Fillet lifting

Bismuth expands upon freezing, whereas tin contracts. A phenomenon called fillet lifting has been reported (see Hg. 45.2). It is mostly associated with bismnth ternary alloys such as Sn-Cu-Bi and Sn-Ag-Bi used in plated throu -hole (PTH) wave soldering, but has also been observed with Sn-Bi system. 

When associated with the primary side of the board, fillet lifting is not considered a defect. 

As pointed out earlier, the appearance and shape of lead-free solder joints differ from eutectic Sn-Pb. However, it should be pointed out that this has not caused an increase in defects requiring rework. This is true even though new defects such as fillet lifting occur with some lead-free alloys. Reliability tests have not indicated these defects, including fillet lifting, have lower quality, and thus are believed to not require repair [67]. More experience is required to verify that this is the case. 

FIG. 50 Fillet lifting of Sn-Cu wave solder at the solder/pad interface of a through-hole fillet. 

Solder Bridge Plating compatibility Void Icicle Fracture mode Crack, % elongation Strength Fillet lifting Impurity res. 

FIG. 48 The incidence of fillet lifting can be significantly reduced by reducing the size of the land on the top of the PCB. (Courtesy of Panasonic.)... 

FIG. 94 Fillet lifting on pins post a wave solder operation utilizing Sn-Cu. (From Ref. 50.)... 

FIG. 5 Optical micrograph of a vertical cross section of a through-hole solder joint exhibiting fillet lifting between the solder and the intermetallic compound layer on the copper land. Note that the fillet lifting extends from the edge of the land to the knee of the through-hole barrel. 

SM joints of high-Sn solders, or in SM or TH joints of eutectic Sn Pb, or eutectic Sn 58Bi. Several aspects of fillet lifting have been identified. 

TABLE 7 Tendency for Fillet Lifting as a Function of Composition... 

CDIP components assembled with Sn 2.6Ag. 8Cu. 5Sb also failed. Examination of cross-sectioned parts suggests that solder joints with both Pb-free alloys were more susceptible to shoulder cracking than eutectic Sn-Pb. Although approximately 25% of Sn-2.6Ag-0.8Cu-0.5Sb as-fabricated joints showed fillet lifting, there was no obvious relationship between fillet lifting and failure. No electrical failures of PDIPs occurred for any alloy at the end of test (i.e., up to 2000 cycles). 

Through-Hole Technology. Most joints exhibit fillet lifting. 

Less susceptible than other high-tin solders to fillet lifting but results with Sn-2.6Ag-0.8Cu-0.5Sb indicate through-hole reliability still may be compromised. 

Phenolic Thermal fatigue test ( 40 to 125 C, 30-min dwell time) Creep test. Fillet lifting (JR-02 board) DIPIC Jumper wire... 

Fillet lifting becomes more severe in the presence of Bi. Even without Bi, fillet lifting occurs by the segregation of Pb from the surface finish of components during solidification. 

The effect of fillet lifting with lead-free solder in wave soldered plated holes, is minimized by the use of near-eutectic Sn 3.8AgA).7Cu-0.25Sb alloy. Tolerance to lead-containing component terminations has been demonstrated for this alloy. 

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