Viscom_Article_SP_3D_AXI_LEDs_EPP_Europe_en.pdf - 第2页

EPP EUROPE November 2018 25 I n electronics manuf actu ring, when there is a need to examine a print ed circuit board rejected b y a customer , an anomaly in production, or a prototype that has just been dev eloped, manu…

EPP EUROPE November 2018

Hidden structures clearly revealed

Fast in-line X-ray

secures quality

High-quality systems for automatic optical inspection (AOI) of electronic

assemblies offer as standard features both high throughput and 3D. For

hidden solder joints, more and more electronics manufacturers want to

employ in-line X-ray systems (AXI) as well. Again, it comes down to fast

assembly handling and first-class 3D image quality. One example is the

secure detection of voids as required in manufacturing of products with

light-emitting diodes (LEDs) and other applications.

COVER

EPP EUROPE November 2018

n electronics manufacturing, when there is a need to

examine a printed circuit board rejected by a customer,

an anomaly in production, or a prototype that has just

been developed, manual X-ray (MXI) is a good choice.

How long the inspection with any optional radiation pen-

etration angle between tube and swiveling detector fin-

ally takes is not a decisive factor. Because it is separate

from the production lines, it does not impact cycle times.

However, if a one hundred percent examination of hidden

solder joints is required, in-line X-ray systems are called

for. Typical examples of components soldered in this way,

where automatic optical inspection meets its limits, are

QFNs (quad-flat no-leads package) and BGAs (ball grid

array) as well as, e.g., the increasingly miniaturized LEDs.

Concept with parallel printed circuit board

handling

In-line X-ray systems must reliably point out defects that

are invisible to the human eye; furthermore, they are con-

stantly developing to keep pace with the rising demands

for speed in electronics manufacturing. For example,

today handling times as low as four seconds can be real-

ized; as a result, up to three printed circuit boards can be

present in the 3D AXI system at the same time. In order

to keep the X-ray inspection from unnecessarily slowing

down line beat, the faster 2D and 2.5D inspections can

be combined with 3D inspections as required. 3D tech-

nology is applied where it is most efficient.

Results without interfering structures

It is helpful for the inspection to gain slice images from

volume information. Both horizontal and vertical slices

can be extracted. Because of the extremely complex

overlapping, especially with printed circuit boards popu-

lated on both sides, this capability is essential. In this

case, with a 3D reconstruction, all significant features can

be made sharply visible in clear slice images – even for

shadowing by components or on multi-layer boards.

While details of the back side are always seen in 2D X-ray

images, the 3D slice image eliminates all such interfering

structures. This greatly simplifies inspection program

generation and enables effective false call prevention. For

improved verification, planar computed tomography of a

3D AXI system can be applied to generate partial or com-

plete volumes of the inspected solder joint, which can

then be tilted, rotated and zoomed on the monitor.

COVER

Viscom AG, from Hanover, Germany,

has developed an extremely fast

printed circuit board transport

especially for in-line X-ray inspection.

Source: Viscom AG

EPP EUROPE November 2018

3D AXI on the example of LEDs

When LEDs are examined more closely, it is immediately apparent

that a 3D AOI on its own is not enough to guarantee high quality in

the final product. In-line X-ray provides the ideal means, especially

to precisely inspect the associated hidden solder joints. Innumer-

able application possibilities from interior lighting, streetlights, and

displays are open for LEDs, and for industrial designers, a wide lati-

tude in implementing their lighting ideas. The automotive branch in

particular was quick to recognize the potential of this technology,

which has long since established itself as the standard in vehicle

lights, flashers, and headlights. Its inherent advantages include a

long service life, high light yields, and low energy consumption.

Heat dissipation is a decisive quality feature

LEDs emit not only light, but heat as well. Quick dissipation of this

heat is critical for their longevity. The electronics association IPC has

not specified any standards for heat dissipation in their acceptance

criteria. These standards state only that for components with con-

nections to heat sinks on their bottom sides, the criteria

for the hidden areas of the solder joints on the thermal

level must be determined by agreement between user

and manufacturer. For the heat transition layer, these

criteria depend on process and design, whereby the com-

ponent manufacturer‘s application notes, solder cover-

age, pores, solder height, etc., must be taken into ac-

count. Formation of pores and bubbles in the thermal

layer when these component types are soldered is nor-

mal. However, hollow spaces in the solder joints (voids) in

particular can prevent optimal dissipation of heat from

chips and semiconductor component barrier layers. The

life span (operating hours), wavelength (color), and even

the light yield (brightness) of the LEDs are strongly de-

pendent on the barrier layer temperature.

High efficiency under strict requirements

Especially in the automotive industry, manufacture and

quality control of printed circuit boards assembled with

LEDs are often sourced to EMS providers (Electronic

Manufacturing Services). They must meet the strict

requirements of their ordering customers and at the

same time, be as efficient as possible to stay competi-

tive. Here, brand manufacturers specify very precisely

just how high a percentage of voids in solder joints is ac-

ceptable. Process ovens with vacuum chambers and

other means can help prevent voids. Those that do form

can be very clearly identified with cutting-edge 3D X-ray.

While a flexibly applicable manual system is adequate for

random inspections, one hundred percent, fully auto-

matic inspection is performed by a system in the production line.

This way, voids can be completely recorded and analyzed, even

while hidden from an outside view.

X-ray and optical inspection supplement each other

Nevertheless, the importance of 3D AOI in inspecting LEDs is by no

means overshadowed. Especially in automotive applications, there

are very specific requirements for the positioning and alignment of

the components. Detecting defects such as minimum non-coplanar-

ity of the ever smaller LED components depends on reliable

measurement values. To support the absolute precision and repeat-

ability of 3D AOI, an exactly manufactured height gauge – measured

with high precision instruments by a specialized laboratory and cer-

tified – can be used as a target standard. Tilted components can be

detected with 3D AOI very well; precise measurements are also

achieved for rotations and X/Y offset. Thus, the AOI also makes a

decisive contribution to quality assurance.

Different inspection methods in the

optimized process

3D AOI and 3D AXI have been optimally incorporated in a single ma-

chine for many years now. Printed circuit boards move quickly and

nearly simultaneously between the two inspection sections and the

combination of different inspection methods can be versatilely ad-

COVER

Details of a BGA shown with the X-ray viewer of Viscom‘s operating software, vVision.

Source: Viscom AG

Comparison of 2D AXI (top row) and 3D AXI slice images (bottom row) with

three different LEDs as examples. The 3D AXI image quality convinces with

excellent void recognizability.

Source: Viscom AG