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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]. The thickness of the stencil is typically 125?µm (5?mil) or 150?µm (6?mil). Stencils with the thickness of 100?µm (4?mil) or thinner have become more popular with the high density and fine pitch components such as 0201/01005 (Imperial) chip components or 0.4/0.3?mm pitch CSP or quad flat no-leads/bottom termination component (QFN/BTC) components. Thicker stencils than 150?µm are typically used when more paste is needed. Stencil thickness and aperture size determine the amount of paste deposited on the pad. In general, stencil aperture must be three times and preferably five times the diameter of the solder particles. To ensure the proper paste release and efficient printing, the aspect ratio should be greater than 1.5, and the area ratio should be greater 0.66.

The aspect ratio is defined by Eq. (1.1), and the area ratio is shown in Eq. (1.2).

Snap off and stencil separation speed are also important for good printing quality. Snap off is the distance between the stencil and the PCB. For metal stencil printing, the snap off should be zero. This is also called contact printing. A high snap off will result in a thicker layer of solder paste. Stencil separation speed is the speed of separation between the stencil and PCB after printing. Traditionally, high separation speed will result in clogging of the stencil apertures or tailing at edges around the solder paste deposited (Figure 1.1). However, lead-free pastes tend to have a higher adherence than tin-lead pastes and may prefer high separation speed than tin-lead solder paste. Separation speed varies depending on the solder pastes and its supplier, and the supplier's recommendation should generally be followed.

Figure 1.1 Example of tailing at the edge of the paste due to high separation speed.

Last but not least, the correct solder paste type and material should be used. The correct type of solder paste should be selected based upon the size of the apertures within the stencil. Type 3 was commonly used in the tin-lead process; however, Type 4 has become a more common lead-free solder paste type in the recent years due to the increase in miniaturized components on the printed circuit board. The release from the apertures of the stencil is affected by the particle size within the selected solder paste. Table 1.1 lists the particle size of different solder paste type.

Table 1.1 General solder paste type and particle sizes.

Both tin-lead and lead-free solder paste should be refrigerated while being stored to maintain its shelf life but must be brought to room temperature before use to maintain quality. Some new lead-free solder pastes require no refrigeration and can be stored at room temperature. The solder paste should be mixed...