Solder bar for high power flip chips
View Patent ↗A solder bar compatible with conventional flip chip technology fabrication methods for high power/high current applications includes first and second generally circular solder pads of diameter D formed upon a substrate and connected by a solder bar pad of width BW. The centers of the generally circular solder pads are spaced apart by distance BL (bar length). A mass of solder having volume VB is formed over the first and second generally circular solder pads and over the solder bar pad to form a dog-bone shaped solder bar. The solder bar reaches height H 1 above the centers of the first and second generally circular solder pads, and reaching height H 2 above the midpoint of the solder bar pad. The values for diameter D, bar length BL, bar width BW, and solder volume VB are selected in such manner that H 1 and H 2 are approximately equal. Conventional circular (as viewed from above) solder bumps can be formed upon the same substrate; in this case, heights H 1 and H 2 are made approximately equal to the height of the conventional solder bumps.
1. A reflowable solder bar formed upon an upper surface of a first substrate, the first substrate having a first electrical contact, said reflowable solder bar being adapted to join the first electrical contact to a second electrical contact on a second substrate, said reflowable solder bar comprising in combination:
a. a first generally circular solder pad formed upon the upper surface of the first substrate, the first generally circular solder pad having a center, and having a first predetermined diameter D;
b. a second generally circular solder pad formed upon the upper surface of the first substrate, the second generally circular solder pad having a center, and having said first predetermined diameter D, the center of said second generally circular solder pad being spaced from the center of said first generally circular solder pad by a predetermined spacing distance BL;
c. a solder bar pad of a first predetermined bar width BW formed upon the upper surface of the first substrate connecting said first circular solder pad to said second circular solder pad, the first predetermined bar width BW being less than the first predetermined diameter D;
d. a mass of tin/lead eutectic composition solder having a solder bar volume VB formed over the first and second generally circular solder pads and over said solder bar pad to form said reflowable solder bar, the solder bar volume VB reaching a height H 1 above the centers of said first and second generally circular solder pads, and reaching a height H 2 above a midpoint of said solder bar pad, the mass of tin/lead eutectic composition solder having a lowermost base region adjacent said solder bar pad, the width of the lower most base region of the solder mass along the solder bar pad being substantially equal to solder bar pad width BW;
e. wherein the values for predetermined diameter D, spacing distance BL, predetermined bar width BW, and solder bar volume VB are selected in such manner that H 1 and H 2 are approximately equal.
2. The reflowable solder bar recited by claim 1 wherein conventional generally circular (as viewed from above) solder bumps are also formed upon the upper surface of the first substrate, the conventional generally circular solder bumps having a height H 3 , and wherein height H 1 and height H 2 of said solder bar are approximately equal to height H 3 .
3. The reflowable solder bar recited by claim 2 wherein the conventional generally circular solder bumps have a particular solder pad diameter Dc, and wherein the diameter D of said first and second generally circular solder pads of said solder bar is in the range of from substantially DC to substantially 2 times Dc.
4. The reflowable solder bar recited by claim 2 wherein the conventional generally circular solder bumps have a particular solder bump volume Vc, and wherein the solder bar volume VB is in the range of from substantially 2 times Vc to substantially 5 times Vc.
5. The reflowable solder bar recited by claim 1 wherein said first substrate is a flip-chip integrated circuit.
6. The reflowable solder bar recited by claim 1 wherein any difference between height H 2 and height H 1 is less than 10% of height H 2 .
7. The reflowable solder bar recited by claim 1 wherein any difference between height H 2 and height H 1 is less than 5% of height H 2 .
8. A reflowable solder bar formed upon an upper surface of a first substrate, the first substrate having a first electrical contact, said reflowable solder bar being adapted to join the first electrical contact to a second electrical contact on a second substrate, said reflowable solder bar comprising in combination:
a a first generally circular solder pad formed upon the upper surface of the first substrate, the first generally circular solder pad having a center, and having a first predetermined diameter D;
b. a second generally circular solder pad formed upon the upper surface of the first substrate, the second generally circular solder pad having a center, and having said first predetermined diameter D, the center of said second generally circular solder pad being spaced from the center of said first generally circular solder pad by a predetermined spacing distance BL;
c. a solder bar pad of a first predetermined bar width BW formed upon the upper surface of the first substrate connecting said first circular solder pad to said second circular solder pad, the first predetermined bar width BW being less than the first predetermined diameter D;
d. a mass of reflowable solder having a solder bar volume VB formed over the first and second generally circular solder pads and over said solder bar pad to form said reflowable solder bar, the solder bar volume VB reaching a height H 1 above the centers of said first and second generally circular solder pads, and reaching a height H 2 above a midpoint of said solder bar pad, the mass of reflowable solder having a lowermost base region adjacent said solder bar pad, the width of the lower most base region of the solder mass along the solder bar pad being substantially equal to solder bar pad width BW;
e. wherein the values for predetermined diameter D, spacing distance BL, predetermined bar width BW, and solder bar volume VB are selected in such manner that H 1 and H 2 are approximately equal.
9. The reflowable solder bar recited by claim 8 wherein said mass of reflowable solder comprises eutectic composition solder.