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(248)957-1170

Elmet LLC is primarily a metallurgical and metallographic laboratory focused on the electrical and electronics industries.  All circuit boards, connectors, clips, cages, and electrical devices have metallic components that can suffer damage or fail due to causes such as fatigue, corrosion, stress overload, dendritic growth, and fretting.  These are metallurgical issues, not electrical issues, and a metallurgical perspective brings insight into the problem.  We provide that insight through investigation and thorough explanation of the metallurgical factors at work, from manufacturing defects to design to conditions of use.

Much of our work is solder joint cross sectioning, and we have a large capacity to handle this type of work.  With five automated polishers we can process more than 20 cross sections per day, day after day.  We have two metallographic studios with studio cameras, stereomicroscopes, and metallurgical microscopes to document the incoming parts, the cross sections, and the microstructures. Our scanning electron microscope assists us in evaluating intermetallic layers, microstructures, fretting, and fracture.  The EDS (energy dispersive spectroscopy) system allows us to analyze solder compositions, contamination, and debris observed on circuit boards.

We have staff holding certificates in IPC-A-610 inspection (CIS), electrical engineering, and metallurgical engineering, and we have a great deal of experience with failure analysis of metallic components. 

Tech Topics

Solder Crack Measurement

Arthur Griebel

Repeated changes in service temperature, due to the weather or engine operation, can cause thermally induced stresses to fatigue crack the solder joints of automotive PCBAs resulting in circuit failure.  Circuit boards are designed to minimize solder fatigue, but in order to verify that PCBAs can survive many years of service use, circuit boards are tested by thermal cycling with the intention of exceeding the number of thermal cycles anticipated for the life of the vehicle.  At the end of the thermal cycle testing the circuit must still be operational and, for added security, any solder cracks that have developed must be less than a stated percentage, often 25%, of the solder joint width.

Because this 25% crack length limit creates a hard divide between pass and fail, or between acceptance and rejection, the method of crack length measurement must be sensible and agreeable to all organizations involved.  While everybody accepts the definition of crack percentage as crack length divided by solder joint width, there is  no universally accepted definition of what crack length or joint width actually mean or how they are measured.  What seems a simple concept in the abstract becomes confused by the details of implementation.

We will discuss some of the issues that create confusion or conflict; we will then discuss how Elmet measures crack length in the absence of contrary customer instructions. We present images of cracked solder joints and present possible interpretations of measurement. 

We will start with an easy one. Consider the solder ball shown in Figure 1.  A crack extends from one side of the solder ball to the other, this solder ball is clearly 100% cracked.  Accepting this obvious statement has implications for how we define the width of the solder joint and how we define the length of the crack.

Figure 1

Since we agree this is 100% cracked, implicitly we accept that crack length is not always measured in a straight line; this crack turns the corner at the edge of the solder pad. Crack length includes the length along the top of the solder pad plus the lengths down the edges of the solder pad. 

Likewise, we must accept that the “width” of the solder joint, in this case the solder ball, is not the width at the centerline or even at the top of the solder pad.  The width is not even a straight line; if it were, the contorted crack length could exceed the length of the solder joint, or crack length could be measured as less than joint width even though the crack clearly splits the joint in two.

The “width” must be defined along the same path as the crack, extending across the top and down the sides of the solder pad. That is the only definition of joint width that leads to (crack length divided by joint width) = 100%.

What if the crack is less than 100%?  The crack in the solder ball shown below extends between A and C - but it does not extend all the way across the solder ball (to simplify the discussion we will ignore the small dark pores at the right end of the solder pad).

Figure 2

What is the crack length?  Consistent with the previous example we would measure the length along the left edge of the solder pad from A to B, written as A-B, and add the length along the top edge of the solder pad from B to C, written as B-C.

Now, what is the solder joint width?  Consistent with the previous example the solder joint width measurement follows the same path as the crack from A to B to C.  We reasonably extend the measurement to D, but what next?  Do we continue measuring horizontally to the right edge of the solder ball or do we measure down the right edge of the solder pad?

Measuring down along the right edge of the pad is consistent with the previous example, AND this distance is slightly shorter than extending the measurement horizontally to the surface at right, so reasoning argues for measuring downward. The crack percentage is given by (A-B-C)/(A-B-C-D-E).

Now consider the QFN solder joint shown in Figure 3. What is the solder joint width? It makes no sense to measure the width of solder along the solder pad because much shorter paths are available for crack propagation. In fact we can see a crack along the vertical face of the QFN lug. To reach 100% cracking, the existing crack must propagate horizontally beneath the lug, so we define solder joint width as the existing crack length plus the horizontal length beneath the lug. Crack percentage is (A-B)/(A-B-C). Using these dimensions the solder joint is 28.1% cracked.

Figure 3

In all three of the examples so far the solder joint width was determined by considering the crack path that is observed.  That is, the crack path always lies along the line that defines joint width.  This is the only way to assure that edge to edge cracking results in a calculated 100% crack percentage.

These examples emphasize that the joint “width” that we are interested in is the distance that a crack would follow, so we must consider the probable crack path before we decide how to measure the solder joint width.

Look at the solder joint in Figure 4. The crack extends from A to A’ at toe of the gull-wing lead.  If this crack propagates further, it will reach the top surface at B’.  The distance from A’ to B’ is much shorter than any other path option, and if the solder joint is to fail, this will certainly be the path of the crack.  Therefore we know the length from A’ to B’, the shortest distance from the crack tip to the surface, is part of the solder joint dimension.

Figure 4

There are three options for measuring joint width to the left of the crack.  We can measure along the gull-wing lead from A to B to C where we reach the surface, or measure from the left end of the crack to the nearest surface at E, or lastly we can measure from A to B to D, connecting the existing cracks or voids at B and D.   

There are two reasons to choose the last option. First, those two small cracks at B and D suggest that the stress field favors cracking along that path, and second, connecting these cracks represents less crack growth than propagating from point A to point E. (distance A-E is 1.16 times the distance A-B plus B-D)  Elmet would measure the solder joint length from B' to A', and also from A to B to D.  The distance from A’ to A is the crack length and is also included in the solder joint width.

In general, the solder joint width is measured as the length of the anticipated crack path. In all three cases so far, the anticipated crack path is the shortest distance from the crack tip to a free surface.  This is usually the case, except when other considerations intervene, such as voids or other crack segments.

Figure 5 shows a solder joint that is 100% cracked with complete separation along the path A-B-C. Considering the crack traverses a large void, what is the crack length and the solder joint width in this example? If the void dimension is included in the crack length but not in the solder joint width, the calculated crack percentage will exceed 100%. If the void dimension is included in the solder joint width but not the crack length, the crack percentage will be less than 100% even though the solder joint clearly has failed and separated. Therefore, to be sensible and to provide meaningful information about the condition of the solder joint, crack percentages must be calculated either with the void included in both the crack length AND in the joint width, or excluded from both.

Figure 5

Crack percentage numbers are intended to reveal something about solder cracking, not the amount of voiding, so typically void dimensions are excluded from the crack length measurement and from solder joint width. The crack percentage of the solder joint in Figure 5 is calculated as (A-B)/(A-B)=100%. The void dimension B-C is not included in either the numerator or denominator.

Figure 6 shows another example. A small crack is visible at the center of the solder joint and the yellow line, labeled A-B, indicates the anticipated crack path.  Length A-B includes a significant amount of voids and a small length of crack.  If we included the voids in the calculations this solder joint would be reported as about 15% cracked.  Excluding void dimensions from the calculations gives the more reasonable interpretation that this solder joint is about 25% cracked.  Voids can have a large influence on crack percentage calculations and, in order to make crack percentages sensible, the void dimensions must be excluded from crack percentage calculations.

Figure 6

Crack percentage measurement is fraught with debate. Disagreement usually arises because a participant has not considered the implications of their assumptions. We hope the above discussion provides some clarity, but comments and other opinions are welcomed in the comment section below.

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