The Detailed Comparison of Lead-Free and Lead Soldering Phenomenon of PCBs

The solder joints are being decreased at a fast pace in the field of micro-electronics. The main reason of reduction of solder joints is to enable Printed Circuit Boards (PCBs) to sustain, thermodynamic, electrical, and mechanical loads. One of the reasons is to increase its reliability. Numerous of the electronics packaging technologies such as Ball Grid Array (BGA), Chip-Scale package (CSP), and Surface Mount Technology (SMT) etc. are in need for the implementation of quality mechanical and electrical connections among various materials or components with the help of soldering for offering a reliable connection ensuring high quality of electronic products. If failure of even a single solder joint happens, there is a possibility that entire product will breakdown. Therefore, insuring the quality of the solder joints is of great importance in the modern day electronic and electrical products.

 

The solder joints

The Agents of Traditional Solder Joints

The traditional or common solder joint are having lead (Pb) mixed with few other chemicals. The results compound is very toxic and its long-term applications are bringing various issues including health hazards to humans and damages environment. In the modern day, the lead-free soldering technique is replacing the lead soldering because of its high merits and non-impacts to humans and environment. However, there is a difference in the manufacturing process of lead-free and lead soldering joints. There are certain parameters which needs to be modified before its application to PCBs.

 

Comparison of Lead and Lead-Free Solder Joints

 

  1. 1. The melting points of the lead-free solder joints is higher than lead solder joints.
  • (1) The traditional lead soldering joint’s melting point is about 183°C.
  • (2) The modern lead-free soldering joint’s melting point is about 217°C.

 

As illustrated from the above facts, the melting point of lead-free soldering joints is almost 33°C higher than that of traditional lead soldering techniques. Therefore, the following are its main concerns.

  • (1) The rise in the temperature after the lead solder is oxidation with various chemical compounds is rapidly growing among metals.
  • (2) Some components such as electrolytic capacitors or those having plastic packages are affected at most because of rise in temperature.
  • (3) The SAC alloys are bringing strain to the components because of having lower dielectric constant tends to have more failures.
  • (4) There are a number of surfaces for soldering on the lead-free surface of components. The use of tin in soldering is because of its lower costs. However, there is a thin layer of oxidation which is tending to generate on the tin’s surface. Moreover, there is a stress as a result of electroplating.

 

  1. 2. Worst Wettability of Lead-Free Soldering

When the lead-free soldering is compared to lead soldering technique, it features lower wettability. The wettability of lead-free soldering is way lower than lead soldering. The bad wettability is tending the soldering joints to perform poorly and are not meeting the requirements when compared to the self-calibration capacity, shear strength, and tensile strength of lead soldering joints. Moreover, the bad wettability of lead-free soldering is also leading to a higher rejection ratio of the soldering joints when there is no possibility of modifications.

Worst Wettability of Lead-Free Soldering

Physical Characteristics Comparison of Lead-Free and Lead Soldering Joints

The following table is illustrating the comparison of various physical characteristics of lead and lead-free soldering joints.

Item Sn37Pb SAC387 Sn0.7Cu
Electrical Conductivity (IACS) 11.5 15.6 Nil
Melting Point (°C) 183 217 227
Density (g/m2) 8.5 3.5 3.31
Resistivity (MΩ-cm) 15 11 10-15
Fatigue Life 3 1 2
Surface Tension 260°C (mN/m) 481 548 491
CTE (×10-4) 23.9 23.5 Nil
Shear Strength (MPa) 23 27 20-23
Thermal Conductivity (W/m·1k·1s) 50 73 Nil

 

The table above is illustrating that the lead-free soldering technique is having a negative influence over the reliability of soldering joints because of the difference in performance when compared to that of traditional soldering. However, when mechanical influence is taken under consideration, the lead-free soldering is very harder when compared to traditional lead soldering. Furthermore, it is generating surface oxide, alloy residue, and flux contaminant and is leading to the bad electrical performance and increase contact resistance. Therefore, it can be stated that the conversion of soldering technique for electronic products from lead to lead-free soldering is not a solution owning to different concerns mentioned in the article.

 

The lead soldering is softer, therefore soldering joints are generated through lead-free soldering mechanism which are relatively harder, which are leading to smaller transformation and high intensity leading to higher reliability of solder joints which are lead-free. Because of the bad wettability of lead-free soldering certain defects are assumed to arise such as tomb standing, vacancy, and displacement etc.

 

Taking everything apart, the most concerning fact is that when transferring from lead to lead-free soldering technique, the bulging difference is lying in higher tin content which is more than 95 percent. This is arising high whisker growing of tin, explained below in detail.

 

Tin Whisker Growing

The tin whisker is growing from the weaker section of oxidation layer as mono-crystal tin and is performing in the cylindrical filament or columnar shape. The following are the expected damages.

  • There is a danger of causing shortcuts among neighboring pins.
  • There is also chance of bad influence to the features of high frequency applications.

 

The Growth of Metal Dendrite

The metal dendrite is featuring in a dissimilar manner than tin whisker. The former is the result of ionic electro-migration in the electrochemistry, whereas the metal dendrite will be leading to various shortcuts which will further cause the circuits to fail.

 

Conductive Anodic Filament (CAF) Generation

The generation of CAF is another failure which is result of the electro-chemical reaction. The CAF is taking place in the internal side of Printed Circuit Board and is caused as a result of the conductive filament which contains copper growing from the side of anode towards cathode.

The CAF is growing to amount when both cathode and anode are connected short circuited that are taking place among two dissimilar poles leading to the catastrophic disaster. CAF is considered as disaster for both PCBs having high density assembly as well as lead-free soldering having elevated temperature, making the issue to occur in abundance and easily.

 

The RayPCB Offers on Lead and Lead-Free Soldering

RayPCB is understanding the requirements of each project. At RayPCB, we are aware of the fact that different projects are having different soldering requirements. Therefore, for meeting the demands of the client, we are offering both lead-free and lead soldering techniques for PCB fabrication.