Metal fusion bonding – Process – With measuring – testing – indicating – inspecting – or...
Reexamination Certificate
2001-03-07
2004-01-06
Edmondson, L. (Department: 1725)
Metal fusion bonding
Process
With measuring, testing, indicating, inspecting, or...
C228S102000, C228S180100, C228S180210, C228S180220
Reexamination Certificate
active
06672500
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and to an arrangement for measuring the cooling rate and thermal differential between the top and bottom of a printed circuit board (PCB). As defined, the invention is intended to facilitate control over the temperature differential which is encountered between the top and bottom of the PCB so as to prevent warpage thereof during the formation of component/module solder joints or fillets.
BACKGROUND OF THE INVENTION
In the implementation of soldering procedures, for example, in reflow soldering ovens, which are employed in the soldering of high-mass PCBs, particularly when these PCBs are equipped with high-mass ceramic column grid array (CCGA) or ceramic ball grid array (CBGA) modules, there have been ascertained unique types of solder joint failures which are encountered in the formation of the module solder joints for producing electrical interconnections, and which are frequently referred to as solidification stress fractures. There are three types of failure mechanisms that can result;
(a) Solidification fractures, time-zero fails or reliability exposure (any solder fillet).
(b) Stretched or disturbed joints, reliability exposure (any solder fillet).
(c) Brittle cracked columns, time-zero fails or reliability exposure (Solder column modules only).
These modes of solder joint failure have been investigated in the technology, and shown to be highly dependent upon the cooling rate and thermal gradient extending through the thickness of the printed circuit board during corner solder joint or fillet solidification. An aspect which has been ascertained in cases of stress fracture solder joint failure has been the occurrence of a clean fracture which is produced between the intermetallic copper-tin (Cu—Sn) material on the circuit board solder joint pad and the solder material which is in the solder joint or fillet. The result of the foregoing can be either an almost immediate time-zero (instantaneous) open electrical interconnection, or a latent reliability fail which necessitates temperature cycling in order to become electrically open. The time-zero opens are characterized by gaps of up to 1 mil (0.001 inches), which may occur on only a single solder joint or fillet which is surrounded by a large number of so-called “stretched” solder joints. The reliability failures may be separated through less than 100% of the soldered cross sectional area and do not evidence any measurable gap until failure during temperature cycling.
In essence, when a thermocoupled profile card is conducted or conveyed, such as on a belt or conveyor, through a reflow oven, for instance, an infrared (IR)/convection oven which is known to produce stress fracture interconnections, there has been indicated the presence of a sharp transient thermal differential spike between the top and bottom surfaces of the profile card, when thermocouple data is collected and plotted/analyzed as prescribed herein. This particular temperature differential spike is believed to be responsible for inducing printed circuit board warpage at a period in time when module solder joints or fillets especially at a corner and periphery of the module, are at the verge of solidification, thereby resulting in the type of failure mode previously described on nearby solder joints shortly after solidification.
For example, in cases where a pull test wire is soldered to a CCGA solder joint pad on a PCB, and shortly after solidification, it has been ascertained that the solder joint strength is extremely low; for example, approaching only a few grams. Moreover, from modeling studies, there has been indicated a variation in board warpage, which may be on the order of 1 mil. Thus, when these experimental observations are combined, this can readily result in an occurrence of stress fracture types of solder joint failure.
Another aspect of these particular solder joint failures which may not be readily apparent after assembly of the PCB components or modules, resides in the application of PCB deformation during solder joint solidification in effect, warping of the PCB during solder joint solidification, can readily weaken any resulting electrical interconnection solder joint or fillet of the components or modules.
The process causing solder joint failure and related influencing factors are essentially as follows:
1. The temperature differential between the top surface and bottom surface of the PCB (&Dgr;T
z
=T
top
−T
bot
) which is created extending through the thickness of the PCB (the Z-axis) during the cooling segment of the conveyor belt or rail driven oven reflow cycle. The temperature on the top surface of the printed circuit board can be significantly higher or lower than that on the bottom surface, depending on which surface cools more rapidly. One surface may cool faster than the other due to oven factors (forced air rate/volume/temperature, belt effects, etc.) and product attributes (component density and thermal mass). Thus, the magnitude of this Z-axis thermal differential is a function of
(a) PCB thickness inasmuch as thicker boards can withstand a larger temperature difference between the top or upper surface and the bottom surface.
(b) The mass, density and placement of components or modules on the top surface of the printed circuit board whereby the greater mass and density retains more heat on the top side or upper surface in comparison with the bottom side or surface.
(c) Cool-down rate wherein higher cooling rates exaggerate the instantaneous temperature differential between the top or upper and the bottom surfaces of the printed circuit board.
(d) The employment of direct impingement fans in order to cool the top and/or bottom PCB surfaces whereby one surface is cooled significantly faster than the other (instantaneously or during the entire cool down period) due to differences in
(1) Design and use of top versus bottom fans, staggered location of top/bottom fans, differences in airflow, and balanced use (some ovens have fans only on one side or the other).
(2) Design and use of oven belt or work board holder on which the PCB is placed, which can impede airflow more from one side than the other.
(3) Layout of components or modules on PCB top and/or bottom surface, which can also impede airflow locally across the PCB.
2. Differential thermal expansion between top and bottom board surfaces due to Z-axis temperature differential causes the printed circuit board to warp practically instantaneously, thereby imparting a load or stress on some of the component/module solder joints assembled thereon.
In the case of a positive &Dgr;T
z
or change in the positive direction, the top side or upper surface of the printed circuit board expands at a greater rate than the bottom surface, causing the PCB to warp or bend concavely downwardly. In effect, below a module or component site, the PCB moves away from the module or component to the greatest extent at corner and edge solder joints.
3. The instantaneous warping of the printed circuit board with respect to the modules or components which are positioned thereon creates a displacement and a resultant load which can readily produce a disturbed or fractured solder joint or fillet, depending upon the timing and the temperature of the solder joints. This can occur in solder columns, balls, or fillets when the temperature range of the solder is from about (T
1
+10 degrees Celsius)>T
2
>(T
1
−25 degrees Celsius) where T
2
is defined as the instantaneous temperature of the solder joint and T
1
is the solidification point of the solder joint upon cooling. The larger temperature range specified below T
2
is intended to account for a phenomenon known as undercooling.
The result of the foregoing can be a time-zero electrical opening of the solder joints or fillets, and/or early life cycle reliability failures during thermal cycling.
Although considerations have been given in the technology towards improving the reliability of component or module solder joints on printed circuit boards, and particularly in
Caletka David V.
Knadle Kevin
Woychik Charles G.
Edmondson L.
International Business Machines - Corporation
Steinberg William H.
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