ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control

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Fig. 55

Typical thermal signatures for acceptable and defective soldered joints inspected by the laser beam

heating technique. Source: Ref 9

In acoustic microscopy, magnified acoustic images of the elastic structure of the surface or interior of a solid are produced

by passing high-frequency focused acoustic pulses through the material and displaying the received signals in image form

as shades of gray (Ref 10). Usually, much higher frequencies are used than in other kinds of acoustic imaging. The

acoustic pulses are focused directly, or the energy used to produce the pulses is focused into a very small region. When a

small area of a specimen is scanned with a series of focused pulses and the transmitted or reflected signals are viewed on

an imaging system (a television picture tube, a gray-scale recorder, or some other scanned display), the magnification of

the resulting image is equal to the ratio at the diameter of the display to the diameter of the area scanned. At present,

acoustic microscopy includes several methods for the formation of magnified acoustic images. The three discussed in the

article "Acoustic Microscopy" in this Volume are scanning acoustic microscopy, C-mode scanning acoustic microscopy,

and scanning laser acoustic microscopy. Acoustic images can also be produced by C-scan ultrasonic imaging as described

in the article "Ultrasonic Inspection" in this Volume. The following example demonstrates the use of combined x-ray

inspection and acoustic microscopy for determining soldered joint integrity.

Example 3: Determination of Silicon-Solder Attachment Integrity.

Two [111] cut silicon wafers, 40 mm (1.5 in.) in diameter and 0.32 mm (0.0125 in.) thick, were vacuum soldered

together. The solder foil was initially 25 m (0.001 in.) thick before melting. The sample was fabricated to study silicon-

solder integrity through the use of acoustic microscopy and x-ray shadow-graphs.

Sample Preparation. The sample was prepared in the following manner:

• A solder foil

was placed between the two prepared silicon wafer surfaces. The assembly was placed in a

vacuum apparatus, on a surface that could be heated electrically, with an axial load applied

perpendicular to the plane of the wafers

• A vacuum was applied (

135 Pa, or 1 torr), the sample was heated, the solder melted, and then the

assembly was cooled under vacuum. The x-ray image shown in Fig. 56

(a) was taken at this step in the

sample preparation. Figure 56(a) clearly shows a number of voids in the solder layer

•

Because of the voids in the solder layer, the sample was reheated, this time in a dry nitrogen glove box,

at atmospheric pressure. An additional x-ray image (Fig. 56

b) indicated that the large voids had now

been closed. This was possib

le at the modest pressure applied by the atmosphere because the voids

contained only vacuum. However, an acoustic microscope image (Fig. 57

), in addition to displaying

grain structure and small voids, indicated that the solder still was not well attached and had probably

never achieved good attachment over most of the initial solder foil area

Fig. 56 X-ray images of silicon-solder sample. (a) X-

ray shadowgraph after initial solder bond in vacuum.

Light-gray areas are voids in the solder. (b) X-

ray shadowgraph of sample after reheating at atmospheric

pressure. Note that most of the major voids have been closed. Courtesy of Robert S. Gilmore, General Electric

Research and Development Center

Fig. 57 Acoustic micrograph made with a 50-

MHz transducer focused through the silicon on the solder bond.

The focused beam diameter is approximately 75

m (0.003 in.). Courtesy of Robert S. Gilmore, General

Electric Research and Development Center

Inspection Procedure/Defect Analysis. The microfocus x-ray images were made at 100 kV with Polaroid film.

The wafer and film were placed at a source-to-target distance of approximately 750 mm (30 in.).

The acoustic micrographs were made with an ultrasonic microscope using a 6 mm (0.25 in.) diam 50-MHz transducer

with a focal length of 13 mm (0.5 in.) in water. Figure 57 was made by focusing the ultrasonic beam on the solder layer

and scanning it at a pulse-to-pulse and line-to-line spacing of approximately 25 m (0.001 in.). The gray-scale display is

of the amplitude of the ultrasonic echo from the solder layer. The solder grains are imaged because their elastic anisotropy

results in a different echo amplitude at the silicon/solder interface.

Conclusions. X-ray images clearly indicated voids where the pores contained substantial volume. However, because the

x-rays produced no difference in density, they could not detect a lack of attachment between a solder layer of uniform

thickness and the two wafers, where the lack of attachment was not accompanied by a pore volume. Small voids also

showed poor detectability when they resulted in an intensity change of less than a shade of gray (-3 dB) in the x-ray

transmission shadowgraph image.

The acoustic image (Fig. 57) clearly displays the attachment integrity of the final assembly as well as evidence of the

stages in its preparation. The grain size in the well-attached portion of the solder layer is clearly displayed, as are the

voids in this layer. The outlines of the original x-ray detected voids are indicated as well as the grain structure of the

solder now filling these areas and the location of a few small voids still remaining. The lack of attachment over large

regions of the wafer is indicated, and the fabricators of the sample verified that these regions clearly outline the original

solder foil area. It seems reasonable to conclude that a combination of x-ray and acoustic evaluations could supply the

most information on silicon-solder attachment, but for this specific geometry the acoustic images provided the most

information.

Cracked Soldered Joints

Cracks occur in soldered joints for a number of reasons, and each type of crack may have a distinctive appearance.

Among the causes of cracks in soldered joints are excessive mechanical loads, differential thermal expansion, foreign

material, improper joint clearances, and intermetallic compound layers.

Excessive mechanical loads produce fractures that are characterized by the nature of the applied stress. As the load

increases, the cracks become wider and deeper until fracture occurs. If the load on the base metal creates excessive tensile

shear loading on the solder, the specimen will exhibit a typical cup-and-cone fracture with the angle of the cone

approximately 45° to the applied load, as shown in Fig. 58(a). If the solder is overloaded in compressive shear, the

fracture surface will be parallel to the applied load, as shown in Fig. 58(b). If the joint was of questionable strength before

soldering, the inspection process itself may overload the solder and produce cracks.

Fig. 58 Fracture mode (a) in excessive tensile shear load of solder; cup-and-

cone fracture line is at 45° to the

tensile axis. (b) Fracture mode in excessive compressive shear load of solder; fracture surface is cylindrical and

is parallel to the load axis.

Differential thermal expansion can cause solder cracks; these generally consist of a series of closely spaced hairline

cracks. To eliminate this type of crack, design specifications concerning the choice of base metals, solder alloys, and

amount of thermal cycling permitted should be revised.

Foreign material such as flux inclusions and gas bubbles can act as nucleation sites for cracks. Small, random cracks

frequently radiate from the inclusion or bubble to the outer surface. The remedy lies in more care and skill in the

soldering operation.

Improper joint clearances may prevent proper flow of molten solder and thus create a joint of subnormal strength,

which is vulnerable to cracks. A clearance of less than 75 m (0.003 in.) may result in flux entrapment, poor solder

penetration, and voids. Small, random cracks may radiate from the voids to the outer surface. On the other hand, a

clearance greater than 125 m (0.005 in.) may impair the capillary flow of molten solder and also form voids. In this

situation, cracks may form parallel to the joint at the point of incomplete fill. The most satisfactory capillary flow of

solder is achieved with joint clearances between 75 and 125 m (0.003 and 0.005 in.).

Intermetallic compound layers formed in the solder may cause embrittlement and subsequent cracking. Generally,

the cracks are parallel to the surface of the part and open to the surface for a comparatively long distance. They may result

from high temperature, or excessive time at temperature, during the soldering operation, or from the alloy combinations

chosen for the joint. In high-reliability systems, for example, the connections are frequently gold plated before soldering

to ensure good corrosion resistance and adequate solderability. However, if the gold finish is only partly dissolved in the

solder, a layered structure may form that is very susceptible to cracking. This structure consists of layers of gold plate, of

the AuSn

intermetallic compound, and of solder. It is the interface between the soft gold and the hard AuSn

that initiates

the cracks. A printed circuit board connection with a crack between the gold plate and the AuSn

is shown in Fig. 59.

Fig. 59 Section through a cracked soldered connection on a printed circuit board. At A is the tin-

lead solder

with a thin layer of the intermetallic compound AuSn

(B) on its underface. C is the crack that opened

at the

interface between the AuSn

and a layer of gold plate at D. E is a layer of nickel plating over two layers of

copper plate (F) on the circuit board (G). 500×

Corrosion of Soldered Joints

Either galvanic or chemical corrosion can attack a soldered joint to a degree that may severely weaken its structural

integrity or completely destroy it. In electrical connections, the occurrence of corrosion in a soldered joint may either

increase the circuit resistance by impairment of contact or cause current leakage through surface deposits of conductive

corrosion products. Problems in relay contacts can arise because of deposits of nonconductive corrosion products on

conductive surfaces. Although the corrosion products due to many causes of corrosion are visible as white or green

powder or dust around the soldered joint, corrosion due to flux residues will not necessarily result in discernible corrosion

products.

Galvanic corrosion will occur when a sufficiently large difference in electromotive force exists between base metals or

between solder and base metal, provided that an electrolyte is present on the soldered joint.

Chemical corrosion of soldered joints is frequently caused by inadequate removal of corrosive flux residues left from

the soldering operation. Other causes of chemical corrosion include residual plating or etching solutions and

contamination from packaging materials, handling procedures (including human perspiration), and environmental

conditions. Additional information on the corrosion characteristics of soldering alloys and soldered assemblies is

available in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook.

Cleanliness in Soldered Joints

It is important that traces of flux or flux residues be removed during postsoldering cleaning procedures. In addition, many

solder operations are by nature contributors of small solder droplets. These small metallic particles can eventually be

dislodged in the assembly and can cause short circuits and other undesirable side effects. When an assembly is improperly

cleaned after soldering or fluxes are not removed, visual inspection is physically hampered. Cleanliness is therefore one

of the items that should be specified in any proper procedure for the inspection of soldered joints.

Developments in high-reliability low-current circuitry (especially printed circuits) have emphasized the importance of

cleanliness of electronic assemblies because of the danger of corrosion. Corrosion damage to conductors can increase

circuit resistance, and high resistance is undesirable. Corrosion can also cause physical failure of the conductors by

weakening and embrittlement. In addition, corrosion products themselves can cause current leakage. Current leakages are

particularly bad because they are not consistent; humidity changes in the atmosphere cause variations in and sometimes

intermittent occurrences of current leakage. Corrosion products also can cause contamination throughout the whole

system in the form of nonconductive deposits on mechanical contacts and relay surfaces. Corrosion is definitely a

problem, but corrosion is not always caused by fluxes. Additional information on the sources of corrosion in electronic

components is available in the articles "Corrosion in the Electronics Industry" and "Case Histories and Failures of

Electronics and Communications Equipment" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals

Handbook.

Cleanliness is easily checked after the proper cleaning procedures are followed. The danger to the assembly stems from

the presence of ionizable materials, mostly chlorides. The easiest and most thorough check is done with a conductivity

cell and distilled and demineralized water, either leaching the ionizable materials off the surfaces in premeasured amounts

of water or submerging the whole assembly into a container of the water and measuring the resistivity of the water. This

indicates the presence of ionizable materials. However, because chlorides are the major contributors in the corrosion

mechanism and are the most abundant form of ionic contamination on the assembly, standardized, simple silver nitrate

tests can be used to establish the presence of chlorides.

Cleanliness after soldering is important because soldering is usually the last step in a long process of assembly. However,

precautions must be taken to prevent recontamination of the surfaces; otherwise, the purpose of cleanliness and of the

check for corrosion products in the final inspection is defeated.

References cited in this section

R. Vanzetti, "Intelligent Laser Soldering Inspection and Process Control, Electronics Reliability and

Measurement Technology," NASA CP-2472, National Aeronautics and Spac

e Administration, June 1986, p

85-94

9. A.C. Traub, Parts Inspection by Laser Beam Heat Injection, NDT Int., Vol 21 (No. 2), April 1988, p 63-69

10.

R.S. Gilmore, Acoustic Microscopy, in Encyclopedia of Materials Science and Engineering,

Pergamon

Press, 1986, p 38-43

Note cited in this section

** Example 3was prepared by Dr. Robert S. Gilmore, General Electric Research and Development Center.

Nondestructive Inspection of Weldments, Brazed Assemblies, and Soldered Joints

References

1. T.M. Mansour, Ultrasonic Inspection of Spot Welds in Thin-Gage Steel, Mater. Eval.,

Vol 46 (No. 5),

April 1988, p 650-658

2. P. Kapranos and R. Priestner, NDE of Diffusion Bonds, Met. Mater., Vol 3 (No. 4), April 1987, p 194-198

P.G. Partridge, "Diffusion Bonding of Metals," Advisory Group for Aerospace Research and Development

(NATO), Aug 1987, p 5.1-5.23

4. G. Tober and S. Elze, "Ultrasonic Testing Techniques for Diffusion-

Bonded Titanium Components,"

Advisory Group for Aerospace Research and Development (NATO), July 1986, p 11.1-11.10

5. Diffusion Welding and Brazing, Vol 3, 7th ed., Welding Handbook, American Welding Society, 198

0, p

311-335

6. J.J. Munro, R.E. McNulty, and W.Nuding, Weld Inspection by Real-Time Radiography, Mater. Eval.,

Vol

45 (No. 11), Nov 1987, p 1303-1309

G.L. Burkhardt and B.N. Ranganathan, Flaw Detection in Aluminum Welds by the Electric Current

Perturbation Method, in Review of Progress in Quantitative Nondestructive Evaluation,

Vol 4A, Plenum

Press, 1985, p 483-490

R. Vanzetti, "Intelligent Laser Soldering Inspection and Process Control, Electronics Reliability and

Measurement Technology," NASA CP-

2472, National Aeronautics and Space Administration, June 1986,

p 85-94

9. A.C. Traub, Parts Inspection by Laser Beam Heat Injection, NDT Int., Vol 21 (No. 2), April 1988, p 63-69

10. R.S. Gilmore, Acoustic Microscopy, in Encyclopedia of Materials Science and Engineering,

Pergamon

Press, 1986, p 38-43

Nondestructive Inspection of Adhesive-Bonded Joints

Donald J. Hagemaier, Douglas Aircraft Company, McDonnell Douglas Corporation

Introduction

ADHESIVE-BONDED JOINTS in which adhesives are used to join and reinforce materials (Fig. 1), are extensively used

in aircraft components and assemblies where structural integrity is critical. Figures 2 and 3 show typical adhesive-bonded

joints used in aircraft structural assemblies. However, the structural components are not limited to aircraft applications,

but can be translated to commercial and consumer product applications as well. In terms of production cost, ability to

accommodate manufacturing tolerances and component complexity, facility and tooling requirements, reliability, and

repairability, adhesive bonding is very competitive when compared to both co-cure and mechanical fastener joining

methods.

Fig. 1 Schematic of a metal-to-metal adhesive-

bonded joint. (a) Adhesive sandwiched between two metal

sheets. (b) Analogy of adhesive-bonded joint components to individual links in a chain

Fig. 2 Typical applications of adhesive-bonded joints in aircraft. (a) Helicopter components. (b) Lockheed C-

transport plane, with various types of honeycomb sandwich structures totaling 2230 m

(24,000 ft

) in area

Fig. 3 Some typical adhesive-bonded joints used to j

oin components in structural assemblies. (a) Skin splices.

(b) Stiffener runout. (c) Bonded doublers. (d) Shear clip

Variables such as voids (lack of bond), inclusions, or variations in glue line thickness are present in adhesive-bonded

joints and will affect joint strength. This article addresses the problem of how to inspect bonded assemblies so that all

discrepancies are identified. Once the flaws are located, engineering judgment and analysis can determine if adequate

strength exists or if the parts need to be reworked or scrapped. When very large and costly assemblies have been bonded,

the importance of good non-destructive inspections is fully appreciated.

This article describes several techniques and presents drawbacks and limitations where they exist. No single method or

procedure can locate every flaw, so it is important for personnel to be aware of the available techniques and their

limitations. Bonded assemblies are inspected immediately after the adhesive cure cycle, after the parts have been cleaned

and removed from the manufacturing jigs or fixtures. The next important time for inspection is after the assemblies have

been in use and the condition of the parts is known. It is necessary to determine if new delaminations have occurred or if

any metal cracks have developed. Inspection in the field requires the instruments to be easily transported and to be used in

confined places.

The commonly used terms describe what is to be done. For example, nondestructive testing (NDT) comprises the testing

principles methodology, nondestructive inspection (NDI) involves inspection to meet an established specification or

procedure, and nondestructive evaluation (NDE) is the actual examination of materials, components, and assemblies to

define and classify anomalies or discontinuities in terms of size, shape, type, and location. This article specifically

addresses the three terms as they apply to adhesive-bonded joints within structures.

Inspection reference standards must be developed for each instrument, and the standards must be made available to and

used by inspectors to standardize their equipment and to permit them to detect anomalies that will indicate a flaw.

Inspectors must also be familiar with the internal details of the assembly being inspected so that they can distinguish

between flaws and legitimate structural details. For example, how many layers of metal were bonded at any one spot?

Note

* Reprinted from Adhesive Bonding of Aluminum Alloys, Marcel Dekker, 1985, p 337-423. With permission

Nondestructive Inspection of Adhesive-Bonded Joints

Donald J. Hagemaier, Douglas Aircraft Company, McDonnell Douglas Corporation

Description of Defects

There are a wide variety of flaws or discontinuities that can occur in adhesive-bonded structures. Table 1 lists possible

generic flaw types and their producing mechanisms. Metal-to-metal voids or unbonds are the most frequently occurring

rejectable flaws, as indicated in Table 2.

Table 1 Generic flaw types and flaw-producing mechanisms

Generic flaw type

Flaw-producing mechanism

Metal-to-metal

Metal-to-core

Core

Surface

Adhesive

Disbonds, internal X

Disbonds, part edge X X

Disbonds, high core X

Porosity X X X

Unremoved protective release film from adhesive

X X

Foam adhesive in film adhesive bond line X X

Cut adhesive X X

Adhesive gaps X X

Missing adhesive X X X

Weak bonds X X X