The Science of Component Heat Damage
How Overheating Damages I.C.s
An integrated circuit can be thought of as a miniature circuit board, as shown in the image above left. Gold wires (2) run from the lead frame (1) to aluminum pads (3) on the silicon or silicon oxide substrate.
The I.C. was soldered with a 750°F iron and the bond (3) was cross–sectioned. The electron microscope scan of that cross–sectioned bond is shown in the right–hand image. The gold bond (A) connects to the aluminum pad (D) have reacted naturally to form an intermetallic (B). Because of its color, the intermetallic is known as "purple plague." The intermetallic has significantly greater electrical resistance than either gold or aluminum, so thicker intermetallics mean degraded electrical properties for the component. Depending on the sensitivity of the component, the increased resistance created by a thicker intermetallic can cause system failure.
The intermetallic formation also results in Kirkendahl voids at the interface between the intermetallic and the substrate. Eventually, breaks occur at the edges of the bond as shown in the photograph and the component fails.
Intermetallic growth and Kirkendahl voiding occurs even at room temperature and all electronic components ultimately fail. However, the time to failure at room temperature is decades.
Exponential Heat Damage
From the Arrhenius equation, we know that the rate of chemical reactions roughly doubles with every 10°C increase in temperature. The thermal aging inflicted by a soldering iron at 350°C-400°C — that is, roughly 660°F to 750°F — can, therefore, be profound. Compared to an approximate room temperature of 25°C, intermetallic formation at 350°C occurs at 8,589,934,592 (i.e. 8.59 BILLION) times the rate. If the component has an operating temperature of 50°C, the difference in the rate of intermetallic formation would be 1,073,741,824 (i.e., slightly more than 1 BILLION) times.
There are 31,536,000 (i.e., 31.536 million) seconds in a year. Therefore, a soldering operation that increases the component's internal bond temperature by 325°C for 1 second causes as much degradation as occurs in 272.385 years at room temperature. If the component would have an operating temperature of 50°C, one second of exposure to the iron temperature would age the component "only" 34.048 years.
The standard instruction given to operators to prevent heat damage is "solder quickly." And "quickly" is generally specified as 3 seconds or less. But we can see that catastrophic damage can be inflicted on an I.C. in just a single second. There is nothing reliable about soldering "quickly."
How Many Failures Are "Acceptable"?
The other instruction is to set the iron at a lower temperature. But the iron must be set to the temperature required to solder the components with the highest thermal mass. So a temperature below 315°C (600°F) is generally not practical. So in the best case, one second exposure to the lower temperature iron will "only" age a component 536,870,912 times as quickly.
Whether soldering "quickly" or at "low" temperature, the damage to components can be disastrous. Components subjected to such soldering conditions may not fail at test but will certainly fail prematurely.
Of course, there is enormous variation in the temperature profile from one manually soldered connection to another. So not every component soldered by hand will fail at test. Some may live long, productive lives. But many will not. The question, then, is how many failures are "acceptable?"
Our Heat Control Methodolgy
Components can be soldered with 370°C (700°F) irons without seriously damaging the components. But it can't be done using the methods which are taught in soldering training.
We devised a simple yet scientific and absolutely reliable way to solder components using that 370°C (700°F) or higher soldering iron without subjecting the component to temperature above 232°C (450°F). There is no special equipment required; it works equally well with a $100 soldering iron as with a $1,000 iron. And it is just as fast as using the traditional techniques found in all other solder training. You will be astonished at simplicity and effectiveness of our heat control technique.
Our scientific heat control technique is at the heart of our Science of Soldering© soldering course — and nowhere else.
We would love to discuss this issue with you. Please give us a call at (01)727–866–6502, extension 21 or use this form to us.