JP2008198687A - Printed wiring board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a printed wiring board having a Kelvin connection pattern capable of more correct current detection. <P>SOLUTION: Respective pads for soldering both electrodes of a surface-mounted type resistor are divided into rectangular main pads 20 and 21 and L-shaped sub pads 24 and 25. Sub outgoing lines 26 and 27 for detecting current extend from specific positions X1 and Y1 on the sub pads 24 and 25 close to a reference point where a standard resistance value of the resistor 1 is exhibited. An area of the main pads 20 and 21 is approximately the same as that of the sub pads 24 and 25. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a printed wiring board, and more particularly to a wiring pattern for mounting a surface mount type current detection resistor.

  When an IC (Integrated Circuit) such as a DC / DC converter is mounted on a printed wiring board, a precision resistor having a low resistance and a small error may be externally mounted in order to detect an output current. FIG. 8 shows a conventional wiring pattern when a surface-mounted resistor is used as such a resistor, and FIG. 9 shows a surface-mounted resistor.

  Referring to FIG. 8, each pad for soldering the surface-mounted resistor 1 is divided into two. That is, each pad is composed of main pads 2 and 3 having a large area and sub pads 4 and 5 having a small area. Referring to FIG. 9, the surface-mounted resistor 1 includes a resistor 6, one electrode 7, and the other electrode 8.

  Referring to FIG. 8 again, the one electrode 7 of the resistor 1 is disposed across the main pad 2 and the sub pad 4 and soldered. On the other hand, the other electrode 8 of the resistor 1 is disposed over the main pad 3 and the sub pad 5 and soldered. The large current output from the IC flows in the order of the main lead wire 9, the main pad 2, the resistor 1, the main pad 3, and the main lead wire 10. The sub lead lines 11 and 12 extending from the sub pads 4 and 5 are connected to a voltmeter, whereby the voltage applied to the resistor 1 is measured and the current flowing through the resistor is detected. In this way, a line in which an actual large current flows and a line for detecting current are separately provided for the current detection resistor are generally called “Kelvin connection”. According to the Kelvin connection, even when a large current flows, the current detection line is less affected by the current, and the current can be detected more accurately than when the pads are integrated.

  However, there are problems with Kelvin connections. Referring to FIG. 9, the standard resistance value of resistor 1 appears between reference points X and Y of resistor 6. On the other hand, when the position immediately below the reference point X where the electrode 7 contacts the main pad 2 is X1, there is a certain distance L1 between the position X1 and the sub lead line 11. If the position immediately below the reference point Y where the other electrode 8 contacts the main pad 3 is Y1, there is a certain distance L1 (for example, 2.0 mm) between the position Y1 and the sub lead line 12. Therefore, in addition to the resistance value on the standard of the resistor 6, a resistance value corresponding to the distance L1 is added, and there is a problem that current cannot be detected accurately. However, if the sub lead lines 11 and 12 are arranged so as to extend from the inside rather than the outside of the sub pads 4 and 5, the distance L2 (for example, 1.25 mm) is slightly shortened. Can not.

  Further, the solder is placed in a lump over the main pad 2 and the sub pad 4, but since the areas of the main pad 2 and the sub pad 4 are different, the solder is biased to the main pad 2 having a large surface tension. The same applies to the main pads 3 and 5 side. Therefore, there is a problem that poor contact is likely to occur.

Japanese Unexamined Patent Application Publication No. 2002-372551 (Patent Document 1) discloses a technique related to a mounting structure of a current detection resistor. According to this technology, the wiring pattern on the substrate connected to the voltage detection terminal (Kelvin terminal) of the current detection resistor is electrically insulated from the resistor of the current detection resistor, and the current path to be measured of the resistor is measured. And then drawn in a direction perpendicular to the current path to be measured. In addition, in a current detection resistor that uses a part of the current electrode to be measured as a voltage detection terminal, the connection point between the voltage detection terminal portion of the current electrode to be measured and the pattern on the substrate is the resistance of the current detection resistor. They are spaced apart from the central axis in the direction along the measured current of the body in opposite directions. With this structure, the measured current path of the resistor and the wiring pattern from the Kelvin terminal are magnetically coupled by mutual inductance, and the voltage detected between the Kelvin terminals has a current due to the influence of the inductance of the measured current path. The voltage formed by the mutual inductance acts to cancel the error voltage proportional to the time change. However, each pad used here is not divided into two.
JP 2002-372551 A

  The objective of this invention is providing the printed wiring board which has a Kelvin connection pattern which can detect an electric current more correctly.

Means for Solving the Problems and Effects of the Invention

  The printed wiring board according to the present invention includes first and second main pads, first and second main lead lines, first and second sub pads, and first and second sub lead lines. Prepare. The first main pad is soldered to one electrode of the surface mount resistor. The second main pad is soldered to the other electrode of the resistor. The first main lead line extends from the first main pad. The second main lead line extends from the second main pad. The first sub-pad is disposed adjacent to the first main pad, and one electrode of the resistor is soldered together with the first main pad. The second subpad is disposed adjacent to the second main pad, and the other electrode of the resistor is soldered together with the second main pad. The first sub lead line extends from a specific position of the first sub pad close to the reference point while the resistance value on the standard of the resistor appears. The second sub lead line extends from a specific position of the second sub pad near the reference point on the other hand, where the resistance value on the standard of the resistor appears.

  In this printed wiring board, since the sub leader line extends from the specific position of the sub pad, the distance between the specific position and the sub leader line is shortened. As a result, the current flowing through the resistor can be accurately detected.

  Preferably, the first main pad has a substantially rectangular shape. The first sub pad has a substantially L shape surrounding two adjacent sides of the first main pad. The second main pad is substantially rectangular. The second sub pad has a substantially L shape surrounding two adjacent sides of the second main pad. Here, the rectangle includes not only a rectangle but also a square.

  In this case, the ends of the main pad and the sub pad are exposed on both sides of the resistor that has been soldered, and the quality of soldering can be visually determined.

  Preferably, the area of the first main pad is the same as the area of the first sub pad. The area of the second main pad is the same as the area of the second sub pad. Here, the areas do not have to be exactly the same, but may be substantially the same. Specifically, there may be a difference of about 10% in the area.

  In this case, the surface tension acting on the solder placed over both pads is the same. Therefore, the solder is evenly spread over both without unevenness. As a result, the resistor can be reliably soldered and contact failure is unlikely to occur.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

  Referring to FIG. 1, generally, a resistor 1 and a power transistor 14 are externally connected to an IC 13 for a DC / DC converter. The resistor 1 is for detecting the output current, and a precision resistor having a low resistance value and a small error is used.

[First Embodiment]
A pad structure for soldering the surface-mounted resistor 1 to the printed wiring board is shown in FIG. Referring to FIG. 2, the printed wiring board according to the embodiment of the present invention includes main pads 20 and 21, main lead lines 22 and 23, sub pads 24 and 25, and sub lead lines 26 and 27. .

  The main pad 20 is soldered to one electrode 7 of the surface-mounted resistor 1. The other electrode 8 of the resistor 1 is soldered to the main pad 21. The main lead line 22 extends from the main pad 20. The main lead line 23 extends from the main pad 21. The sub pad 24 is disposed adjacent to the main pad 20, and one electrode 7 of the resistor 1 is soldered together with the main pad 20. The sub pad 25 is disposed adjacent to the main pad 21, and the other electrode 8 of the resistor 1 is soldered together with the main pad 21. The sub lead line 26 extends from a specific position X1 of the sub pad 24 close to the reference point X while the resistance value on the standard of the resistor 1 appears. The sub lead line 27 extends from a specific position Y1 of the sub pad 25 close to the reference point Y on the other side where the resistance value on the standard of the resistor 1 appears.

  The main pad 20 has a substantially rectangular shape. The sub pad 24 has a substantially L shape surrounding two sides of the main pad 20 adjacent to each other. The main pad 21 is substantially rectangular. The sub pad 25 has a substantially L shape surrounding the two adjacent sides of the main pad 21. The area of the main pad 20 is substantially the same as the area of the sub pad 24. The area of the main pad 21 is substantially the same as the area of the sub pad 25.

FIG. 3 is an equivalent circuit of Kelvin connection. R _OUT is the value of the resistor 1, r _F1 is the internal resistance on the main pad 20 side, r _F2 is the internal resistance on the main pad 21 side, r _S1 is the internal resistance on the sub pad 24 side, and r _S2 is the internal resistance on the sub pad 25 side. Internal resistance. Both electrodes 7 and 8 of the resistor ₁ are connected to a voltmeter via internal resistances r _S1 and r _S2 . Therefore, the smaller the internal resistances r _S1 and r _S2 are, the more accurately the current flowing through the resistor 1 can be detected.

In this printed wiring board, since the sub lead lines 26 and 27 extend from the inside of the sub pads 24 and 25 closest to the specific positions X1 and Y1, the distance from the specific positions X1 and Y1 to the ends of the sub lead lines 26 and 27. L3 (for example, 0.25 mm) is shorter than before. As a result, since the internal resistances r _S1 and r _S2 are reduced, the current flowing through the resistor 1 can be accurately detected.

  In addition, when soldering the resistor 1 by reflow, cream solder is preliminarily placed in a lump so as to straddle the main pads 20 and 21 and the sub pads 24 and 25. The area of the main pads 20 and 21 is the sub pad 24. , 25, the surface tension acting on the solder is almost the same. Therefore, the solder is not biased to any of the main pads 20 and 21 or the sub pads 24 and 25, and is evenly spread over both. As a result, the resistor 1 can be reliably soldered to the main pads 20 and 21 and the sub pads 24 and 25, and contact failure is unlikely to occur.

  Further, since the main pads 20 and 21 are substantially rectangular and the sub pads 24 and 25 are substantially L-shaped, the main pads 20 and 21 and the sub pads 24 and 25 are arranged on both sides of the resistor 1 which has been soldered. The end of the metal is exposed, and the quality of soldering can be visually determined.

In the DC / DC converter using the LTC3728LX made by Linear Technology, a precision resistor is used for current limiting. According to the standard, the limit current Imax can be obtained by the following equation (1) when the value of the precision resistor is Rsense.
Imax = 50 mV / Rsense (1)

  In order to set Imax = 5.000 A, it is necessary to set Rsense = 0.01Ω. This is a design ideal value when the resistance of the pad is ignored.

When the limiting current is obtained for the case where the pad shown in FIG. 1 is used and the case where the pad shown in FIG. 8 is used, the following table 1 is obtained.

  Since the resistance value per 1 cm (= 10 mm) of a copper foil having a width of 1 mm and a thickness of 35 μm is generally 0.0048Ω, the resistance of the conventional pad is 0.00096Ω (= 2.0 mm × 0.0048Ω / 10 ), The resistance of the pad of the present invention is 0.00012Ω (= 0.25 mm × 0.0048Ω / 10). The limiting current is 4.562 A in the conventional case, but is 4.941 A in the present invention. Therefore, the detection error with respect to the design ideal value is 7.5% in the conventional case, but 1.2% in the present invention. Thus, the limiting current of the present invention is close to the design ideal value, and the current can be detected more accurately than in the past.

[Second Embodiment]
In the first embodiment, the area of the main pad and the area of the sub pad are substantially the same. For example, as shown in FIG. , 33 may be rectangular and arranged inside the main pads 30, 31. However, in this case, there is a possibility that the solder placed over the main pads 30 and 31 and the sub pads 32 and 33 is biased toward the main pads 30 and 31 due to a difference in surface tension. Further, since the sub pads 32 and 33 are hidden after the resistor 1 is mounted, it is impossible to visually determine whether the soldering is good or bad.

[Third Embodiment]
Further, as shown in FIG. 5, for example, the main pads 34 and 35 and the sub pads 36 and 37 may all be rectangular with the same area and arranged adjacent to each other. However, in this case, since the sub pads 36 and 37 are almost hidden after the resistor 1 is mounted, it is difficult to visually determine whether the soldering is good or bad.

[Fourth Embodiment]
The first to third embodiments have patterns suitable for the surface-mounted resistor 1 shown in FIG. 9, but the fourth embodiment is different from the surface-mounted resistor 40 shown in FIG. It is a suitable pattern. In the surface-mounted resistor 40, the reference points X and Y at which standard resistance values appear are at the centers of both end faces. The one electrode 41 and the other electrode 42 extending from both end faces are bent along the resistor body 43 and attached to the resistor body 43. The resistor main body 43 is made of resin, and a resistor is enclosed therein.

  In order to mount the surface-mounted resistor 40, as shown in FIG. 7, the substantially rectangular main pads 44 and 45 are disposed on the inner side, and the substantially L-shaped sub pads 46 and 47 are disposed on the outer side. The main lead lines 48 and 49 extend from the inside of the main pads 44 and 45. The sub lead lines 50 and 51 extend from specific positions X 1 and Y 1 outside the sub pads 46 and 47.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

It is a circuit diagram of a general DC / DC converter. It is a top view which shows the Kelvin connection pattern of the printed wiring board by the 1st Embodiment of this invention. It is an equivalent circuit of Kelvin connection. It is a top view which shows the Kelvin connection pattern of the printed wiring board by the 2nd Embodiment of this invention. It is a top view which shows the Kelvin connection pattern of the printed wiring board by the 3rd Embodiment of this invention. It is a top view which shows the Kelvin connection pattern of the printed wiring board by the 4th Embodiment of this invention. It is a perspective view which shows the external appearance structure of a surface mount type resistor. It is a perspective view which shows the external appearance structure of the surface mount type resistor different from FIG. It is a top view which shows the conventional wiring pattern for mounting the resistor for electric current detection.

Explanation of symbols

1,40 Surface mount type resistors 20, 21, 30, 31, 34, 35, 44, 45 Main pads 24, 25, 32, 33, 36, 37, 46, 47 Sub pads 7, 41 One electrode 8, 42 Other electrodes 22, 23, 48, 49 Main leader lines 26, 27, 50, 51 Sub leader lines X, Y Reference points X1, Y1 Specific position

Claims (3)

A first main pad to which one electrode of the surface mount resistor is soldered;
A second main pad to which the other electrode of the resistor is soldered;
A first main lead line extending from the first main pad;
A second main lead line extending from the second main pad; and a second main lead line disposed adjacent to the first main pad, wherein one electrode of the resistor is soldered together with the first main pad. 1 subpad,
A second subpad disposed adjacent to the second main pad, wherein the other electrode of the resistor is soldered together with the second main pad;
A first sublead line extending from a specific position of the first subpad close to a reference point while a resistance value on the standard of the resistor appears;
A printed wiring board, comprising: a second sub lead line extending from a specific position of the second sub pad near the reference point on the other side where a resistance value on the standard of the resistor appears. The printed wiring board according to claim 1,
The first main pad has a generally rectangular shape, the first sub pad has a generally L-shape that surrounds two adjacent sides of the first main pad, and the second main pad has a generally rectangular shape. The printed wiring board is characterized in that the second sub pad has a substantially L shape surrounding two adjacent sides of the second main pad. The printed wiring board according to claim 1 or 2,
The area of the first main pad is the same as the area of the first sub pad, and the area of the second main pad is the same as the area of the second sub pad. Wiring board.

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