Screen And Stencil Printability

Published Date: 02 Nov 2017

CHAPTER 1

Solders are generally described as fusible alloys with liquids temperature below 400°C (750°F). Solder is a filler metal with a low melting point. In welding and metallurgical processes, solder is used to join two or more metallic surfaces. This means that, using metals that have a low melting temperature to adhere the surfaces to be soldered together. Solder materials are used for many applications, including electronic assemblies, in which solder covers mechanical and physical properties like thermal and electrical. In general, elements which frequently used in solder alloys are Plumbum (Tm =328°C), Silver (Tm =961ºC), Bismuth (Tm =271.5ºC), Indium (Tm =156.6ºC), Antimony (Tm = 630.5 ºC), and Cadmium (Tm =321.2 ºC). Pb-Sn alloys are widely used as solder in the electronics industry. High percentages of Pb in conventional solder alloys (95Pb-5Sn or 90Pb -10Sn) are used for high temperature application such as die attach solders in power semiconductor packaging. The application of power electronics has been extended to a variety of automotive, aerospace, and energy production industries. However, low percentages of Pb are used for normal consumer electronics like as laptops, cell phones, and electronic toys. Majority of these products because of having short service life, are releasing in landfill within a few months or years. The lead can penetrate into the drinking water and poisoning humans’ health. In July 2006, The European Parliament and Council agreed to the Restriction on the Use of Hazardous Substances in Electrical and Electronic Equipment Directive, which bans the use of lead, or plumbum (Pb). Also, since Japan is one of the paramount electronic products producers, which required all new electronic products to be lead free solder from January 2005. These directives encourage the development of high-temperature lead-free solders. There are some alternatives for replacing lead in solder alloys but, according to Jennie S. Hwang on 2004, solder paste should represent following parameters for chemical and physical characteristics [1] :

Physical appearance

Stability and shelf life

Viscosity

Screen and stencil printability

Tack time

Adhesion

Exposure life

Quality and consistency

Suitability with surfaces to be joined

Flow property before becoming molten

Wettability

Wicking, Dewetting and bridging phenomenon

Quantity and properties of residue

Residue cleanability and corrosivity

Solder-joint appearance

High Temperature Solder

In recent years, there has been an increasing need within the avionics, military, oil exploration, telecommunications, and automotive industries for solders that execute reliably at ever-higher temperatures, which come close to or exceed the melting point of tin-lead eutectic. Such harsh environment applications require solders with melting points higher than that of tin-lead eutectic in order to achieve the required reliability. Table 1 demonstrates the temperature range of high temperature electronic market [2]

High temperature solder alloys are used in power electronic systems which attach between the silicon die and the basic substrate. At high operating temperatures, the usual Sn-Pb eutectic solder cannot be used, because it would be melted before completing service requirements (Tm=183°C).Moreover, reducing the load on cooling systems is another advantage for high temperature solder usage . High temperature alternatives have higher shear modules than high-lead solders, but high-lead solders are cheaper and ordinarily provide much better resistance against thermo-mechanical fatigue than the eutectic gold alloys [2].

Table 1: High Temperature Electronic Markets [2]

Well Logging Instrumentation

Oil and gas wells

75 °C to 225 °C

Steam injection wells

200 °C to 300 °C

Geothermal wells

200 °C to 350 °C

Aerospace

Electronic braking system

Up to 250 °C

Engine control / monitoring

Up to 300 °C

Automotive

Engine compartment

Up to 165 °C

On-engine and on-transmission

Up to 165 °C

Wheel mounted components

Up to 250 °C

Others

Exploration vehicles

Up to 500 °C

Space systems

Up to 500 °C

Nuclear reactor monitoring

Up to 550 °C

Gas Turbine Engines

Up to 1200 °C

High Temperature Solder Selection

A wide variety of fusible alloys are suitable for soldering. Selecting a solder alloy for a specific application , following specificities should be considered [2]

Metallurgical and morphological observations

Mechanical properties

Temperature consistency

Electrical characteristics

Alloy-substrate consistency

Linear coefficient of thermal expansion

Repeatability of manufacture / Consistency in melting point

Availability

High Temperature lead- Free Solders

Selecting an alternative high temperature lead-free solution involved two main properties, mechanical and electrical behavior .A typical led-free high temperature solder replacement is eutectic Au20Sn.This alloy is hard solder with a melting temperature of 280°C which would be within range of applications for high temperature lead free solders [3]. Hard solders have a higher melting temperature and higher yield stress than pb5sn.The cost of gold and lack of suitable mechanical properties however have limited its implementation. Other lead-free solutions such as Bi5Sb have the proper thermodynamic properties, but lack sufficient mechanical and electrical properties to be considered as a suitable high temperature replacement [4]. The addition of antimony to bismuth did little to increase the strength of the material while increasing its brittleness and resistivity. A number of promising lead-free materials (Table 4) SnAgCu-alloys i.e., have been developed as replacement for the (near-) eutectic Sn-Pb solders in mainstream applications, despite the fact that there is still no ‘drop-in’ alternative for the traditional Pb-Sn alloy [5].

Table 4.Review on different recently developed and/or manufactured LFS alloys [5].

Problem Statement

Current high temperature solders (Pb-Sn) are used for lots of applications, such as attachment of power semiconductors, flip chip packages, heat resistant vehicles packages, aerospace, and variety of intrusive procedures are using in the medical. Environmental concerns after used and throw away are also driving research in this area. The concern about toxicity and health hazards means that lead-free solders should be replaced instead of leaded solders as soon as possible.

Prohibiting the use of lead containing solders in many industries was applied from starting 1st of July 2006. Recently, a new alloy system, Bi-Ag, has been considered as the replacement for high Pb solders for high-temperature applications (e.g., 95Pb-5Sn, with a melting range from 308 °C to 312 °C) [6]. Therefore, this research will be focused on several candidate alloys (Bi-1.5-2.5-3.5Ag) as alternative solders in order to replace the high lead solders. For reaching to consequence, the morphology and mechanical properties of candidate alloys should be investigated under real conditions and must cautiously apply these solders for high temperature applications step by step.

Research Objectives

The main objectives of this research are:

Investigate the morphology Bi-Ag (three different percentages of Ag 1.5, 2.5, and 3.5 ) solders on Cu substrates with the effect of isothermal aging on 160°C

Mechanical and morphological evaluation of solder joints using single lap-shear test method by 10 KN tensile test machine.

Scope of Study

In this research, candidates of Bi-Ag alloys with different compositions were put in oven at 160°C and various times. Characteristics and morphological investigations such as, grain growth behavior, Cu-rich particles size behavior in solder bulk and wetting angle formed between the solder alloys on copper plate, during aging process, were conducted using Optical Microscope (OM) and Scanning Electron Microscope (SEM). Area elemental analysis was performed using Energy Dispersive X-ray (EDX). Moreover, candidates of Bi-Ag alloy joint’s behavior and resistance were investigated using single lap-shear test method by 10 KN tensile test machine. Also failed areas were investigated by SEM and EDX.

Layout of the Thesis

This thesis is primarily divided into five chapters:

Chapter 1 introduces the explanation of solder and soldering, high temperature solder, high temperature solder selection, and high temperature lead-free solders. In addition, this chapter describes the problem statements, research objectives and scope of this research.

Chapter 2 provides a literature review …

Chapter 3 explains in detail the methodology …

Chapter 4 explains the results, discussions and analyses on the experimental data in this study…

Chapter 5 concludes the main findings in relations to the objectives of the research…