Advances in solder ball placement for surface-mountable packages
uniformity outweigh the cost, so many would prefer to place onto much simpler, Alignment accuracy is an important
assemblers are now looking in this two-dimensional, flat substrates. consideration, too. Usually some kind
direction. of vision recognition system drives
Singulated substrates mechanical assemblies to the correct
Throughput Potentially quite small to handle in a location for solder ball transfer. Clearly
As volumes increase, more and more conveyorised machine (can be less than this needs to be performed to an accuracy
pressure is placed on the equipment 10x10 mm), these parts will often be better than 20 µm to ensure minimal
vendors to offer faster cycle times or larger loaded into JEDEC trays or Auer boats for reflow defects for the smaller diameter balls
placement areas whilst maintaining the processing. They can have components on discussed earlier.
same level of placement accuracy. Cost both sides, overmold or a lid on the non- Another factor to consider is that
of ownership models are used by assemblers ball side, and be based on ceramic, plastic, smaller pitches and ball diameters will
to lever price concessions or throughput or fibreglass/resin substrates. usually result in higher I/O counts per
enhancements from the equipment vendors placement cycle. A system whose modus
to reduce the capital equipment cost Silicon wafers operandi involves step and repeat placing
per part. With the growth of the WLCSP, high part of a wafer or panel each time may lose
volume ball placement onto wafers is out here, whereas a system employing the
Warpage proliferating amongst original equipment stencil printing method, whose throughput
Particularly problematic for overmolded manufacturers (OEMs) and outsourced and cost of ownership are relatively
strips, but also for laminated wafers, some semiconductor assembly and test (OSATs) unaffected by the ball count per component
singulated parts (especially for PoPs) and alike. Wafer challenges include 300 mm will have a clear advantage.
even panels have potential issues with wafers, as well as handling and tooling for
warpage, which is usually caused by the thinned or warped wafers. Throughput
different thermal expansion rates of the Production economics are pushing Most placement machines will have four
various materials used in the component, for more die per wafer. This also increases distinct process stages and throughput gains
most likely is the overmold or encapsulant the ball count per wafer and reduces the can be made on all of them:
which, due to the aforementioned ‘die-to-die’ ball pitches down to levels 1. Transport in & out
throughput requirements, may not be below the ‘within die’ ball pitch, creating 2. Vision alignment
given sufficient cooling time in the mold. further problems. 3. Flux print
This warpage must be either accommodated 4. Ball placement
or temporarily removed during the Technical solutions
placement process. Miniaturisation Throughput gains can be made in all
Another factor influencing warpage Unless a novel placement method is to four areas by the following means:
is the trend towards miniaturisation. be invented every few years, the most 1. Rapid transit, three stage conveyors for
As components and their substrates get important point to consider in order to high speed loading & unloading
thinner, their inherent rigidity is reduced, meet the challenge of the ever-shrinking 2. Linear motors and high-speed graphics
presenting further transport and handling IC is the build-up of mechanical tolerances processing for fast, fiducial capture and
problems. in the ball placement machine and its accurate alignment
associated tooling. A placement machine 3. A proven high-speed screen printing
Ball-side SMT that works well for 300 µm balls can be platform, optimised over many years in
Many of today’s packages, particularly SiPs made to work equally well for 200 µm SMT, to provide flux print cycle times of
and BGA modules have components on balls if all the potential inaccuracies in only a few seconds
the ball side of the substrate as well as, or the system are reduced proportionally. Of 4. I/O count independent placement
instead of, the other side (Figure 14). course this is easier said than done and method with a large placement area that
A similar challenge is created by some serious engineering is required, along facilitates larger panels, strips or wafers,
the window encapsulant of the WBGA with new materials and subsystems. Figure giving a higher part throughput
(Figure 5). Both of these conditions create 15 shows the result of such efforts, a 200
an additional hurdle for the ball place mm wafer placed with ~270,000 solder balls Warpage
equipment suppliers who, given the choice, of 200 µm diameter on a 300 µm pitch. There are really only three main ways
to deal with component warpage, which
can be surprisingly bad before component
singulation. Of course a system that can
use more than one of these methods
will be the most flexible and maybe the
most successful:
1. Vacuum this needs to be optimised for
the task at hand. For example, before
the part is fully captured on the tooling
(whilst there is a lot of vacuum leakage),
a high flow/low vacuum system can
be used to snatch it onto the tooling.
Assuming the capture is successful, then
Figure 15. 200 mm wafer placed with ~270,000 solder
later a low flow/high vacuum is better to
Figure 14. OSE Bluetooth module with ball-side SMT) balls of 200 µm diameter on a 300 µm pitch.
hold the part in place.
16 – Global SMT & Packaging - August 2008 www.globalsmt.net
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