Post: QSOP-28 Package: Dimensions and Land Pattern

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QSOP-28 Package: Dimensions and Land Pattern

The QSOP-28 package holds 28 gull-wing leads on 0.635 mm pitch in a body 9.80 mm to 10.01 mm long and 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-AF. Two very different packages are sold as SSOP-28, and published land patterns for this one range from 6.400 mm to 6.800 mm across the rows.

Key takeaways

QSOP-28 is JEDEC MO-137-AF: 0.635 mm pitch, 0.150 in nominal body width, 1.75 mm maximum seated height, 9.9 mm nominal body length.
A part sold as SSOP-28 may be this package or the 5.30 mm wide, 0.65 mm pitch one. Their land patterns differ by 0.25 mm in span and 0.245 mm in row length.
Published pad widths run from 0.300 mm (Richtek) to 0.419 mm (Analog Devices GN28). Maximum lead width is 0.300 mm, so the narrowest of those allows no side fillet at all.
The KiCad SSOP-28 footprint spans 6.400 mm, which is tighter than an IPC-7351B Level C calculation on the same inputs.
Board area no longer favours QSOP at this lead count: 62.1 mm² against 62.7 mm² for a 28-lead TSSOP.
Over 13 pitches, the 0.015 mm difference between 0.635 mm and 0.65 mm accumulates to 0.195 mm end to end.

[IMAGE 1: dimensioned QSOP-28 outline, top and side views, with D, E, E1, A, A1, b, c, L and e called out | alt: “QSOP-28 package dimensions diagram showing 0.635 mm lead pitch and 9.80 to 10.01 mm body length”]

QSOP-28 dimensions per JEDEC MO-137-AF

A QSOP-28 (quarter size outline package, 28 leads) is a plastic surface-mount package carrying fourteen 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-28 and marks the drawing compliant to JEDEC MO-137-AF.

Symbol	Dimension	Min (mm)	Max (mm)	Min (in)	Max (in)
D	Body length	9.80	10.01	0.386	0.394
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-28 outline limits, Analog Devices RQ-28 drawing rev 04-12-2021-B, compliant to JEDEC MO-137-AF. Coplanarity 0.10 mm max, lead angle 0° to 8°.

Vendor drawings agree closely. Renesas M28.15 rev 1 6/04, headed both SSOP and QSOP, gives 9.81 mm to 10.00 mm body length and a lead span of 5.80 mm to 6.19 mm. The Analog Devices GN28 drawing, LTC DWG 05-08-1641 rev B, quotes 9.804 mm to 9.982 mm excluding mould flash. Any of the three will do for a courtyard; use the one in your part’s datasheet for the foot length.

Two different packages are sold as SSOP-28

This is the trap that costs boards. The JEDEC registration behind QSOP is titled Plastic Shrink Small Outline Package (SSOP) Family, so vendors are free to call this outline SSOP-28, and many do. A separate and much older SSOP-28 exists on a 5.30 mm body at 0.65 mm pitch. The two are not footprint compatible and are not even close.

28-lead package	Body L × W (mm)	Pitch (mm)	Lead span (mm)	Max height (mm)	Published land span Z (mm)	Pad row length (mm)
QSOP-28 / narrow SSOP-28	10.01 × 4.01	0.635	5.79 to 6.20	1.75	6.800	8.555
SSOP-28, 5.30 mm body	10.20 × 5.30	0.65	not stated on drawing	not stated	7.050	8.800
TSSOP-28	9.80 × 4.50	0.65	6.40	1.20	7.100	8.800

Table 2. Narrow-body figures from the ADI RQ-28 drawing and the Richtek SSOP-28 footprint file. Wide-body figures from the Union Semiconductor SSOP28 land pattern 111-1078-28 rev 02, June 2016. TSSOP-28 from the Renesas M28.173 drawing rev 1, 5/10 (JEDEC MO-153), whose recommended land pattern is 1.45 mm × 0.35 mm pads at 5.65 mm centre to centre.

Read the pitch first, every time. If the drawing says 0.635 mm you have this package. If it says 0.65 mm you have one of the other two, and the pad row is 0.245 mm longer.

Three published land patterns for the narrow body

Land pattern source	Pad width X (mm)	Pad length Y (mm)	Outer span Z (mm)	Inner span G (mm)	Toe beyond E max (mm)
Richtek footprint file, SSOP-28	0.300	1.300	6.800	4.200	0.30
KiCad SSOP-28_3.9×9.9mm_P0.635mm	0.400	1.200	6.400	4.000	0.10
Analog Devices GN28, 05-08-1641 rev B	0.419	1.143	not resolved	not resolved	not resolved
IPC-7351B Level A (most)	0.473	2.489	7.324	2.345	0.562
IPC-7351B Level B (nominal)	0.433	2.189	6.924	2.545	0.362
IPC-7351B Level C (least)	0.393	1.889	6.524	2.745	0.162

Table 3. Toe projection measured against the 6.20 mm maximum lead span. The Richtek column mapping checks out twice: (A − B) / 2 equals the stated 1.300 mm pad length, and 13 × 0.635 + 0.300 equals the stated 8.555 mm row length. The GN28 span dimension could not be resolved from the drawing’s text layer; its pad size is unambiguous.

[IMAGE 2: the three published land patterns and the IPC-7351B Level B result drawn to scale on one grid | alt: “QSOP-28 land pattern comparison showing Richtek, KiCad and IPC-7351B pad geometry”]

Note where the KiCad entry lands. Its 6.400 mm span is below the 6.524 mm Level C result, and Level C is the minimum-material geometry IPC reserves for handheld products. At the 6.20 mm maximum lead span it leaves 0.10 mm of pad outside the toe. That is enough copper for the joint to work and not much for an inspector to grade.

The Richtek pattern is the opposite choice on span, at 6.800 mm, which sits between Level C and Level B. Its inner edge stops 0.095 mm clear of the maximum body outline, so no copper hides under the moulding. On span alone it is the one to copy.

The side fillet problem at 0.300 mm pad width

Pad width is where these patterns really separate. Maximum lead width b in MO-137-AF is 0.300 mm. The Richtek pad is also 0.300 mm, so at maximum material condition the land and the lead are the same width and there is no side fillet to form. KiCad’s 0.400 mm gives 0.050 mm per side. The Analog Devices GN28 pad, 0.0165 in with a tolerance of 0.0015 in, works out at 0.419 mm and gives 0.059 mm per side.

IPC-7351B asks for 0.03 mm of side fillet at Level B and 0.05 mm at Level A. Running the equations on the MO-137-AF band returns a pad width of 0.393 mm at Level C, 0.433 mm at Level B and 0.473 mm at Level A. Every published pattern here is narrower than the Level B result, and one of them is narrower than Level C.

Does it matter? For reflow, a zero side fillet is legal and inspectable under IPC-A-610 as long as the toe and heel are formed. It matters for two other things: optical inspection, because a side fillet is what an AOI algorithm usually measures, and placement tolerance, because a pad no wider than the lead turns every 0.05 mm of placement error into exposed lead edge. If your line is running a 0.05 mm placement spec, widen the pad to at least 0.40 mm.

Why 0.635 mm and 0.65 mm rows do not interchange at 28 leads

A 0.015 mm pitch difference sounds like nothing. It accumulates. Fourteen leads per side means thirteen pitches, so the pad row is 13 × 0.635 = 8.255 mm on this package and 13 × 0.65 = 8.450 mm on a 0.65 mm part. That is 0.195 mm of difference end to end, or 0.098 mm at each outermost pad.

Lead count	Pitches per side	Row length at 0.635 mm	Row length at 0.65 mm	Accumulated error	Offset at end pad
20	9	5.715 mm	5.850 mm	0.135 mm	0.068 mm
24	11	6.985 mm	7.150 mm	0.165 mm	0.083 mm
28	13	8.255 mm	8.450 mm	0.195 mm	0.098 mm

Table 4. Pitch accumulation across the QSOP lead counts. The end-pad offset is half the accumulated error, since the rows are centred on the same point.

At 0.098 mm the outer pin sits a quarter to a third of a pad width off centre before you add placement tolerance. Combine it with the 0.235 mm gap between lands on a KiCad-style pattern and the bridging margin at the ends of the package is gone. The 28-lead case is the worst in the family, which is why this substitution survives at 16 leads and fails here.

The area argument runs out at 28 leads

QSOP earns its name at low lead counts. Compare placement envelopes, taken as maximum body length times maximum lead span from each drawing: QSOP-28 is 62.1 mm², the Renesas M28.173 TSSOP-28 is 62.7 mm². Within about 1%, they are the same footprint area.

That is a reversal. On the 20-lead comparison the QSOP is roughly a quarter larger than the TSSOP; at 24 leads it is about 6% larger; at 28 leads the TSSOP has caught up and slightly passed it. The reason is that the QSOP body length grows with lead count from 8.65 mm at 20 and 24 leads to 9.9 mm at 28, while the TSSOP started wider and grew more slowly. If you are choosing a package for a new 28-pin die, area is no longer an argument for QSOP. Height still is: 1.75 mm against 1.20 mm goes the other way.

Stencil, mask and coplanarity over a 10 mm body

Paste release is the one place the 0.300 mm pad bites. A 0.300 mm by 1.300 mm aperture returns an IPC-7525 area ratio of 0.96 in a 0.127 mm stencil, which is fine. Put the same part on a board that needs a 0.20 mm stencil for a heavy connector and the ratio falls to 0.61, below the 0.66 floor. That board needs a step stencil, not a compromise thickness.

The web between lands is 0.335 mm on the Richtek pattern and 0.235 mm on the KiCad one. Ask your fabricator for its minimum solder-mask dam before choosing, because the wider pad that helps the side fillet is the one that eats the dam.

Coplanarity deserves a mention at this body length. MO-137-AF allows 0.10 mm of lead coplanarity across a package that is up to 10.01 mm long. Spread over fourteen leads per side, that is a permitted 0.010 mm of height variation per millimetre of body. A part sitting at the limit, on a board with any warp of its own, is how open joints appear at pin 1 and pin 14 while the middle of the package looks perfect. If you see end-of-row opens on this package, measure the part before you touch the stencil.

Lifecycle and second sourcing

QSOP-28 is a legacy-heavy population. Much of what ships in it is long-lived analog: the Analog Devices GN28 body carries parts such as the LTC1405, LTC1414, LTC1420 and LTC3789, and the KiCad footprint for it cites an LTC datasheet as its source. On the logic side, the Renesas QS32390QG, a 16:8 multiplexer in 28-QSOP, is listed by distributors as obsolete.

Two consequences for a new design. First, expect the second source to be a different package rather than a different vendor, usually the TSSOP-28 version of the same die, which needs its own footprint. Second, if you are respinning an old board, check the part status before you reuse the library part. The footprint outliving the component is the normal failure mode here.

One library note worth acting on: KiCad’s Package_SO library ships SSOP-28_3.9×9.9mm_P0.635mm but no QSOP-28 entry, although it does ship QSOP-20 and QSOP-24 entries. Search your library by pitch and body size, not by the name on the datasheet, or you will conclude the footprint does not exist.

Five QSOP-28 mistakes that cause rework

Reusing a 0.65 mm pitch SSOP-28 or TSSOP-28 footprint. The pad row is 0.195 mm too long and the end pins pay for it.
Copying a 0.300 mm pad width without checking your placement spec. That pad allows no side fillet at maximum lead width.
Accepting the KiCad 6.400 mm span on a board that will be AOI-graded on toe fillet.
Choosing QSOP-28 over TSSOP-28 to save board area. At this lead count there is no area to save.
Blaming the stencil for end-of-row opens before measuring lead coplanarity across a 10 mm body.

QSOP-28 FAQ

What is a QSOP-28 package?

A QSOP-28 is a 28-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 9.80 mm to 10.01 mm long, no more than 1.75 mm tall. JEDEC registers it as MO-137-AF. Fourteen leads sit on each long side.

What are the QSOP-28 dimensions?

Body length 9.80 mm to 10.01 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, pitch 0.635 mm basic, foot length 0.41 mm to 1.27 mm, coplanarity 0.10 mm maximum. Those are the JEDEC MO-137-AF limits from the Analog Devices RQ-28 drawing.

Is QSOP-28 the same as SSOP-28?

Only sometimes. The JEDEC family behind QSOP is registered as an SSOP family, so this 3.9 mm body on 0.635 mm pitch is often labelled SSOP-28. A separate SSOP-28 exists on a 5.30 mm body at 0.65 mm pitch. Check the pitch on the drawing before you trust either name.

What land pattern should I use for a QSOP-28?

Start with the drawing in your part’s datasheet. Failing that, the Richtek 6.800 mm span with 1.300 mm pads is the better published choice on toe fillet, but widen the pad from 0.300 mm to about 0.40 mm so a side fillet can form. Confirm the mask dam with your fabricator.

Can you hand solder a QSOP-28?

Yes. At 0.635 mm pitch, drag soldering with flux and a chisel tip works, and the leads stay visible for inspection. Fourteen leads per side makes it slower than a QSOP-20, and the longer body is more prone to lifting at the ends, so tack both corner pins before you run the row.

QSOP-28 or TSSOP-28 for a new design?

TSSOP-28 in most cases. Placement area is effectively identical, 62.1 mm² against 62.7 mm², and the TSSOP is 0.55 mm shorter at 1.20 mm maximum height. Choose QSOP-28 when the function you need only exists there, which for 28-lead parts often means a legacy analog device.

What to do next

Confirm the pitch on your part’s drawing before anything else, because SSOP-28 alone does not identify the package. Then take the Richtek span of 6.800 mm and the Level B pad width of 0.433 mm rather than either published pad width, which gives a pattern with real toe and side fillets and still keeps copper clear of the body outline. Check the resulting aperture against your stencil thickness, not against a nominal one. Last, before you commit, look up the part’s lifecycle status and add the TSSOP-28 sibling footprint to the library while the schematic is still open.

External references consulted for this article:

Analog Devices RQ-28 package drawing: https://www.analog.com/media/en/package-pcb-resources/package/pkg_pdf/qsoprq/rq_28.pdf

Renesas M28.15, 28-lead shrink small outline plastic package, 0.150 in wide body: https://www.renesas.com/en/document/psc/m2815-28-lead-shrink-small-outline-plastic-package-0150-wide-body-ssopqsop

Richtek SSOP-28 footprint file: https://www.richtek.com/assets/podfiles/Footprint-SSOP-28.pdf

Renesas M28.173, 28-lead TSSOP outline and recommended land pattern: https://www.renesas.com/package-image/pdf/outdrawing/m28.173.pdf

Union Semiconductor SSOP28 land pattern 111-1078-28: https://www.union-ic.com/upload/Package/Land%20Pattern-SSOP28.pdf

KiCad Package_SO SSOP-28 footprint: https://github.com/KiCad/kicad-footprints/blob/master/Package_SO.pretty/SSOP-28_3.9×9.9mm_P0.635mm.kicad_mod