 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| EPSI INC. - Electronics Packaging Solutions International Inc. Contact us at: P.O. Box 1522, Montclair, NJ 07042 USA, tel.: +1 (973)746-3796, fax: +1 (973)655-0815 e-mail: jpclech@aol.com |
| NOVEMBER 2004 An OEM perspective on lead-free solder reliability, first appeared in Hobbs Engineering's November 2004 newsletter, posted with permission of the author, Larry Edson of General Motors, and Hobbs Engineering. LEAD-FREE SOLDER AND THE AUTOMOBILE by Larry Edson, GM Technical Fellow for Reliability and Accelerated Testing. |
The automobile industry will be facing a formidable challenge over the next 5 years as it moves away from
lead based solders and becomes lead-free. The challenge will be greatest in electronics as the weaknesses of
lead-free solder encounter the severe environment of the automobile. It would be easy to digress into the
debate of why circuit boards must be lead free when less than 1% of the lead in our environment emanates
from circuit board products. However, we all need to be active participants for the total benefit of society,
especially when our children and grandchildren will be most affected. Young people absorb and retain lead
at a much higher rate than adults and are thus the most at risk. Many electronic industries have already made
the move to becoming lead-free and the automotive industry can now benefit from their experiences.
We ask a lot of solder in electronics. We ask the solder to be ever compliant in order to absorb the relative
movement between materials with different thermally driven expansion rates. We ask the solder to provide a
basic level of strength as it retains components on circuit boards during the effects of mechanical shock and
vibration. Mechanical shock in automobiles occurs with the “slamming” of car doors and the jolting impacts
of potholes in the road. Furthermore, we ask that the solder remains compliant and strong for at least 10
years while enduring an environment that spans –40C to >85C over thousands of thermal cycles. Pretty
demanding aren’t we? In light of what is being asked of the solder, let’s review some of the major concerns
when using lead-free solder in an automotive application:
* The thermal fatigue life of lead-free solder is less than that of leaded solder. Much confusion exists around
this fact, as lead-free solder requires significantly greater dwell periods during thermal cycling to produce the
same degree of damage that would occur with leaded solder. Fast thermal cycling without adequate dwell
periods portrays lead-free solder as having a greater thermal fatigue life than leaded solder. However, this
erroneous conclusion is the result of testing without adequate dwell periods.
* Kirkendall voids are formed as copper diffuses into the tin. These voids accumulate and can significantly
reduce the strength of attachment under conditions of mechanical shock.
* Increased processing temperatures resulting from the higher melting point of lead-free solder can lead to
“popcorning” of plastic encapsulated components. Moisture trapped within the plastic matrix can violently
expand and damage the plastic encapsulation. Humidity control prior to processing becomes increasingly
important with exposure to these higher temperatures.
* Bismuth is sometimes alloyed with tin to reduce the melting point of solder. The contamination of lead into
the tin-bismuth alloy will result in a ternary phase material with the low melting point of 96C. This low melting
point is a serious concern in an automotive application with its high temperatures.
* Lead-free solder exhibits an abrupt transition in its loss of ductility at –30C. This rapid transition from a
ductile material to a brittle material could represent a failure mode not previously seen with leaded solder.
* Tin-whisker formation and tin-pest are additional concerns for in-service products as well as long-term
storage of replacement parts.
lead based solders and becomes lead-free. The challenge will be greatest in electronics as the weaknesses of
lead-free solder encounter the severe environment of the automobile. It would be easy to digress into the
debate of why circuit boards must be lead free when less than 1% of the lead in our environment emanates
from circuit board products. However, we all need to be active participants for the total benefit of society,
especially when our children and grandchildren will be most affected. Young people absorb and retain lead
at a much higher rate than adults and are thus the most at risk. Many electronic industries have already made
the move to becoming lead-free and the automotive industry can now benefit from their experiences.
We ask a lot of solder in electronics. We ask the solder to be ever compliant in order to absorb the relative
movement between materials with different thermally driven expansion rates. We ask the solder to provide a
basic level of strength as it retains components on circuit boards during the effects of mechanical shock and
vibration. Mechanical shock in automobiles occurs with the “slamming” of car doors and the jolting impacts
of potholes in the road. Furthermore, we ask that the solder remains compliant and strong for at least 10
years while enduring an environment that spans –40C to >85C over thousands of thermal cycles. Pretty
demanding aren’t we? In light of what is being asked of the solder, let’s review some of the major concerns
when using lead-free solder in an automotive application:
* The thermal fatigue life of lead-free solder is less than that of leaded solder. Much confusion exists around
this fact, as lead-free solder requires significantly greater dwell periods during thermal cycling to produce the
same degree of damage that would occur with leaded solder. Fast thermal cycling without adequate dwell
periods portrays lead-free solder as having a greater thermal fatigue life than leaded solder. However, this
erroneous conclusion is the result of testing without adequate dwell periods.
* Kirkendall voids are formed as copper diffuses into the tin. These voids accumulate and can significantly
reduce the strength of attachment under conditions of mechanical shock.
* Increased processing temperatures resulting from the higher melting point of lead-free solder can lead to
“popcorning” of plastic encapsulated components. Moisture trapped within the plastic matrix can violently
expand and damage the plastic encapsulation. Humidity control prior to processing becomes increasingly
important with exposure to these higher temperatures.
* Bismuth is sometimes alloyed with tin to reduce the melting point of solder. The contamination of lead into
the tin-bismuth alloy will result in a ternary phase material with the low melting point of 96C. This low melting
point is a serious concern in an automotive application with its high temperatures.
* Lead-free solder exhibits an abrupt transition in its loss of ductility at –30C. This rapid transition from a
ductile material to a brittle material could represent a failure mode not previously seen with leaded solder.
* Tin-whisker formation and tin-pest are additional concerns for in-service products as well as long-term
storage of replacement parts.