Richard D. Pettegrew

National Center for Microgravity Research, Cleveland, Ohio

Peter M. Struk

Glenn Research Center, Cleveland, Ohio

John K. Watson

Johnson Space Center, Houston, Texas

Daniel R. Haylett

National Center for Microgravity Research, Cleveland, Ohio

Experimental Methods in Reduced-Gravity

Soldering Research

NASA/TM—2002-211993

December 2002
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Richard D. Pettegrew

National Center for Microgravity Research, Cleveland, Ohio

Peter M. Struk

Glenn Research Center, Cleveland, Ohio

John K. Watson

Johnson Space Center, Houston, Texas

Daniel R. Haylett

National Center for Microgravity Research, Cleveland, Ohio

Experimental Methods in Reduced-Gravity

Soldering Research

NASA/TM—2002-211993

December 2002

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

Second International Brazing and Soldering Conference

sponsored by the American Society for Metals

San Diego, California, February 17–19, 2003
Acknowledgments

The authors would like to thank the following individuals for their valuable contributions to this project: Gregory

Fedor, Michael Dobbs, Jack Kolis, Anthony Butina, Michael Conley, Robert Lowe, William O’Hara, Edward Van

Cise, Allison Bahnsen, Owen Farmer, and Julio Estrada.

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identification only. This usage does not constitute an official

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findings, subject to revision as

analysis proceeds.

Available electronically at http://gltrs.grc.nasa.gov
EXPERIMENTAL METHODS IN REDUCED-GRAVITY

SOLDERING RESEARCH

ABSTRACT

The National Center for Microgravity Research, NASA’s Glenn Research Center, and NASA’s

Johnson Space Center are conducting an experimental program to explore the influence of

reduced gravity environments on the soldering process. An improved understanding of the

effects of the acceleration environment is important to application of soldering during current

and future human space missions. Solder joint characteristics that are being considered include 

solder fillet geometry, porosity, and microstructural features. Both through-hole and surface

mounted devices are being investigated. 

This paper focuses on the experimental methodology employed in this project and the results of

macroscopic sample examination. The specific soldering process, sample configurations,

materials, and equipment were selected to be consistent with those currently on-orbit. Other

apparatus was incorporated to meet requirements imposed by operation onboard NASA’s KC135 research aircraft and instrumentation was provided to monitor both the atmospheric and

acceleration environments. The contingent of test operators was selected to include both highly

skilled technicians and less skilled individuals to provide a population cross-section that would

be representative of the skill mix that might be encountered in space mission crews.

INTRODUCTION

Future long-duration human exploration missions will be challenged by constraints on mass and

volume allocations available for spare parts. Addressing this challenge will be critical to the

success of these missions. As a result, it is necessary to consider new approaches to spacecraft 

maintenance and repair that reduce the need for large replacement components. On the

International Space Station the maintenance concept for avionics has evolved from removal and

replacement of entire Orbital Replacement Units (ORU’s) to removal and replacement of circuit

cards when possible. The next step to reducing the size of the items being replaced would be to 

implement component-level repair, which is the repair mode used by the U.S. Navy on vessels at 

sea. Factors in the decision to implement component level repair include operational issues

(such as skill requirements and crew training), systems issues (including fault diagnosis and postrepair verification), and process issues (including potential influences of a reduced gravity

environment on the metallurgical characteristics and configuration of solder joints). The process 

issues are the thrust of this project.

NASA/TM—2002-211993 1

Richard D. Pettegrew

National Center for Microgravity Research

Cleveland, Ohio 44135

Peter M. Struk

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

John K. Watson

National Aeronautics and Space Administration

Johnson Space Center

Houston, Texas 77058

Daniel R. Haylett

National Center for Microgravity Research

Cleveland, Ohio 44135
flight analyses were not performed, were not reported, or only considered gross macroscopic

effects. The dominance of surface tension was noted (allowing filling of wider gaps than would

be possible in normal gravity), as well as an increased entrapment of flux in the solidified joints. 

These experiments were conducted in vacuum; their applicability to practical repair operations is 

therefore limited since such operations would be carried out in a pressurized “shirtsleeve”

environment.

A manual soldering experiment was conducted on STS-57. In this effort, a crewmember

soldered and de-soldered a variety of samples mounted on circuit cards. These tests were started 

in an onboard glove box, but the crewmember concluded the tests in the open cabin environment. 

The samples included various electronic devices and wire junctions. A review of videotapes

made during the course of the experiment yields some interesting qualitative observations based 

on the crewmember’s comments. First, the molten solder alloy appeared to solidify at a slower 

rate than during 1-g training. Second, the solder fillets appeared to be more “convex” than in 1-g.

 

This paper will compare the external joint geometry, cooling rates, and flux entrapment

characteristics of solder joints produced in reduced gravity to those produced in normal gravity.

TECHNICAL APPROACH

Aircraft Facilities

Flying an aircraft (NASA’s KC-135) in a

parabolic trajectory generates the reduced gravity

environment (Ref. 2, 3). The maneuver starts

with a full power climb, after which the nose is

lowered to trace the parabolic arc (Figure 1). 

From the time that the nose begins to lower, until 

the pullout (from the ensuing ‘dive’ on the back

side of the trajectory), the experiment and crew

experience a 20-25 second period of ‘reduced

gravity’, relative to their surroundings. While the

acceleration levels experienced during these

maneuvers are qualitatively very low, some

residual accelerations (engine and airframe

vibrations, etc) remain. 

Experimental Hardware

The experimental apparatus (Figure 2) provides accommodations for a test operator, who is

seated and strapped to a seat, to manually solder on a circuit board in an enclosed glove box. 

The test operator places their hands into the glove box via two access holes. The soldering iron 

is a Weller® TCP 12P with a PTP7 tip and is the same model as currently flown in the soldering 

kit aboard the International Space Station. The glove box is vented between tests via an

overboard vent valve. The apparatus includes video recording of each solder joint, glove box

temperature, relative humidity, and ambient pressure. On select joints, thermocouples were spot 

welded or soldered to the circuit board pad to obtain heating and cooling profiles of the solder 

Figure 1

Schematic of aircraft trajectory during 

reduced-gravity maneuvers

BACKGROUND

Several automated on-orbit soldering experiments have been conducted since the beginning of

the Shuttle program with experiments flying on STS-4, -7, -11, -17, and –40 (Ref. 1). Some of 

these experiments malfunctioned and yielded no results. Others functioned nominally but postNASA/TM—2002-211993 2
1.59 approx. 

A ñ A 

typ. 8 plcs. 

apply solder 

this side 

PRESSURE 

 X-DUCER CIRCUIT 

BOARD 

joint. Additionally, the Space 

Acceleration Measurement 

System (SAMS) program at 

NASA GRC provided a 3-axis 

accelerometer for local 

acceleration measurements. 

A ìplated through-holeî 

configuration (Figure 3) has been 

used as the standard test 

configuration. This arrangement 

is representative of a typical 

service joint on an electrical 

circuit board. The components consisted of resistors that were fastened to the sample card using 

an epoxy. Future tests will examine surface mount device (SMD) configurations. 

The primary solder alloy consisted of Sn/Pb 60/40 with a 

resin flux core. Select samples used a eutectic 

composition of Sn/Pb 63/37 solder again with a resin flux 

core. Additional tests were conducted using a solid core 

solder (Sn/Pb 60/40). During these tests, liquid flux was 

applied to the solder joint prior to heating and adding 

solder to the joint. Application of the solder flux was 

primarily done during the 2-g pull-up just prior to the 

reduced gravity portion of a parabola on the KC-135 

aircraft. During select tests, the liquid flux was applied 

in reduced gravity to observe the flowing characteristics 

of the flux. Results of the solid-core soldering are 

currently being investigated and are not reported. 

In addition to low-gravity testing, samples were also 

soldered under normal gravity conditions while onboard 

the KC-135 aircraft to determine a baseline sample set 

for comparison. Initially, normal gravity tests were 

conducted in the same orientation as during reduced 

gravity tests with the circuit board at approximately a 53°

angle relative to the horizontal (as shown in Figure 2). 

During all subsequent testing, the sample card was 

positioned in the horizontal plane for the normal gravity 

tests, allowing the gravity vector to be perpendicular to 

the circuit board. 

Multiple operators were used during the testing. The effect of operator skill was characterized 

by using multiple ëunskilledí operators (all of whom attended a common training class, similar to 

the training given to astronauts), as well as a ìskilledî technician, who is qualified to solder 

space flight hardware. Since this effort is designed to evaluate the practical aspects of soldering 

Figure 2

Experimental Hardware used on KC-135 Aircraft 

Figure 3

Plated through-hole sample configuration 

(all dimensions in millimeters). 

6.35 typ.

65.9

 65.9

6.35 typ.

A

A

NASA/TM—2002-211993 3
by an astronaut, control of operational parameters (dwell time of soldering iron on joint, amount 

of solder added, etc) were at the test operatorís discretion. The mass of solder added, while not 

controlled, was determined by measurement (before and after the test) of the mass of the solder 

piece used. 

Post-flight analysis (done jointly at NASA Glenn and NASA Johnson) consists of a visual 

inspection, photography, and leg-length measurements of the soldered joints, followed by crosssectioning / porosity measurements and optical metallographic characterization. Metallographic 

analysis will be presented in a future paper. 

RESULTS 

Testing was performed with the plated through hole configuration using five test operators. A 

total of 297 reduced gravity samples and 143 normal gravity samples were obtained using the 

resin-flux cored solder. In these tests, a higher incidence of porosity occurred in reduced gravity 

compared with normal gravity (see below). In an attempt to mitigate the increase in porosity, 

tests were conducted using a solid core wire with flux applied externally to the joint producing 

160 reduced gravity samples and 64 normal gravity samples. The results of the solid core wire 

soldering are currently under analysis. 

Acceleration Environment

The acceleration environment was measured 

during the test using a 3-axis accelerometer 

system. While ëzero-gravityí is the goal, 

small amplitude, high frequency 

accelerations are routinely observed, with 

larger amplitude, low frequency 

accelerations observed at times. Figure 4 

shows an acceleration plot typical of those 

observed throughout the test series. The 

horizontal lines indicate acceleration levels 

of 0.02, 0.0, and ñ0.02 g/ge, respectively. 

Parabolas where the acceleration level was 

between +/- 0.02 g/ge for the duration of the 

time between solder application and 

solidification were judged to be acceptable. 

Visual Inspection

A visual inspection on the solder joints from the first two flight weeks yielded the results shown 

in Table 1. The inspection followed the standard set forth by NASA-STD-8739.3 (Ref. 4). 

Workmanship failures include excess or insufficient solder added to the joint, cold solder joint 

(insufficient heating which exhibits poor wetting), overheated solder joint, and poor or nonwetting joints. Void failures describe solder joints that have visually apparent holes or evidence 

of subsurface voids. 

Cause of failure Normal Gravity Reduced Gravity 

Workmanship 4.7% 10.7 % 

Voids 3.1% 10.0 % 

Figure 4

Typical acceleration environment aboard reduced gravity 

aircraft along the vertical axis of the aircraft 

Table 1: Visual Inspection ñ Failure rates in normal and reduced gravity samples 

+0.02 g/ge

-0.02 g/ge

NASA/TM—2002-211993 4
Geometry

Each sample was photographed and the leg length of the ìtopî and ìbottomî of the solder joint 

was measured as shown in Figure 5. The ratio of the leg lengths (top to bottom) is shown in 

Table 2. This table shows that the acceleration environment affects fillet geometry. In reduced 

gravity, the upper and lower legs are about the same length. In normal gravity, the lower leg is 

longer than the upper leg (the 20% change in normal gravity leg length between Week 1 & Week 

2 is attributed to the sample orientation change, previously discussed). It is possible that the 0-g 

solder joints may respond differently than 1-g solder joints to operating stresses. We are 

currently investigating the implications of this result. 

During the STS-57 Shuttle experiment, the 

crewmember made the observation that the 

shape of the solder joint appeared more 

convex than was expected. Samples from 

the current experiment were examined 

(subjectively) for this characteristic by means 

of visual inspection of the sample crosssections. The fillet geometry of samples 

produced in reduced gravity tended to exhibit a more convex shape than those produced in 

normal gravity. However, this change in overall shape may be attributable (at least in part) to an 

increase in the mass of solder used in the reduced gravity tests. Measurements of the solder mass 

used indicate that test operators tended to add more solder in the reduced gravity tests than in the 

normal gravity tests (Table 3). This trend was evident in 4 of the 5 operators in this data subset. 

Overall, the mass of the reduced gravity samples was about 9% greater than those soldered in 

normal gravity. No explanation of this observation is offered at this time, but this factor, 

combined with the changes in leg length (previously described) are likely to account for the 

perceived change in the overall shape. 

Week 1 1.14 ±0.057 Reduced gravity Week 2 1.08 ±0.081 

Week 1 0.87 ±0.071 

Normal gravity Week 2 0.70 ±0.040 

Figure 5

Front illuminated photograph of a solder joint used for leg length measurements 

Table 2: Vertical Leg Length (LT/LB) ratios 

LT LB Resistor

ìTopî ìBottomî 

Solder applied to this side

Circuit Board 

NASA/TM—2002-211993 5
Porosity Measurements

Samples from the first two flight weeks 

were examined for internal porosity 

content by mounting the samples in 

ëmetallographicí mounts and grinding 

them down to approximately the sample 

centerline. The percentage of porosity 

exposed by this was then measured by 

imaging the sample, designating (via 

computer program) what areas were 

ëvoidí (porous) areas, and dividing that 

area by the total area of the sample. 

This technique complies with ASTM 

Standard E1245-00 (Ref. 5), which 

addresses porosity measurements. This 

technique was carried out for both 

normal gravity and reduced gravity 

samples for the first two flight weeks. 

Figure 6 illustrates an example of a 

sample (after mounting and grinding) 

along with a digitized image showing 

the area measurement. 

Results of this analysis are displayed in 

Figure 7, which is a bar chart showing 

the occurrence of porosity as a function 

of the percentage of porosity. The 

colored bars (blue for reduced gravity, 

red for normal gravity) indicate the rate 

of occurrence of samples with a given 

porosity content. For example, the set 

of bars on the far left of the plot 

indicate that about 63% of the normal 

gravity samples showed 2% or less 

porosity, while about 29% of reduced 

gravity samples displayed 2% or less 

porosity. 

 Operator 1 Operator 2 Operator 3 Operator 4 Operator 5 

Normal gravity: 

Avg. mass added 

0.023 g 0.026 g 0.032 g 0.028 g 0.030 g 

Reduced gravity: 

Avg. mass added 

0.031 g 0.027 g 0.037 g 0.022 g 0.032 g 

Occurrence of Porosity

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

<2 2 - 5 5-10 10-15 15-20 20-25 25-30 30-35 35-40

Percent Porosity

Percent Occurrence

Low g

1 g

Digitally enhanced image of 

joint, with yellow areas 

indicating pores & white areas 

indicating solid material. 

Image of joint, prepared for 

analysis. Pores are seen as 

circles in the joint. 

Figure 6

Figure 7

Rate of occurrence of porosity in normal and reduced 

gravity samples 

Table 3: Mass of solder used by each operator in normal and reduced gravity tests.

NASA/TM—2002-211993 6
Overall, this plot indicates that about 96% of normal gravity samples show porosity rates of 10% 

or less, while only about 58% of the reduced gravity samples show 10% or less porosity. The 

hypothesized reason for this is that liquid flux vaporized and was trapped in the liquid solder (in 

the reduced gravity cases); without the presence of gravity to drive these gas bubbles out, they 

solidified into the pores seen in these measurements. 

Thermocouple Data & Cooling Times

Thermocouple junctions spot-welded or soldered to the joint provided temperature traces for a 

subset of both the normal and reduced gravity tests. Figure 8 shows representative temperature 

data for both cases. 

Events such as solder application and 

solidification time were determined through 

examination of the video data, and 

temporally correlated to the thermocouple 

data. The cooling time (for solidification) 

determined from this data was compared to 

the video data by looking for evidence of 

phase change. The temperature data 

correlated well with the visual data, 

allowing the image data to give cooling 

times for all tests (including samples 

without thermocouples). This data is 

compiled in Table 4, along with the 

average mass of the samples. 

Normal gravity Reduced gravity 

Average Sample Mass 0.0261 g 0.0285 g 

Average Solidification Time 2.79 sec 2.96 sec 

This shows that, although the solidification time was about 7% greater in the reduced gravity 

samples (compared to the microgravity samples), this is accounted for by the increased mass of 

the reduced gravity samples, with the slightly larger samples having a greater thermal inertia. 

DISCUSSION AND CONCLUSIONS 

The effect of reduced gravity on the soldering process was examined in a plated through-hole 

configuration aboard a reduced gravity aircraft. In the absence of gravitational forces, geometric 

changes (as measured by the ratio of the ëlegí lengths of the joint on the top and bottom of the 

board) were observed; in reduced gravity, the joint ëlegí on the top surface (where solder was 

applied) was longer than that on the bottom side, whereas in normal gravity, the bottom side had 

greater leg lengths. 

Table 4: Average sample mass and solidification times for normal and reduced gravity samples 

Figure 8

Representative temperature measurements of the 

soldering process 

NASA/TM—2002-211993 7
Samples soldered in reduced gravity were (subjectively) observed to be more convex than those 

produced in normal gravity. However, measurement of the mass of solder applied revealed that 

on average, about 9% more solder was applied during reduced gravity tests. This additional

mass, combined with the measured changes in leg length of the joint, may account for the

slightly more convex shape of the reduced gravity samples. The increased mass is also likely to 

account for the slight increase in solidification times (as measured by thermocouples and video

analysis) for the reduced gravity samples.

Significantly more visual defects and internal porosity were also observed in the reduced gravity

samples. A reasonable explanation for this could be that flux is trapped in the liquid solder, and 

after vaporization, the bubbles are unable to migrate to the surface due to the lack of buoyant

forces. These trapped bubbles are then preserved as porous voids in the joint after solidification.

Due to this observed increase in porosity in reduced gravity samples, efforts were made to

mitigate the porosity through a procedural change. Instead of using a flux-cored solder (as is 

currently manifested on ISS), solid-cored solder (along with liquid flux) was used. Liquid flux

was applied first, and (after application of the soldering iron) allowed to vaporize before

application of the solid solder. If the hypothesis that entrapped flux is responsible for the

increased porosity is correct, this approach may help mitigate the problem. Data from this flight 

week is currently under analysis, and will be presented in a future paper. The possibility of an

on-orbit ISS experiment is also being examined, to validate the recommendations for porosity

abatement that will come from the current efforts.

REFERENCES

1. Winter, C. and Jones, J. C. 1996. The Microgravity Research Experiments (MICREX) 

Database, NASA TM-108523, (http://mgravity.itsc.uah.edu/microgravity/micrex/).

2. Lekan,J.; Neumann,E.S.; and Sotos, R.G. 1992. Capabilities and Constraints of NASA’s 

Ground-Based Reduced Gravity Facilities, NASA CP-10113

3. Operations User’s Guide: JSC Reduced Gravity Program User’s Guide, Johnson Space 

Center, JSC Document 22803. 

4. Soldered Electrical Connections. 1997. NASA Technical Standard, NASA-STD-8739.3

5. ASTM Committee E-4. 2001. Standard Practice for Determining the Inclusion or SecondPhase Constituent Content of Metals by Automatic Image Analysis. Annual Book of ASTM 

Standards 2001 Sect. 3, Vol 03.01 July.

NASA/TM—2002-211993 8
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E–13664

WU–101–58–0F–00

14

Richard D. Pettegrew, Peter M. Struk, John K. Watson, and Daniel R. Haylett

Microgravity applications; Microgravity; Soldered joints; Fluxes; Soldering

Unclassified -Unlimited

Subject Category: 29 Distribution: Nonstandard

Experimental Methods in Reduced-Gravity Soldering Research

Prepared for the Second International Brazing and Soldering Conference sponsored by the American Society for Metals,

San Diego, California, February 17–19, 2003. Richard D. Pettegrew and Daniel R. Haylett, National Center for

Microgravity Research, Cleveland, Ohio 44135; Peter M. Struk, NASA Glenn Research Center; and John K. Watson,

NASA Johnson Space Center. Responsible person, Peter M. Struk, organization code 6712, 216–433–5948.

The National Center for Microgravity Research, NASA Glenn Research Center, and NASA Johnson Space Center are conducting an experimental program to explore the influence of reduced gravity environments on the soldering process. An improved understanding of the effects of the acceleration environment is important to application of soldering during current

and future human space missions. Solder joint characteristics that are being considered include solder fillet geometry,

porosity, and microstructural features. Both through-hole and surface mounted devices are being investigated. This paper

focuses on the experimental methodology employed in this project and the results of macroscopic sample examination. The

specific soldering process, sample configurations, materials, and equipment were selected to be consistent with those

currently on-orbit. Other apparatus was incorporated to meet requirements imposed by operation onboard NASA’s KC–135

research aircraft and instrumentation was provided to monitor both the atmospheric and acceleration environments. The

contingent of test operators was selected to include both highly skilled technicians and less skilled individuals to

provide a population cross-section that would be representative of the skill mix that might be encountered in space mission

crews.