Solution to the brittleness of lead-free solder joints

- Aug 28, 2018-

A variety of lead-free solders have emerged for use in the electronic assembly industry as a result of the recently implemented hazardous substance limitation act (RoHS).SnAgCu (SAC) alloy is the most common choice due to its excellent physical, mechanical and fatigue properties as well as acceptable welding methods and costs.However, as shown by the drop test results, the application of this kind of alloy appears to be deficient due to the high brittleness of the solder joint (as compared to 63Sn37Pb).This is especially important for portable products that use array packaging (such as BGA and CSP). 

SAC weld break occurs mainly at the interface between solder paste and pad.It is obvious that any problems that occur at the interface can be solved by taking measures on one or both sides of the interface, such as solder pad metallization and solder alloys, or by reducing the impact on the solder joints.Other ways to improve solder joint reliability include:

(1) improve surface treatment

(2) improve the solder alloy

(3) strengthening relay

(4) improve packaging

Surface treatment

The composition of the surface treatment directly affects the type of IMC formation.It is known that in some cases, the dropping test performance of the ni-au pad is worse than that of the OSP.This condition appears to be related to the formation of coarse-grained IMC fan-shaped matter (scallop) on nickel.If the coating thickness is not controlled within 0.2 micron, the result of immersion silver drop test will also be affected by small voids. 

The quality of surface treatment can affect wettability and the structure of intermetallic compounds.According to IBM observations, the brittleness of SAC solder joints may be related to changes in OSP surface treatment between batches, although the exact nature of the changes is unclear. 

Solder alloy

Reduction in the Ag content of SAC alloys, such as SAC 105, is helpful, but is still inferior to eutectic tin-lead.In this regard, a kind of SnAgCu alloy modified with a few additives is developed and studied.This alloy solution is often referred to as SAC+X, where X is one or more elements added.This improvement reduces the hardness of the solder alloy and thus its brittleness.One concern about this approach is that it may affect thermal fatigue reliability.A recent initay report on various alloys, including SAC+Mn, and their drop tests performed better than Sn63.In addition, the creep rate of this metal is also lower than Sn63, so the results of drop test and thermal fatigue performance evaluation provide promising results. 

Reinforce the relay

The fragility of brittle solder joints can be compensated by the use of an adhesive.The adhesive can increase the connection strength between BGA and PCB.The commonly used methods are capillary bottom filling and corner strengthening.The disadvantage of bottom filling is that there are many steps, and the residual flux will affect the adhesion, but its advantage is greater bonding strength.Corner reinforcement has limited effect on improving connection strength.There is a new approach that has generated a lot of interest in the industry, which is called "non-liquid bottom filling".This method is fully compatible with SMT process and has the best effect on improving reliability of finished products. 

Improve the packaging

This method reduces the impact on solder joints by providing some kind of cushion in the packaging of portable products.Design changes include replacing hard shell materials with rubber or more foam materials.The downside is that it increases the size and cost of the device.