Lead-free Soldering Process Development and Reliability

Jasbir Bath, Jasbir Bath

• Editore: Wiley
• Anno: 2020
• ISBN:
  9781119481935
• Edizione: 1
• Argomento: Tecnología e ingeniería
• Categoria: Control de calidad en ingeniería

Covering the majortopics in lead-free soldering

Lead-free Soldering Process Development and Reliability provides a comprehensive discussion of all modern topics in lead-free soldering. Perfect forprocess, quality, failure analysisand reliability engineersin production industries, this reference will help practitioners address issues inresearch, development andproduction.

Among other topics, the book addresses:

· Developments in process engineering(SMT, Wave, Rework, Paste Technology)

· Lowtemperature, hightemperature andhighreliabilityalloys

· Intermetallic compounds

· PCB surface finishesandlaminates

· Underfills, encapsulants and conformal coatings

· Reliability assessments

In a regulatory environment that includes the adoption of mandatory lead-free requirements in a variety of countries, the book'sexplanations ofhigh-temperature, low-temperature, andhigh-reliabilitylead-free alloysin terms of process and reliability implicationsare invaluable to working engineers.

Lead-free Soldering takes a forward-looking approach, with an eye towards developments likely to impact the industryin the comingyears. These will include the introduction of lead-free requirementsin high-reliability electronics products in themedical, automotive, and defense industries.The book provides practitioners in theseand other segments of theindustrywith guidelinesand informationtohelpcomplywith these requirements.

1
Lead‐Free Surface Mount Technology

Jennifer Nguyen¹ and Jasbir Bath²

¹Flex, Milpitas, California, USA

²Bath Consultancy LLC, San Ramon, CA, USA

1.1 Introduction

Surface mount technology (SMT) involves the assembly or attachment of surface mount devices (SMDs) onto the printed circuit board (PCB). Today, the majority of the products are built using surface mount technology and lead‐free process. This chapter will review the surface mount process for lead‐free soldering, including printing, component placement, reflow, inspection, and test. The chapter also discusses some advanced miniaturization technologies used in the SMT process.

1.2 Lead‐Free Solder Paste Alloys

Today, there are a variety of lead‐free solder paste alloys available in the market. SnAgCu (SAC) materials with 3.0–4.0% Ag and 0.5–0.9% Cu and remainder Sn are widely accepted within the industry. Among them, Sn3.0Ag0.5Cu (SAC305) is still the most common alloy used in the SMT process. These SnAgCu alloys have the liquidus temperature of around 217 °C. As the cost of Ag has increased over the past years, the use of low Ag alloy materials such as Sn0.3‐1.0AgCu or SnCu/SnCuNi has increased. These alloys have approximately 10 °C higher melting temperature than SAC305 and may need to be processed at slightly higher temperature during the reflow process.

Low temperature lead‐free alloys which contain SnBi/SnBiAg are also used. These alloys have melting temperature around 140 °C and can be processed at 170–190 °C. These low temperature alloys usually have high bismuth content and they create some reliability concerns, especially on mechanical reliability. These low temperature alloys are used on certain applications such as light‐emitting diode (LED)/TV products. In recent years, there is a desire for low temperature lead‐free alloy alternatives with better reliability. The drivers for these low temperature alloys include component warpage, low energy consumption, and component or board sensitivity to the higher temperature lead‐free process. These alloys typically have higher liquidus temperature than traditional SnBi/SnBiAg alloys, but they still have lower liquidus temperature than SAC305. These alloys have gained a lot of interest in the industry in the recent years, and some are available in the market and used in production.

1.3 Solder Paste Printing

1.3.1 Introduction

One of the most important processes of the surface mount assembly is the application of solder paste to the PCB. This process must accurately deposit the correct amount of solder paste onto each of the pads to be soldered. Screen‐printing the solder paste through a foil or stencil is the most commonly used technique, although other technique such as jet printing is also used.

There is no major change to solder paste printing for lead‐free processes. The same printer can be used for tin‐lead and lead‐free printing. In general, the same stencil design guidelines can be used for lead‐free process.

1.3.2 Key Paste Printing Elements

Solder paste printing process is one of the most important processes in surface mount technology. This process can account for the majority of the assembly defects if it is not controlled properly. For effective solder paste printing, the following key factors need to be optimized and controlled:

• PCB support
• Squeegee (type, speed, pressure, angle)
• Stencil (thickness, aperture, cleanliness, snap off, separation speed)
• Solder paste (including type, viscosity)

PCB support is important to the printing process. Good PCB support holds the PCB flat against the stencil during the screen‐printing process. PCB support is generally provided with the screen‐printing machines. If the board is not properly supported, solder defects such as bridging, insufficient solder, and solder smearing can be seen. For fine pitch printing such 0.3/0.4 mm pitch chip scale package (CSP), 0201/01005 (Imperial) chip component, a dedicated custom‐made fixture for printing or vacuum support should be used.

Squeegees, squeegee pressure, and speed are other critical parameters in the screen‐printing process. Metal squeegees are commonly used for printing solder paste, and rubber or polyurethane squeegees are used for epoxy printing. A squeegee angle of 60 °C to the stencil is typically used [1]. Squeegee speed and squeegee pressure are critical for good printing. The speed of the squeegee determines how much time the solder paste can roll and settle into the apertures of the stencil and onto the pads of the PCB. In the beginning of lead‐free conversion, a slower printing speed was used because the lead‐free solder paste was stickier than tin‐lead solder paste. Today, many lead‐free solder pastes can print well at high speed.

The speed setting is widely varied from a typical range of 20–100 mm/s⁻¹ depending on the size of the aperture, the size of PCB, and the quantity of boards being assembled, etc. Printing speed used depends on the solder paste supplier or is optimized by a Design of Experiment (DOE). It is typically between 40 and 80 mm s⁻¹. During the solder paste printing, it is important to apply sufficient squeegee pressure and this pressure should be evenly distributed across the entire squeegees. Too little pressure can cause incomplete solder paste transfer to the PCB or paste smearing. Too much pressure can cause the paste to squeeze between the stencil and the pad.

Stencil is another key factor in the solder paste printing. Metal stencils are used in solder paste printing. Stainless steel material is commonly used; however, metal stencils can be made of copper, bronze, or nickel [2]. There are several types of screen‐printing stencil, including chemical etch, laser cut, and electroformed [2...