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Overview
Hermeticity of Electronic Packages is a book about the integrity of sealed packages to resist foreign gases and liquids penetrating the seal or an opening (crack) in the packageùespecially critical to the reliability and longevity of electronics. The author explains how to predict the reliability and the longevity of the packages based on leak rate measurements and the assumptions of impurities. Non-specialists in particular will benefit from the author's long involvement in the technology. Hermeticity is a subject that demands practical experience, and solving one problem does not necessarily give one the background to solve another. Thus, the book provides a ready reference to help deal with day to day issues as they arise.
The book gathers in a single volume a great many issues previously available only in journalsùor only in the experience of working engineers. How to define the ""goodness"" of a seal? How is that seal measured? How does the integrity of the seal affect circuit reliability? What is the significance of the measured integrity of the seal? What is the relationship of Residual Gas Analysis and the seal integrity? The handbook answers these questions and more, providing an analysis of nearly 100 problems representative of the wide variety of challenges that actually occur in industry today.
Product Details
| ISBN-13: | 9781437778786 |
|---|---|
| Publisher: | Elsevier Science |
| Publication date: | 10/05/2011 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 360 |
| File size: | 6 MB |
Read an Excerpt
Hermeticity of Electronic Packages
By Hal Greenhouse Robert Lowry Bruce Romenesko
William Andrew
Copyright © 2012 Elsevier Inc.All right reserved.
ISBN: 978-1-4377-7878-6
Chapter One
Gas Kinetics
CHAPTER OUTLINE 1.1 General Considerations 1 1.1.1 Boyle's Law 3 1.1.2 Charles's Law (1787) or Gay-Lussac's Law (1802) 3 1.1.3 Dalton's Law (1801) 3 1.1.4 Avogadro's Law (1811) 3 1.1.5 Avogadro's Number 3 1.1.6 Loschmidt's Number 4 1.2 Mathematical Relationships 4 1.3 Problems and Their Solutions 7
1.1 General Considerations
This book is primarily about the movement of gases between a sealed package and its external environment. There are many properties of gases, which are dependent upon the gas pressure, the temperature, and the type of gas.
The father of the Kinetic Theory of Gases is usually considered to be Gassendi. In 1658, he proposed that gases were composed of rigid atoms made from a similar substance, different in size and form. Gassendi was able to explain many physical phenomena based on these simple assumptions. In 1678, Hooke took Gassendi's theory and expanded it to explain the elasticity of a gas. The elasticity was said to be due to the impact of hard, independent particles on the material which surrounded it. The next advancement was made by Newton in 1718. Newton proposed that the pressure of a gas was the result of the repulsive forces between molecules. This was later proven to be only a secondary effect. The first major improvement to the theory was made by Daniel Bernoulli in 1738.
The Kinetic Theory of Gases was well established by the year 1870. It was based on classical physics such as Newtonian mechanics. Some of the proofs of the theory did not occur until the beginning of the twentieth century but they did not alter the classical theory. The classical theory accounts for the motion and energy of gases, and only has deviations when the pressure of the gas is greatly different from one atmospheric pressure.
Although the Kinetic Theory of Gases explained the gases macroscopically, the theory did not include their vibration, rotation, and the effect of these motions on translation. These later microscopic properties were explained by quantum and statistical mechanics. The classical theory suffices for the purpose of this book; the only exception is the topic of Residual Gas Analysis that is based on mass spectrometry. The flow of gases between a container and the container's environment can be completely understood by the classical theory.
Molecules move very quickly, at a rate of about 500 meters per second. This rapid motion results in a very large radius of curvature so that the molecular motion can be considered to be in a straight line. The effect of gravity is negligible. The classical theory assumes perfect elasticity of a completely rigid molecule whose size is negligible.
Consider a gas in a sealed container. The fast moving molecules strike the walls of the container, as well as colliding with each other. The molecular collisions with the walls exert a pressure on the walls in proportion to the number of impacts. If we now decrease the size of the container by half, while keeping the number of molecules in the container the same, the density of the molecules will double. The number of molecules striking the walls in a given time has now doubled, so the pressure at the wall has also doubled.
The Mean Free Path (mfp) of a gas is the average distance a molecule travels before it collides with another molecule. The mfp is therefore inversely proportional to the density (or pressure) of the gas as well as the size of the molecule. If a gas is not confined, the mfp is proportional to the absolute temperature. The mfp of a mixture of gases considers the differences in diameters of each gas. The different diameters of the gases cause a greater variation from the mean than in the case where only one kind of gas is present.
The opportunity of gases to have different densities and different mfps gives rise to three different kinds of gas flow: viscous, molecular and transitional (diffusive).
The flow is viscous when the molecular collisions most often occur between the molecules, as opposed to collision of the molecules with the walls of the gas container, and there is a difference in the total pressure across the interface. Molecular flow takes place when the majority of the molecular collisions are with the walls of the container. The transitional flow is when both types of collisions are common and extend over two magnitudes of pressure. For example: viscous flow would be applicable at pressures greater than 10-3 atmospheres, molecular flow at pressures less than 10-5 atmospheres, and transitional flow between the pressures 10-3 and 10-5 atmospheres. The type of flow can be defined in terms of the mfp and the diameter of the channel.
• When the mfp/D is less than 0.01, the flow is viscous.
• When the mfp/D is greater than 1.0, the flow is molecular.
• When the mfp/D is between 0.01 and 1.0, the flow is transitional.
Another condition for defining molecular flow is the relationship between the smallest dimension of the leak channel and the average pressure.
• When DPave > 600 µ-cm, the flow is viscous.
• When DPave < 10 µ-cm, the flow is molecular.
• Where Pave is the average pressure in (microns of Hg).
• Where D is the diameter or smallest dimension of a channel or tube in cm.
It is seen from the latter condition that molecular flow is possible even at a pressure of one atmosphere, if the minimum leak dimension is small enough. This is the typical condition for fine leaks in packages where the collisions with the walls predominate.
The aforementioned is a general description of molecular motion. The specific laws by which the molecules behave are attributed to several scientists.
1.1.1 Boyle's Law
Robert Boyle in 1662, and E. Mariotte in 1676, showed experimentally that at a constant temperature, the volume of a definite mass of gas is inversely proportional to its pressure. There is a deviation from this law at high pressures. The deviation is related to the temperature at which the gas can be liquefied. Boyle's Law at 50 atm has a 3.3% error for hydrogen, and an error of 26% for carbon dioxide. At 10 atm, the error for carbon dioxide drops to less than 10% and hydrogen to less than 1%. The pressure under consideration in the study of hermeticity is seldom greater than 5 atm, so that Boyle's Law is adequate.
1.1.2 Charles's Law (1787) or Gay-Lussac's Law (1802)
The pressure of any gas in a fixed volume is directly proportional to the absolute temperature of the gas. This law implies that the cubical coefficient of expansion is the same for all gases. This is not true for high pressure (similar to the deviations to Boyle's Law).
1.1.3 Dalton's Law (1801)
The pressure exerted by a mixture of gases is the sum of the pressures exerted by the constituents (partial pressures) separately. Here again, we have deviations at high pressures that will be of no concern to the present topic.
1.1.4 Avogadro's Law (1811)
The number of molecules in a gas of a specific volume, temperature, and pressure is the same for all gases, and is therefore independent of the nature of the gas.
1.1.5 Avogadro's Number
The number of molecules in a gram molecular weight (the weight equal to the molecular weight, or one mole) for any substance is 6.023 × 1023.
1.1.6 Loschmidt's Number
The number of molecules per cubic centimeter needed to produce one atmosphere pressure at the standard temperature of 0°C is the same for all gases and equals 2.685 × 1019.
1.2 Mathematical Relationships
Boyle's Law in mathematical terms is V [varies] 1/P or P [varies] 1/V. From the law of Charles and Gay-Lussac, P [varies] T. Combining these two equations, gives: P [varies] T/V or PV = constant × T.
Applying Avogadro's law for one molecular weight (one mole), leads to the conclusion that the constant is independent of the type of gas. Designating this constant as R0, the equation is:
PV = R0T Eq. (1-1)
For n moles:
PV = nR0T Eq. (1-2)
where: P = the pressure
V = the volume
R0 = the gas constant
T = the absolute temperature in °K
R0 = the gas constant for one mole and is standardized at one atmosphere and at 0°C (273.16°K).
One method of verifying Avogadro's law was to measure the volume of different gases when they consisted of one mole at 0°C. This volume was determined to be 22.414 liters.
From Eq. (1-1):
R0 = 1 atm/mole 22:144 liters/273:16°K = 0.08205 liter atm/mole°K
and since 1 atm = 760 torr,
R0 = 0.08205 × 760 = 62.358 torr-liters deg-1 mole-1
R0 can also be evaluated in ergs, where 1 erg is 1 dyne cm, and 1 dyne is 1 gram cm sec-2. Here the pressure is 76 cm of mercury and the volume is in cubic centimeters.
R0 = 8.314 × 107ergs/mole deg
Since 4.184 × 107 ergs = 1 calorie:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
The pressure of a homogeneous gas is:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
where: v2s = the root mean square of the average velocity
ρ = the density of the gas.
The derivation of Eq. (1-3) is lengthy but can be found in Glasstone and in Jeans.
The density of a gas is:
ρ = γ/V Eq. (l-4)
where γ is the mass of the gas in volume V.
Combining Eq. (1-3) with Eq. (1-4):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
Note that 3PV = v2sγ, and if we divide by 2:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eq. (1-5)
The last term is the well-known equation for kinetic energy.
The velocity of a molecule is related to the Boltzmann constant.
v2s = 3kT/m Eq. (1-6)
where: v2s = the root mean square of the velocity
k = Boltzmann constant = 1.38 × 10-16 ergs deg-1, and is the gas constant for a single molecule
T = absolute temperature
m = the mass of one molecule
Putting Eq. (1-6) into Eq. (1-5),
(3=2)PV = (1=2) γ3kT/m; or PV = γkT/m
Now γ/m = N, the number of molecules, so that:
PV = NkT Eq. (1-7)
If we consider only one mole, then N becomes NA, Avogadro's number, and V is the volume for one mole = 22.41 liters. Therefore:
PV = NAkT Eq. (1-8)
The average velocity is va.
va = &radius;8kT/πm Eq. (1-9)
or
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eq. (1-10)
where: R0 = 8.3146 × 107 ergs/(degree K)(mole)
1 erg = 1 dyne cm
1 dyne = gram cm/sec2
so that:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
where T is in degrees K and M is the molecular weight.
In any gas, there is a distribution of velocities (Maxwellian). The mfp for multiple velocities is:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eq. (1-11)
where: N = the number of molecules in volume V
σ = the diameter of the molecule
(Continues...)
Excerpted from Hermeticity of Electronic Packages by Hal Greenhouse Robert Lowry Bruce Romenesko Copyright © 2012 by Elsevier Inc.. Excerpted by permission of William Andrew. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
1. Gas Kinetics2. Viscous and Molecular Conductance of Gases3. The Flow of Gases4. The Flow of Gases into Sealed Packages5. Understanding Fine Leak Testing in Accordance with Method 1014, MIL-STD-8836. Fine Leak Measurements Using a Helium Leak Detector7. Gross Leaks8. The Permeation of Gases Through Solids9. Water in Sealed Packages10. Residual Gas Analysis (RGA)11. Residual Gas Analysis (RGA) and Failure Modes
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