Patent Application
20220310497
BGA (ball grid array) packages are used for many applications. Package mutual parasitics may come mainly from balls rather than from the substrate itself, especially for flip chip BGA (FCBGA) packages based on copper pillar technology, and contribute to injecting noise into balls carrying sensitive signals (e.g., but not limited to, feedback nodes in switching converters or other circuits). Any noisy current into an inductance creates an undesired change in voltage. Depopulating balls from the array helps with reducing mutual capacitance but not inductance. Staggering of all the package balls (interstitial BGAs) may be done to ease substrate routing, but not for the purpose of reducing localized noise injection from the package. Some packages might use localized staggering just on package corners to reduce mechanical stress on corners and improve reliability. Other packages might also use localized staggering to isolate radio frequency (RF) blocks as a whole, i.e. on the package there would be an RF island with its own bailout and layout. Some packages might use additional ground balls to reduce noise injection from the package.
U.S. Patents 9,490,227 (Dedic et al), 7,831,949 (Tokunga et al), and 6,791,177 (Miller et al) and U.S. Patent Applications 2019/0304899 (Park et al) all discuss noise-sensitive circuitry in BGA packages.
It is the primary objective of the present disclosure to reduce injected noise into balls carrying sensitive signals in a BGA package.
It is a further objective of the present disclosure to reduce injected noise into balls carrying sensitive signals in a BGA package (e.g., but not limited to, feedback nodes in switching converters or other circuits) without the need of extra ground balls.
Yet another objective is to reduce injected noise into balls carrying sensitive signals in a BGA package by staggering the layout locally of those balls carrying sensitive signals.
In accordance with the objectives of the present disclosure, a BGA package having reduced injected noise into balls carrying sensitive signals is achieved. An array of solder balls is mounted on a substrate wherein the solder balls provide package output. Solder balls adjacent to a noise-sensitive solder ball package output are shifted to equalize mutual parasitic inductance around the noise-sensitive solder ball package output.
Also in accordance with the objectives of the present disclosure, a method for fabricating a BGA package having reduced injected noise into balls carrying sensitive signals is achieved. A plurality of solder balls is provided on a substrate wherein the solder balls provide package output. An array layout of the plurality of solder balls is designed wherein for each solder ball that is a noise-sensitive package output, adjacent balls to the noise-sensitive solder ball are shifted so as to equalize mutual parasitic inductance around the noise-sensitive solder ball.
Also in accordance with the objectives of the present disclosure, a package having reduced injected noise into balls carrying sensitive signals is achieved. An array of solder balls is mounted on a substrate wherein the solder balls provide package output. Solder balls adjacent to a noise-sensitive solder ball package output are shifted by one-half the pitch of the array of solder balls to equalize mutual parasitic inductance around the noise-sensitive solder ball package output.
In the accompanying drawings forming a material part of this description, there is shown:
The present disclosure solves the problem of non-equal mutual parasitic inductance for noise-sensitive signals in a Ball Grid Array (BGA) package by partial staggering of balls, that is, staggering applied locally to specific balls which carry sensitive signals. The partial staggering achieves equal mutual inductance for the sensitive ball of interest, relative to the neighboring balls which carry switching signals. This is achieved without the need for extra balls and can be applied to any ball carrying sensitive signals.
For example, rows E and F are GPIOs and Comms related balls. Rows A to D and G to K are power stages so duplication of PVDD, PGND and LXX balls. The arrangement / pin out depends on the number of phases used for the switching converter. Depending on the number of phases needed, one can have more or fewer power stage balls. Circuitry other than switching converters can utilize this package arrangement so unlabeled balls could be other signals too.
Depending on the ball assignment, the noise sensitive signals (FBP 22 and FBN 26 in this example) might get assigned in the center of the array. This is usually the case for multi-phase switching converters where the overall solution size imposes constraints in terms of form factor and size. The center of the array could also have other balls carrying noise sensitive signals other than FBP and FBN. Other circuits than switching converters could also be implemented.
Using the example shown in
In copper pillar based technology (FCBGA), the major contributor to the mutual inductance is the ball. Reducing the diameter of the balls would reduce their parasitic inductance contribution, but in high current applications a maximum current per ball needs to be maintained to achieve the required performance, especially for high power applications, so reducing ball diameter is not a viable option.
The relative difference in mutual inductance from FBP/FBN to the neighbor balls causes an undesired voltage (noise) in the FBP-FBN signal which is amplified by the control loop of the switching converter, for example, leading to unwanted oscillations and/or instability of the switching converter.
The movement of balls also creates room for two additional balls 30 and 32, which can be assigned to a PCB ground that would act as a further shield for the FBP 22 and FBN 26 balls. The PGND balls 24, 28 are used in this example, but the staggered arrangement can contain signals other than PGND, for example, AVDD or others.
A pitch for the X dimension of the package (rows) could have a different pitch than for the Y dimension of the package (columns). From a staggering perspective, the shift in balls would be based on the pitch used in the noise-sensitive balls for which the mutual inductance is being equalized. That is, if a row of balls is being shifted, it is shifted by one-half the X-dimension pitch and if a column of balls is being shifted, it is shifted by one-half the Y-dimension pitch.
Switching converter simulations that include package parasitics show a significant reduction in the peak-to-peak noise induced into FBP-FBN when using the partially staggered ball arrangement of the present disclosure. Below is a table summarizing simulation results with a standard BGA package such as shown in
The PWM control voltage, shown in the third column of Table 1, controls output voltage, current duty cycle, and jitter in a switching converter so a reduction of the noise on the PWM control voltage, as shown in the table above, is also beneficial and comes as a consequence of the reduction in FBP-FBN noise.
The present disclosure provides a BGA package with reduced noise and a method for making such a package. Localized staggering of the layout of balls that are associated with noise-sensitive signals will equalize parasitic mutual inductance and thus, reduce noise to those signals. The ball layout is shifted by half a pitch only in rows or columns in the localized area of the noise-sensitive signals to equalize the mutual inductance of signals. With the staggering or shifting of a row or rows (or column or columns) of balls, additional balls could be added for additional ground, if desired, but it is not necessary to add extra balls.
The package and process of the present disclosure can be used for other types of circuit and for any type of BGA package, with or without copper pillars, or other packages such as Wafer Level Chip Scale Packages (WLCSP).
Although the preferred embodiment of the present disclosure has been illustrated, and that form has been described in detail, it will be readily understood by those skilled in the art that various modifications may be made therein without departing from the spirit of the disclosure or from the scope of the appended claims.
