(6)The tension between solder points produces good self-centration effect in solder-
reflow, allowing 5% error of patch precision.
(7)Compatible with original SMT mount technology and machines. The original
screen printer machine, mounter and solder-reflow machine all can be used. The
comparison between BGA and QFP is shown in Table 1. The main defect of BGA is
the necessity of X-ray detection after welding.
c. Welding of BGA
The success of welding BGA is affected by the assembly of PCB. So 3 key
factors must be considered in the layout of PCB.
Features BGA QFP
Package size S/m
2
525 1600
Pin space L/mm
1.27
0.50
fault rate (×10
- 6 / p c s
）
0.60 100
(1)Thermal management
Thermal management must be considered when we design the assembly of PCB.
If many pieces of BGA gather on a small area of PCB, it will cause the imbalance of
PCB in the reflow oven. If many big pieces of BGA gather on a certain area of PCB,
longer heating period will damage some of the BGA. The areas with fewer
components have reached the welding temperature, while those areas with BGA are
still in low temperature. So the welding period is over before the flux paste
withdraws from the welding spots, thus there may be poor soldering or no-melting
of welding balls.
(2)Rabbet
The design of rabbet on PCB should stick to relevant standards strictly. Any
rabbet near BGA welding pad must cover the solder mask very well. If not, the extra
welding paste will flow from the welding pad to the rabbet, then leads to short
circuit between welding pad and rabbets nearby.
The geometric shape and diameter of welding pad:
The compactness of pins has immediate impact on the geometric shape and
diameter of welding pad. Similarly, BGA has different sizes, shapes and complexity.
As the package size decreases, the geometric shape and diameter of the welding pad
calls for higher precision in detection.
3. Inspection standards of BGA
For BGA assembled on PCB, inspection standard is a very important issue.
Before BGA is applied to the PCB, most BGA manufacturers don't use X-ray in
process of inspection. Instead, they employ some traditional methods to test PCB,
such as automatic optical inspection, man-eye inspection or on-line function test.
However, these methods can't detect the invisible problems very exactly, such as
poor soldering, cold welding and bridging. The X-ray can discover these problems
very efficiently. At the same time, it can do live monitoring, guarantee the quality,
and realize the in-time feedback of the process control,
a.
Evaluation of X-ray
When BGA is first used on BGA, X-ray can evaluate the solder-reflow according
to the situation of un-solder area or areas around it. Open-circuit, non-touching and
other similar situations show the solder paste has not reflow enough. Bridging may
result from the fact that the temperature is too high for the solder to liquefy. So the
solder flows from the pad to the interval between them, causing short-circuit.
Poor welding should be evaluated objectively: it is not so terrible; the key
4
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