Case Study Comparing the Solderability of a Specific Pb Free No Clean Paste in Vapor Phase and Convection Reflow (Part 2)

Experiment Process

One of the major goals of this study was to control as much as possible the previous factors with the purpose of observing any difference in wetting behavior caused by the reflow technique. The samples used as the solderable metal surfaces were 10 Cu Blocks. Each Cu Block had the dimensions: 38 mm X 38 mm X 3.2 mm. Figure 4 shows an example of the Cu Block Sample.

Five of the Cu Blocks went through Vapor Phase Reflow and five of the blocks went throug. Convection Reflow. A separate Cu Block had three thermocouples (TC’S) attached to it and was used to set up the recipes of the thermal reflow processes.

The recipes developed were optimized for Pb Free SAC soldering parameters. All three TC’s were attached to the topside surface and were adhered from one corner to the opposite corner. The Vapor Phase Reflow machine was manufactured by R&D Technical Services. The Vapor Phase machine was a batch unit that had a fully enclosed topside/bottomside convection heat pre-heat zone, and a reflow zone containing the vapor well. Both in the pre-heat and reflow zone Nitrogen (Inert) gas was supplied. The following was the recipe used:

• Boiling Fluid – 240 perfluorinated heat transfer fluid
• Pre-Heat - Oven at 290°C for 175 Seconds
• Reflow Dwell – 115 Seconds
• Vapor Flash Off Dwell – 25 Seconds
• Cool Dwell – 400 Seconds

Figure 5 shows the corresponding thermal profile on the Cu Block.

The Convection Reflow machine used was an inline unit from Heller Industries. The Convection

Reflow oven had 12 Topside/Bottomside Zones and had Nitrogen (Inert) gas atmosphere. The Nitrogen source was same for both the Vapor Phase and Convection Reflow Machines. The following was the recipe used:

Figure 6 shows the corresponding thermal profile on the Cu Block. Table 1 compares thermal profile measurements between the two reflow processes.

As shown in Table 1, the Convection Reflow profile peaked about 5 degrees hotter and stayed about 15 seconds longer above 217°C then the Vapor Phase Reflow profile. Note for the heating ramps and cooling ramps, both reflow processes were equivalent. For this experiment, the greater amount of heat and time above the liquidous was assumed to have minimal effect in reflowing the solder and solder solidification.

For sample preparation, each Cu Block was cleaned using a Scotch Brite (Trademark of 3M Corporation) Pad, followed by Xylene and Isopropanol wipe, and allowed to dry. Figure 4 shows the Cu Block metal surface after cleaning. A SAC 305 Pb free solder paste was used for the solder deposition. Here are the data specifications for the solder paste (Indium, 2008):

• 96.5Sn/3.0Ag/0.5Cu (SAC305)
• Type 3 Mesh
• No clean Flux Chemistry – Flux Type R0L0
• Halide-Free
• Typical Solder Paste Viscosity (Malcom 10 RPM) – 1700 poise
• Typical Tackiness – 35 g

A manual mini-stencil was used to deposit the solder paste on each Cu Block. The 0.152 mm thick stencil had 0.508 mm diameter apertures on 1.27 mm pitch for a total of 624 apertures. Figure 7 shows an example of the solder paste deposition on a Cu Block Sample. After solder dispensing the Cu Samples were immediately run through the corresponding reflow process.

Optical Results of Solder Bumps Post Reflow

Figures 8 and 9 show the typical solder bump results on each Cu Block sample post reflow. Each figure includes an (a) and (b) picture, where the (a) picture was from the Vapor Phase Reflow, and the (b) picture was from the Convection Reflow.

From an optical / qualitative perspective, the Vapor Phase deposits appear to have larger area coverage than the corresponding Convection Reflow samples. Also with the Vapor Phase Reflow Solder deposits a larger halo of silver/gray material formed around the bump, which is shown in Figure 9. This halo was confirmed by EDX as Sn and is shown in Figure 10. One hypothesis is that at some point during the vapor phase reflow process the solder liquid had spread further in area but during solidification the solder perimeter pulled back leaving Sn metallization on the Cu Surface. Another hypothesis is that Vapor Phase Reflow caused a greater slumping/spreading of the solder paste, then the coalescence of solder paste occurred followed by a pull back of the molten solder boundary to a state of stable equilibrium (Bielick, 2010). This Sn haloing was not observed as much on the Convection Reflow solder deposits. Also observed with the Vapor Phase samples was a greater amount of transparent residues and the Cu Blocks appeared to be more tarnished from Vapor Phase than Convection Reflow. One last visual observation about all the samples was that there was variability in the amount of solder, specifically that the solder bumps were smaller near the corner and edges than the solder bumps in the middle of the array.

INSPECTION TO IMPROVE LEAD-FREE SOLDER TECHNOLOGIES (Part 2)

Detection of Voids

X-ray imaging in detection of voids is helpful to a certain extent. A Digital Microscope should be considered to work alongside, and not in place of or as alternative inspection equipment. Manufacturers striving to gain the highest quality should consider X-ray and Digital Microscope equipment as a pair. Detecting voids is a helpful example of this relationship.
During pre-heating, voids can occur due to deteriorated flux. This can result in low flowing solder causing gas pockets.
The surface of flux residue and shine of solder joints may indicate possible voids (Figure 4A and 4B).

During reflow, air current that is too hot and strong deteriorates flux and causes voids (Figure 5A). The specialized BGA inspection lens for the Digital Microscope can work side by side with X-ray inspection, detecting possible voids. Adjusting air current can contribute to the reduction of void occurrence (Figure 5B). X-ray is an important aid for void detection and until now has been considered an alternative and separate solution apart from both stereo and digital microscopy. Inspection of BGA properties and defects should be considered from a combination of potential solution providers.

Judgment of Defects by Exterior Inspection
BGA exterior inspection allows access to the hardest to reach places on the PCB. The highly accessible prism adapter for the Digital Microscope BGA Lens can squeeze between components and look directly at the BGA from the side on heavily populated boards with minute pitches. Utilizing a metal halide bulb (discussed below), will greatly enhance the information retrieved from this style of observation. The following section as well will provide further examples of how specific reflections further support conclusions and information gathering. Without even the aid of X-ray technology it is possible to determine defects caused by over extended pre-heating and reflow (Figure 6).


Common among these issues of over-heating are oxidization and visible damage to the ball itself (Figure 7). Exterior inspection goes beyond defects, aiding as well in the correction of

temperature profiles.

Inspection to Improve Lead-Free Solder (Part 1)

INSPECTION TO IMPROVE LEAD-FREE SOLDER TECHNOLOGIES

Kazuo Kawai
Hirox-USA and Seika Machinery, Inc.
River Edge, NJ, USA and Torrance, CA, USA
info@hirox-usa.com and info@seikausa.com


ABSTRACT

Lead-free solders are said to cause various problems such as lack of self-alignment, bridges, solder balls, insufficient wetting, dendrites, pits, voids and peeling of soldering land. Also, on miniaturized lands, there may occur insufficient melting of solder paste. Even though these problems have not yet been resolved, lead-free soldering technologies have already been introduced at mass production sites. Most of these problems basically arise from heightened packaging temperature requirements in line with higher melting points of soldering materials.

Inspection of manufactured products to determine quality of the temperature profile and proper solder connection needs to be done with a high performance inspection system.

This technical paper and presentation will go over defects that can be uncovered by a high performance inspection system and merits of capturing images of such defects early during the assembly process. Without detection there may be a risk of damaging PCBs or parts that require rework. As well, serious quality problems or failures in the market after production may occur. The longer the lag time in detection of the defects, the higher the percentage of PCBs that may be scrapped. Therefore, inspection should take place at the earliest possible stage. Current technology available with a high end digital microscope will allow for immediate and thorough discovery of these defects which is of vital importance for SMT assembly.

Since lead-free solders were introduced to PCB assemblies, many soldering issues have arisen and for many, remain unsolved. This case study introduces the advantages of utilizing the digital microscope to identify soldering defects and root causes to improve productivity of lead-free soldering.
Although leaded and lead-free solders are different in their melting point, the basic theory in the SMT process, including temperature profiles, is still the same. However, the spreading property of lead-free solder is inferior to leaded solders. Therefore, determining good lead-free solder joints requires more detailed inspections than lead ones.
The appearance of flux residue helps to provide enough information for the temperature conditions of the solder joints being inspected. If wrong temperature conditions are used during the soldering process, there is a good chance ofvoids and insufficient spreading of solder. Observation of flux residue in addition to light reflection and shape of solder fillets are essential in the inspection of lead-free solder joints.
The following will provide the reader with many examples and various uses of a Digital Microscope as an exterior inspection as well as defect analysis tool for SMT. It is important as well to consider in the following pages, a comparison between Stereo Microscopy, lighting techniques, and use of adapters to aid inspection.
Key words: Digital Microscope, 360° Rotational Adapter, BGA Inspection Lens

LEAD-FREE SOLDER AND DIGITAL MICROSCOPY
Exterior Observation Using a Digital Microscope System
Excluding temperature profile graphs and X-ray imaging, all of the following images provided for visual aid of PCB components and BGA’s were captured with a Digital Microscope. The multiple oblique angles shown in the images are made possible by a 360° rotational adapter attached to the end of the Digital Microscope Lens itself. A specialized BGA lens allows for 90° inspection of BGA’s by means of a thin, flexible, plastic prim chip adapter. The benefit of using the 360° rotational adapter over stereo microscopy is in the rotation. Varying the angle of inspection on lead-free solder enhances assessment of solder shape and provides changes in reflection to help determine temperature profiles. This technique is not possible on lenses other than a Digital Microscope without configuring the lens, stage, and sample to fit the desired angle. In addition, by using a specialized BGA inspection lens, the same process described above can be applied when determining BGA shape and temperature profile. This simple yet beneficial approach to exterior observation can improve product quality on the production line. The addition of a Digital Microscope as a part of production rather than post- production Quality Control or Failure Analysis can support and expand the foundation of a company’s philosophy of quality and drive down costs on the product assembly line. The following nine sections will discuss the usefulness of a Digital Microscope as a part of PCB production and the potential effect its use can impart to product quality.

BRINGING THE DIGITAL MICROSCOPE TO THE PRODUCTION FLOOR

Any inspection done in-line during product assembly has to be quick, easy, and accurate in order to be seamlessly inserted into a company’s current standards for production. A Digital Microscope can cover both of these necessities while providing high quality images wherein observations and judgments can be made in seconds, observing defects before they reach Quality Control or Failure Analysis Divisions. This can potentially save time and money by integrating aspects of QC/FA into the production line, as well as help Process Engineering Teams attain solutions to problems more quickly.
The first portion to follow begins with examples concerning flux residue, and the ability of the 360° rotational adapter to gain information that up until now could not be easily gathered and stored by a stereo microscope. The sections following that will further support the use of a Digital Microscope, its rotational adapter, and the BGA inspection lens directly on the production line.

Judgment of Thermal Balance with Inspection of Flux Residue
Note: To see flux residue easily, leads were intentionally shifted from the lands (Figure 1A, 1B).
Access to these views have not been obtained by shifting or angling the PCB in any way, but were identified and captured using the 360° rotational adapter without having to manipulate the PCB in any way. Such rotation can easily show the flux residue as in these examples: showing flux spattering, Figure 1A (before pre-heat fan speed change), normal flux residue, Figure 1B (after pre-heat fan speed slow down), normal flux residue condition, Figure 2A and flux bridging, Figure 2B.

Solder melts and flows in accordance with the flux only reaching areas that the flux exists. Flux moves from the resist to the heated land by its tension, and if kept from deteriorating in pre-heating, lead-free solder will be more likely to self-align to the heated land by its surface tension (Figure 3).