Post: QSOP-24 Package: Dimensions and Land Pattern

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QSOP-24 Package: Dimensions and Land Pattern

The QSOP-24 package puts 24 gull-wing leads on 0.635 mm pitch in an 8.56 mm to 8.76 mm body that is 3.81 mm to 4.01 mm wide, with a 5.79 mm to 6.20 mm lead span and 1.75 mm maximum height. JEDEC registers it as MO-137-AE. Four published land patterns for this outline disagree by 0.51 mm in overall span, and two of them ship inside the same KiCad library under different names.

Key takeaways

QSOP-24 is JEDEC MO-137-AE: 0.635 mm pitch, 0.150 in nominal body width, 1.75 mm maximum seated height.
It uses the same body as the 20-lead variation, so four extra pins cost no board area. End margin simply drops from 1.47 mm to 0.83 mm per side.
Published outer land spans run 6.400 mm (onsemi), 6.600 mm (NXP), 6.909 mm (KiCad QSOP-24). KiCad also ships an SSOP-24 footprint at 6.400 mm for the identical package.
A strict IPC-7351B Level B calculation returns a 2.19 mm pad. Substituting a vendor’s tighter foot-length band cuts that to 1.81 mm.
QSOP-24 occupies only about 6% more board area than a 24-lead TSSOP, against 24% for the 20-lead comparison.
Thermally it sits between SO-24 and TSSOP-24: 72 °C/W against 60 °C/W and 85 °C/W on one ST datasheet.

[IMAGE 1: dimensioned QSOP-24 outline, top and side views, with D, E, E1, A, A1, b, c, L and e called out | alt: “QSOP-24 package dimensions diagram showing 0.635 mm lead pitch and 3.81 to 4.01 mm body width”]

QSOP-24 dimensions per JEDEC MO-137-AE

A QSOP-24 (quarter size outline package, 24 leads) is a plastic surface-mount package carrying twelve gull-wing leads per side on 0.635 mm pitch, in a body 3.81 mm to 4.01 mm wide and no more than 1.75 mm tall. Analog Devices publishes it as package option RQ-24 and marks the drawing compliant to JEDEC MO-137-AE.

Symbol	Dimension	Min (mm)	Max (mm)	Min (in)	Max (in)
D	Body length	8.56	8.76	0.337	0.345
E	Lead span, toe to toe	5.79	6.20	0.228	0.244
E1	Body width	3.81	4.01	0.150	0.158
A	Overall height	1.35	1.75	0.053	0.069
A1	Standoff	0.10	0.25	0.004	0.010
A2	Body thickness	1.24	1.63	0.049	0.064
b	Lead width	0.20	0.30	0.008	0.012
c	Lead thickness	0.15	0.25	0.006	0.010
L	Foot length	0.41	1.27	0.016	0.050
e	Lead pitch	0.635 BSC	0.635 BSC	0.025 BSC	0.025 BSC

Table 1. QSOP-24 outline limits, Analog Devices RQ-24 drawing rev 04-12-2021-B, compliant to JEDEC MO-137-AE. Coplanarity 0.10 mm max, lead angle 0° to 8°.

Individual vendors qualify tighter subsets. onsemi case 492B-01 states D, E and E1 as basic dimensions of 8.65 mm, 6.00 mm and 3.90 mm. STMicroelectronics, in the STP16CPP05 datasheet rev 5, holds the foot length L to 0.40 mm to 0.89 mm instead of the JEDEC 0.41 mm to 1.27 mm. That single difference matters more than any other number on this page, for reasons shown below.

The extra four pins are free

Run the row arithmetic. Twelve leads per side on 0.635 mm pitch occupy 11 × 0.635 = 6.985 mm. Inside an 8.65 mm body that leaves 0.83 mm of moulding at each end. The 20-lead variation of the same registration uses the same body length for a 5.715 mm row, so its end margin is 1.47 mm, a figure Diodes prints as ZD on its QSOP-20 drawing.

The practical consequence: moving from a 20-lead to a 24-lead QSOP changes the land pattern row length and nothing else. Body, lead span, height, courtyard width and reel handling are unchanged. If you are choosing between the two lead counts for a new part, board area is not a tiebreaker.

Four published QSOP-24 land patterns, compared

Every pattern below targets the same MO-137-AE outline. They are not interchangeable, and the two KiCad entries are the awkward case: both live in the Package_SO library, both describe a 3.9 × 8.7 mm body on 0.635 mm pitch, and they differ.

Land pattern source	Pad width X (mm)	Pad length Y (mm)	Outer span Z (mm)	Inner span G (mm)	Toe beyond E max (mm)
onsemi case 492B-01, 06 May 2008	0.42	1.12	6.400	4.160	0.10
NXP SOT556-1, 8 Feb 2016 (SSOP24)	not resolved	1.200	6.600	4.200	0.20
KiCad SSOP-24_3.9×8.7mm_P0.635mm	0.40	1.20	6.400	4.000	0.10
KiCad QSOP-24_3.9×8.7mm_P0.635mm	0.41	1.60	6.909	3.709	0.354
IPC-7351B Level B, JEDEC L band	0.433	2.189	6.924	2.545	0.362
IPC-7351B Level B, ST L band	0.439	1.806	6.924	3.313	0.362

Table 2. Toe projection is measured against the 6.20 mm maximum lead span. NXP pad width: the symbol assignment could not be resolved from the PDF text layer, which lists 0.450 mm and 0.500 mm as the candidate values. Read it off Figure 2 of the NXP document before you use it.

[IMAGE 2: the four land patterns drawn to scale on one grid with the package body outline overlaid | alt: “QSOP-24 land pattern comparison showing onsemi, NXP, KiCad and IPC-7351B pad geometry”]

The KiCad pair deserves a closer look. Its SSOP-24 footprint cites NXP SOT556-1 as the source, and it does copy the 1.2 mm pad length. But it places the pad centres at ±2.600 mm, while the NXP drawing’s own 6.600 mm and 4.200 mm spans put them at ±2.700 mm. The library footprint is shifted 0.1 mm inboard per side against the document it names. That comes straight off the toe fillet, which is already the tightest part of this joint.

The KiCad QSOP-24 entry, sourced from a Microchip packaging specification, goes the other way with 1.6 mm pads and a 6.909 mm span, and it is the only vendor or library pattern here whose inner edge tucks under the body outline, by 0.15 mm per side at maximum material condition. That buys heel fillet and costs optical access.

Choose by process. AOI with a toe-fillet criterion favours the KiCad QSOP-24 or IPC geometry. A crowded board that needs the routing channel back favours onsemi or NXP. Hand rework favours the longest pads you can fit.

Fixing the IPC-7351B heel calculation

The IPC-7351B gull-wing equations combine component limits with a solder-joint goal. Level B goals for a SOIC-family gull-wing lead are JT 0.35 mm, JH 0.35 mm and JS 0.03 mm, with 0.10 mm each for fabrication and placement. Feeding the JEDEC MO-137-AE band in:

Z max = L min + 2 × JT + √(CL² + F² + P²) = 5.79 + 0.70 + 0.434 = 6.92 mm

G min = S max − 2 × JH − √(CS² + F² + P²) = 5.38 − 0.70 − 2.135 = 2.55 mm

A 2.19 mm pad. The culprit is the foot length band: L runs 0.41 mm to 1.27 mm, so the heel span S swings from 3.25 mm to 5.38 mm and the root-sum-square term alone consumes 2.13 mm. The equation is not wrong, the input is loose.

Now substitute the ST STP16CPP05 numbers, where L is held to 0.40 mm to 0.89 mm and E to 5.80 mm to 6.20 mm. S swings only from 4.02 mm to 5.40 mm, the RSS term drops to 1.39 mm, and G min rises to 3.31 mm. Pad length falls from 2.19 mm to 1.81 mm. Z is unchanged at 6.92 mm because it never depended on the foot length.

The rule that follows: never run an IPC wizard on the family registration when you have the specific vendor drawing. Half a millimetre of pad length, and the routing channel underneath the part, hangs on which tolerance band you typed in.

QSOP-24 against the other 24-lead options

The STP16CPP05 is a useful reference because ST offers the same die in four packages and publishes thermal resistance for all of them in one table.

24-lead package	Body L × W (mm)	Pitch (mm)	Lead span (mm)	Max height (mm)	θJA (°C/W)	Board area (mm²)
QSOP-24	8.76 × 4.01	0.635	5.80 to 6.20	1.73	72	54.3
SO-24	15.60 × 7.60	1.27	10.00 to 10.65	2.65	60	166.1
TSSOP-24	7.90 × 4.50	0.65	6.25 to 6.50	1.10	85	51.4
TSSOP-24 exposed pad	7.90 × 4.50	0.65	6.20 to 6.60	1.20	37.5	52.1

Table 3. All values from the ST STP16CPP05 datasheet, DocID15379 rev 5, March 2017. Board area is computed here as maximum body length × maximum lead span, so it is a placement envelope, not a courtyard.

Two results are worth keeping. First, at 24 leads the QSOP is only about 6% larger in placement area than the TSSOP, against roughly 24% at 20 leads, because the QSOP body length stops growing while the TSSOP body keeps stretching. Second, the QSOP-24 is thermally better than the TSSOP-24 and worse than the SO-24, which is the opposite of the ordering most engineers assume from size alone.

Work an example with the same part. Sixteen output channels at 40 mA with a 0.725 V typical dropout, the value ST tabulates at 40 mA, dissipate 0.46 W. In the QSOP-24 that is a 33 °C junction rise; in the TSSOP-24 exposed pad version the same power gives 17 °C. If the design sits near the 125 °C operating limit, that difference decides the package, not the footprint area.

Stencil, mask and packing at 0.635 mm pitch

Copper is not the constraint here. The web between adjacent lands is 0.215 mm on the onsemi pattern, 0.225 mm on the KiCad QSOP-24 and 0.235 mm on the KiCad SSOP-24. After the fab applies solder-mask expansion there is very little dam left, and many shops will gang the openings rather than attempt one. Ask before you specify the widest pads.

Paste release has margin. IPC-7525 asks for an area ratio above 0.66. A 0.42 mm by 1.12 mm aperture in a 0.127 mm stencil returns 1.20; the 0.41 mm by 1.60 mm KiCad aperture returns 1.29. Print 1:1 with the land and spend the process budget on registration and on stencil cleaning between prints.

Packing differs by vendor, which matters for feeder setup. ST ships the STP16CPP05PTR in QSOP-24 at 2,500 parts per reel, while the Renesas QS3861QG8 in the same package is listed at 3,000 per reel. The legacy Hittite QS24 datasheet rates the package MSL1 with a 235 °C peak reflow for the Sn/Pb lead finish and 260 °C for the matte tin QS24E version, so confirm the finish before you set the profile.

What actually ships in QSOP-24, and the second-source picture

The package is mature rather than dead, and it clusters in bus switches, LED drivers and interface parts. Three examples, each of which happens to have a same-die alternative in another package:

Renesas QS3861QG and QS3861QG8, a 10-bit bus switch, with the QS3861PAG offering the identical function in TSSOP-24.
ST STP16CPP05PTR, a 16-channel constant-current LED sink driver, also sold as SO-24, TSSOP-24 and TSSOP-24 exposed pad.
NXP CBTD3384, a 10-bit level-shifting bus switch, in SSOP24 (SOT556-1) and TSSOP24 (SOT355-1).

That pattern is the second-source strategy. For most QSOP-24 parts the drop-in escape route is not another vendor’s QSOP-24; it is the same vendor’s TSSOP-24, which needs a different footprint. Laying out both patterns on the board, or at least keeping both in the library from day one, is cheaper than a respin during an allocation.

Five QSOP-24 mistakes that cause rework

Assuming the KiCad SSOP-24 and QSOP-24 footprints are duplicates. They differ by 0.51 mm in span and 0.4 mm in pad length.
Running an IPC wizard against the JEDEC family band and accepting the 2.19 mm pads it returns.
Treating a 24-lead QSOP as a bigger part than a 20-lead one when specifying keepouts. Same body, same courtyard width.
Copying a QSOP-24 footprint onto a TSSOP-24 second source. Different pitch, different span, different body.
Setting one reflow profile for mixed lead finishes. The same package outline carries both 235 °C and 260 °C ratings depending on plating.

QSOP-24 FAQ

What is a QSOP-24 package?

A QSOP-24 is a 24-lead plastic surface-mount package with gull-wing leads on 0.635 mm pitch, a body 3.81 mm to 4.01 mm wide and no more than 1.75 mm tall. JEDEC registers it as MO-137-AE. Twelve leads sit on each long side, and the body is the same length as the 20-lead version of the same registration.

What are the QSOP-24 dimensions?

Body length 8.56 mm to 8.76 mm, body width 3.81 mm to 4.01 mm, lead span 5.79 mm to 6.20 mm, overall height 1.35 mm to 1.75 mm, lead pitch 0.635 mm basic, foot length 0.41 mm to 1.27 mm. Those are the JEDEC MO-137-AE limits; individual vendors publish tighter subsets.

Is QSOP-24 the same as SSOP-24?

Sometimes, which is the problem. NXP registers the identical 3.9 mm body on 0.635 mm pitch as SSOP24, outline SOT556-1, and cites JEDEC MO-137. But other vendors sell an SSOP-24 on 0.65 mm pitch with a 5.3 mm body under JEDEC MO-150. Check the pitch and the lead span, never the name.

What land pattern should I use for a QSOP-24?

Start from the drawing in the datasheet of the part you are buying. If it has none, the onsemi 492B-01 pattern is the conservative choice at 6.40 mm span, and the KiCad QSOP-24 footprint at 6.909 mm is closest to IPC-7351B Level B. Confirm the mask dam with your fabricator either way.

Can you hand solder a QSOP-24?

Yes. At 0.635 mm pitch, drag soldering with plenty of flux and a chisel tip is routine, and the gull-wing leads stay visible for inspection afterwards. Pick the longest published pad you can fit, since pad sticking out past the toe gives the iron somewhere to work. QSOP-24 to DIP-24 adapter boards exist for bench work.

QSOP-24 or TSSOP-24 for a new design?

Area barely separates them at this lead count, roughly 54 mm² against 51 mm². Choose TSSOP-24 for height, 1.10 mm against 1.73 mm, or for a thermal pad if the part offers one. Choose QSOP-24 when the function only exists there, or when your line already qualifies 0.635 mm pitch.

What to do next

Open the datasheet for your exact part number and copy its foot-length and lead-span band into Table 1’s place. If the vendor gives a tighter L than the JEDEC 0.41 mm to 1.27 mm, rerun the Level B calculation with it before you generate anything. Then pick the pattern by inspection method: onsemi or NXP geometry if copper must stay clear of the body, KiCad QSOP-24 or IPC Level B if toe fillet is what gets measured. Last, add the TSSOP-24 sibling footprint to your library at the same time. On this package the realistic second source is a different package, and finding that out during a shortage is expensive.

External references consulted for this article:

Analog Devices RQ-24 package drawing: https://www.analog.com/media/en/package-pcb-resources/package/pkg_pdf/qsoprq/rq_24.pdf

onsemi mechanical case outline QSOP24 NB, case 492B-01: https://www.onsemi.com/pub/Collateral/492B-01.PDF

NXP SOT556-1 package information, SSOP24: https://www.nxp.com/docs/en/package-information/SOT556-1.pdf

STMicroelectronics STP16CPP05 datasheet, DocID15379 rev 5: https://www.st.com/resource/en/datasheet/stp16cpp05.pdf

KiCad Package_SO QSOP-24 footprint: https://github.com/KiCad/kicad-footprints/blob/master/Package_SO.pretty/QSOP-24_3.9×8.7mm_P0.635mm.kicad_mod

KiCad Package_SO SSOP-24 footprint: https://github.com/KiCad/kicad-footprints/blob/master/Package_SO.pretty/SSOP-24_3.9×8.7mm_P0.635mm.kicad_mod