Le PLUS DE PUS

Listening to market changes, PUS understood that specific alloys were needed for each type of soldering. The PUS atomizer is particularly suitable for small and medium volume manufacturing for these specific alloys.

PUS wants to be an independent and pragmatic advice on the true performance of these alloys, far from the commercial fashion or patents.

LOW SAC

OUR VIEW: Silver cost is roughly half of the metal value of the well known SAC305 . Reducing silver amount is leading to a softer alloy with slightly improved behavior in drop test, with lower thermo cycling behavior. Shininess will also be improved

OUR ADVICE: Can be used in many low cost application. Several micro-alloying can improve behavior with minor cost addition

Reliability investigation of Sn/Cu/Ni solder joints >

INTERMETALLIC LAYER

OUR VIEW: On copper pads, the classical IMC that is formed after soldering using a SAC alloy is Sn5Cu6. The shape of this IMC is usually a needle shape. Furthermore, IMC is always a brittle material. This is actually harmful for drop test behavior where breakage will occur near this IMC layer.

OUR ADVICE: Minor Ni addition into solder will have a significant influence both on IMC thickness and shape, leading to a noticeable improvement on drop test behavior.

THERMOCYCLING

OUR VIEW: For high quality solder joint, thermo-cycling behavior is generally a key-point. But be careful! Which kind of thermo-cycling are we speaking of? Soak time, temperature gap will change the rating of an alloy. For example SAC is better than SnPb on -40°C/60°C temperature range while SnPb is the best on -40°C/125°C gaps!

OUR ADVICE: For high gaps, SAC is known to have poor results in thermo-cycling. Inter-dendrite zone is the area to reinforce. Lowering size of precipitates and increasing the number of them by micro-alloying can improve dramatically the behavior of a SAC alloy.

Effect of Ag content on the microstructure of Sn‐Ag‐Cu based solder alloys >

LEAD FREE HIGH TEMPERATURE SOLDERS

OUR VIEW: Lead Rich alloys are still widely used, especially for die attach. A simple lead free alternative doesn’t exist into nature where BiAg based alloys will lead to poor wetting, SnSb with Sb lower than 10% may have a too low melting point and Zn based alloys can lead either to poor wetting or large IMC.

OUR ADVICE: Two promising solutions may merge. One proposes to use a BiAg alloy powder mixed with a small quantity of a classical lead free powder. This one will be only and fully used for wetting, thus the IMC created will have a higher melting temperature. TLP is near this idea where the alloy used will be fully turned into IMC after melting with an higher melting point.

UNDERCOOLING OF SAC ALLOYS

OUR VIEW: SAC Alloys and the most famous of them, the SAC305 generally shows a huge undercooling effect. While the melting point of this eutectic is 217°, the solidification starts at 195°C when using a 10°C per minute cooling rate. Using higher cooling rate is always possible, but not always practical when dealing with massive substrates because it can lead to thermal stress. This undercooling effect is directly related to the size of Ag3Sn precipitates into the substrate than can influence badly the behavior of the solder joint.

OUR ADVICE: Lowering silver content can reduce both undercooling and Ag3Sn precipitates size. Nevertheless, with comparable size of precipitates, a higher silver content will increase thermocycling behavior. It is therefore very important to control from the powder production the size of these precipitates as if big ones are already into the solder powder, they won’t dissolve totally during the reflow process and will lead to even bigger precipitates.

Melting and Freezing Characteristics of Common Lead-free Alloys >