Page 142 - Building A Succesful Board-Test Strategy

128 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

Solder Volume by Assembly

xcess solder
UQL
(bridge)
E
UCL|

Insufficient
solder

Figure 3-32 Looking at both upper and lower control limits (UCL and LCL) as well
as upper and lower quality limits (UQL and LQL) allows the feeding of information back
into the process to prevent future defects.

3.4 Summary

The task of finding manufacturing defects in today's boards has become more
difficult than ever before. In-circuit and other bed-of-nails techniques suffer from
lack of access. For cellular-phone boards and other communications systems, the
bed-of-nails itself creates problems because of its RF characteristics. Product
evolution and the pace of change combined with skyrocketing complexity make
fixture construction and program generation even more painful than in the past.
Those same trends drastically reduce the likelihood that we will reach the "perfect
process" within the foreseeable future.
Adding inspection to the "test" strategy permits verification when you cannot
perform conventional test. It examines the board's structure, ensuring that you have
built it correctly. Test determines on some level whether the board works.
Inspection can occur after paste printing, after parts placement, and after
reflow. Visual inspection, automated-optical inspection (AOI), and laser and white-
light methods work best after paste. After parts placement, most companies select
manual inspection or AOI. After reflow, AOI and x-ray inspection—including
automated x-ray inspection (AXI)—produce the best results. Choosing among
inspection methods, as with test, depends on the nature of the boards and the
manufacturing process.