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SMT craft — formation of inter-metallic compound

164 Published by FASTPCBA 6月 05,2019

SMT craft — formation of inter-metallic compound

Inter-metallic Compound, or IMC for short, we usually use the IMC generated by the reaction between the solder of the SMT processing plant and the interface of the soldered metal as a sign of good solder joints.

In the solder alloys of various SMT processing plants, a large amount of Sn is the leading role, which is the main element involved in the formation of IMC, and the other elements only play a supporting role, mainly in order to reduce the melting point of solder and suppress the growth of IMC, a small amount of Cu and Ni will also participate in the structure of IMC.

SMT

The morphology of inter-metallic compound (IMC) in SMT processing plant is related to the aging time after welding.Common interface reactions and IMC morphology are as below.

1) Interface reaction between Sn and Cu

Sn-Pb, SAC, Sn-Cu solder and OSP, Im-Ag, Im-Sn and HASL interface reaction is the same, the essence is the interfacial reaction between Sn and Cu.

In the range of 200~ 350 C, the interface reaction between Sn and Cu will always form Cu6Sn5 and Cu3Sn double-layer structure, In the range of 240~ 330 C, Cu6Sn5 and Cu3Sn grow at the same time. Cu6Sn5 is mainly formed at the Cu/Sn boundary. Cu3Sn generally forms at the Cu6Sn5 and metal Cu boundary.And in the rich Sn phase, Cu6Sn5 grows much faster than Cu3Sn.In addition, during the reflow soldering process in SMT processing plant, Cu6Sn5 grows as the form of scallop, and the grain coarsening and diffusion processes also occur in Cu6Sn5.Cu3Sn is generally very thin, about 0.2 ~ 0.5um, if the magnification time is less than 1000, – generally can not be seen.Cu3Sn is generally considered to be bad tissue, which makes welds seam very fragile.

2) Interface reaction between Sn and Ni

The interface reaction between Sn-Pb solder and ENIG in SMT processing plant belongs to the interface reaction between Sn and Ni.Because Ni is relatively stable, the interface reaction layer is generally much thinner than that of Cu.The reaction structure was predicted to be Ni3Sn/Ni3Sn2/Ni3Sn4 according to the phase diagram. However, Ni3Sn was not seen at the actual brazing interface, but Ni3Sn4 was easily observed in NiP alloy coating.

3) Sn-Cu-N interface reaction

SAC and SnCu solder in SMT processing plant are similar to the interface reaction of ENIG essentially , which belongs to the interface reaction between Sn, Cu and Ni.

N reacts with SAC for one time reflow soldering, generally forming (Cu, Ni)3Sn4 single-layer IMC.If through many times reflow soldering will form a double-layer IMC structure of (Ni,Cu) 3sn4 and (Ca, Ni) 6sn5. The section diagram of this structure has significant characteristics, namely, thin, continuous, gray (Ni,Cu) 3sn4 and independent block (i.e., discontinuous) and dark gray (Cu, Ni)6Sns.Many cases have shown that this structure has low solder joint strength and is not shock resistant.

The formation process is as follows: Sn and Ni form Ni3Sn4 →the substrate Cu diffuses to Ni3Sn4 through the Ni grain boundary, and forms (Ni,Cu)3Sn4 →the Cu enriched in the solder reacts with Ni3Sn4 to form (Cu, Ni)6Sns. With the increase of the number of reflow soldering, it continues to grow. At the same time, (Ni, Cu) 3Sn4 basically maintains the original size.

4)Bad interface IMC section diagram

(1)Blocky IMC is not a technical term. It is used by the author to describe(Slice appearance) an ultra-thick, ultra-wide and discontinuous IMC shape — scallop shape IMC with thick tissue (W, h 5 m), very thin continuous layer and even disconnection of individual places (section diagram, magnification times over 1000).

The section diagram of solder joint formed by high temperature and long time reflow soldering shows typical blocky IMC structure.Its BGA is SAC welding ball and OSP welding pad processing craft, and the welding adopts SnPb solder paste (mixed packing craft). The peak welding temperature is 235 C, and the time above 217 C is 70s.The test results show that the shear strength is over 20% lower than the normal solder joint.

The normal IMC profile should have a relatively thicker continuous layer, and the scalloped IMC is grow on continuous layer, which is the result of Cu diffusion in the solder.

In the Ni/SAC interface, blocky IMC will be formed if the reflow soldering takes a long time.The blocky IMC (Ni,Cu)3Sn4 was formed ,BGA processed by nickel gold plating technology under the welding peak temperature of 243 ºC and welding time of 95s above 217 ºC .This section diagram is from the BGA dropped sample, so the BGA carrier plate welding plate cannot be seen.

The IMC structure is not large, but it accords with the characteristic of blocky.The IMC of this kind of morphology is not resistant to mechanical stress. If the PCBA of SMT processing plant is not standardized in production turnover and transportation process, It is easy to cause cracking of solder joints of BGA-type stress sensitive components.

The formation mechanism of blocky IMC is still not clear, which may be the result of high temperature melting and recrystallization of IMC, which can explain the morphology of thin continuous layer and blocky morphology .

(2) Pb-rich weld seam formed by QFN containing Ag coating. Due to the existence of Pb – rich phase tissue in the weld seam, the strength of the weld was reduced, resulting in failure in early stage.

(3) The size of Ag3Sn particles in solder joints of SMT processing plants is generally 1μm, and evenly distributed in the Sn parent phase, if with the increase of Ag content, above 3.5%, it can appear Ag3Sn grain coarsening, so that a needle occur, the alloy is prone to cracking if the external force at this point.

When Ag content is below 3.0%, Ag3Sn can hardly be seen in the weld seam.Obvious Ag3Sn can only be formed when Ag content reaches above 3.5% and under the condition of long welding time and slow cooling.

(4) Influence of Cu on IMC

With the increase of Cu content, the structure of interface IMC changes

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