground planes. The BGA rework technician must also understand flux
chemistries and how this impacts the kind of cleaning which can be done underneath the device. The
BGA rework technician must also be able to understand how a variety of conformal coatings can be
removed from the PCBA as well as underneath the BGA. BGA rework technicians must also
understand how device which neighbor the BGA to be reworked can or could be impacted by the BGA
rework process profile.
  As BGA packages have become more widely-accepted the BGA rework process has become more
difficult. This has meant that both the equipment used in BGA rework as well as the technicians doing
the rework have had to become more sophisticated.
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