A datasheet’s package drawing labels every measurement with a letter, not a name, and getting IC package body size wrong by confusing body with lead span is how footprints fail review. This guide decodes the standard letter system — D, E, A, A1, e, b, L — and the BASIC-versus-toleranced rules, so you can pull the right numbers straight from any mechanical drawing.
Key takeaways
- D is package length, E is package width, A is total height — body size, not lead span.
- e is lead pitch, b is lead width, L is the foot length, A1 is the standoff above the board.
- Boxed BASIC (or REF) values carry no tolerance; the toleranced min/max columns are what you inspect to.
- Body dimensions exclude mold flash, which a note caps at 0.15–0.20 mm per side (Analog Devices QFN outlines).
- On QFN/DFN, D2 and E2 are the exposed thermal pad, sized separately from the body.
- Two parts sharing a name can differ; match the JEDEC MO-/MS- outline code, not just “SOIC-8.”
The dimension letter system every datasheet uses
Package drawings follow a shared alphabet defined through JEDEC (JEP95) and JEITA. Learn eight letters and you can read almost any outline. D is the body length; E is the body width. On the International Rectifier SO-8 outline, D is 4.80–5.00 mm and E is 3.80–4.00 mm.
Height splits into three. A is the overall height from the seating plane to the top; A1 is the standoff, the gap between the board and the package bottom; A2 is the molded-body thickness alone. For that same SO-8, A runs 1.35–1.75 mm and A1 is 0.10–0.25 mm.
The lead-side letters finish the set. e is the lead pitch (1.27 mm BASIC on SO-8), b is the lead width (0.33–0.51 mm), L is the foot length that lands on the pad (0.40–1.27 mm), and c is the lead thickness (0.19–0.25 mm). H or E1 often marks the lead-to-lead span across the body.
Body size versus lead span — the error that ruins footprints
What do D and E mean in a package? D and E are the plastic body’s length and width, measured at the body edges. They are not the tip-to-tip lead span. Confusing the two oversizes or undersizes a footprint and is the single most common dimension-reading mistake.
The lead span is a separate letter — commonly H, E1, or a lead-to-lead callout. On a narrow SOIC-8, the body width E is about 3.9 mm, but the lead span H reaches roughly 6.0 mm (JEDEC MS-012). Build the land pattern to the lead span and the foot L, not to E. Use E only for keep-out and courtyard checks.
BASIC, REF, and toleranced dimensions
Not every number on the drawing is inspected. Three types appear, and telling them apart tells you which value is authoritative.
A BASIC dimension (boxed, sometimes written BSC) is theoretically exact and carries no tolerance of its own — pitch e is almost always BASIC, like the 0.50 mm BSC on a fine-pitch BGA. Under ASME Y14.5 GD&T, the tolerance instead lives in a position callout referencing the datums. A REF (reference) value, shown in parentheses, is for information only and is never inspected.
Everything else is a toleranced dimension, given as min/max or nominal ± range. When you need the worst-case size for clearance or a footprint, use the toleranced max — not the BASIC nominal. This is why a datasheet lists D as 4.80–5.00 mm rather than a single 4.90.
Where body size is actually measured
Body length and width are defined at a datum, and the drawing notes tell you where. JEDEC outlines place datum plane -H- at the mold parting line, coincident with where the leads exit the plastic, and D/E are taken there — not at the widest bulge of the molding.
Two notes protect that definition. Dimensions D and E1 do not include mold flash or protrusions, and a companion note caps that flash. Analog Devices QFN outlines state mold flash “shall not exceed 0.20 mm on any side” (0.15 mm on smaller bodies). Miss that note and you will over-tolerance your keep-out by up to 0.4 mm across the part.
Reading QFN and DFN drawings
QFN and DFN outlines add the bottom terminals and the exposed pad. The body D and E read the same way, but two new letters appear: D2 and E2, the exposed thermal pad’s length and width. On a 5 mm QFN the body might be 5.00 ± 0.10 mm while D2/E2 is a separate, smaller value you size the paste stencil against.
Terminal letters shift too. b is the terminal width, measured 0.25–0.30 mm from the tip per the drawing note, and L is the terminal length on the bottom. Because these packages have almost no standoff, A1 is tiny — often 0 to 0.05 mm — so your stencil and paste design, not lead compliance, set the joint. QFN outlines live under JEDEC MO-220; DFN under MO-229.
Reading BGA drawings
BGA outlines drop leads for a ball matrix, so the language changes again. D and E remain body length and width. e is the ball pitch — 0.8 mm on a 12 × 12 mm, 196-ball LFBGA, down to 0.4–0.5 mm on fine-pitch parts. The balls are located by a row-letter, column-number grid with the A1 corner marked by an index.
Height is dominated by two callouts: the overall A (body plus ball) and coplanarity, a flatness zone for the ball tips, often 0.08 mm. A CSP-BGA drawing may list ball diameter 0.25–0.35 mm and coplanarity 0.08 mm as the numbers that decide whether your board’s pad and solder-mask design will yield. BGA outlines fall under JEDEC MO/MS families such as MO-195.
IC package dimension reference
| Letter | Meaning | Example value (source) |
| D | Body length | 4.80–5.00 mm, SO-8 (IR outline) |
| E | Body width | 3.80–4.00 mm, SO-8 (IR outline) |
| A | Overall height | 1.35–1.75 mm, SO-8 (IR outline) |
| A1 | Standoff above board | 0.10–0.25 mm, SO-8 (IR outline) |
| A2 | Molded body thickness | package-specific |
| e | Lead / ball pitch | 1.27 mm BSC (SO-8); 0.8 mm (LFBGA-196) |
| b | Lead / terminal width | 0.33–0.51 mm, SO-8 (IR outline) |
| L | Lead foot length | 0.40–1.27 mm, SO-8 (IR outline) |
| c | Lead thickness | 0.19–0.25 mm, SO-8 (IR outline) |
| D2 / E2 | QFN/DFN exposed pad | separate, smaller than D/E |
| Coplanarity | BGA ball-tip flatness zone | 0.08 mm (CSP-BGA, MO-195) |
[IMAGE 1: annotated SO-8 outline drawing with D, E, A, A1, e, b, L, c labeled | alt: “IC package body size dimensions D E A A1 e b L labeled on an SO-8 outline drawing”]
Worked example: are these two packages interchangeable?
A name match is not a footprint match. Suppose a BOM lists “SOIC-8” but two candidates cite different JEDEC outlines. The narrow body under MS-012 has a 3.9 mm body width and roughly 6.0 mm lead span; the wide body under MS-013 has a 7.5 mm body width.
Compare the toleranced maxima, not the names. If the incumbent footprint was drawn for MS-012 (E ≈ 3.9 mm, span ≈ 6.0 mm) and the second source is MS-013 (E ≈ 7.5 mm), the leads land nowhere near the pads. The parts are electrically equivalent and mechanically incompatible. Confirm the outline code and the D/E/H maxima before approving any alternate.
Decoding the package name and outline code
The short name — SOIC-8, QFN-32 — is not a specification. The specification is the JEDEC registered outline in JEP95, referenced on the drawing as an MS- or MO- code (MS-012 for narrow SOIC, MO-220 for QFN). JEP95 holds roughly 3,000 pages of these outlines maintained by the JC-11 committee.
Two takeaways follow. First, when a datasheet cites a specific MS-/MO- number, that code — not the friendly name — defines the interchangeable footprint. Second, notes like “compliant to MO-195 with exception” (seen on some CSP-BGAs) warn you that one or more dimensions deviate from the standard, so read the toleranced table, not the assumed standard.
Frequently asked questions
What does D and E mean in a package?
D is the package body length and E is the body width, both measured at the body edges at datum plane -H-, excluding mold flash. They describe the plastic body only, not the lead-to-lead span, which is a separate letter such as H or E1.
What is A1 in a package dimension?
A1 is the standoff height — the gap between the bottom of the package and the PCB surface after mounting. On an SO-8 it is 0.10–0.25 mm; on a QFN it can be near zero. A is the total height; A2 is the molded body thickness alone.
What does BASIC mean in a datasheet dimension?
BASIC (or BSC) marks a theoretically exact value with no tolerance of its own, typically used for pitch e. Under GD&T the tolerance is applied through a position callout to the datums, not to the number itself. Reference (REF) values in parentheses are informational and never inspected.
How do I know if two packages are interchangeable?
Match the JEDEC outline code (MS- or MO-), not the friendly name. Compare toleranced maxima for body size D/E, lead span, and pitch e. A narrow SOIC-8 (MS-012, ~3.9 mm body) and a wide one (MS-013, ~7.5 mm) share a name but not a footprint.
What is D2 and E2 on a QFN?
D2 and E2 are the length and width of the exposed thermal pad on the package bottom. They are specified separately from the body D and E, and you size the solder-paste stencil aperture to them, usually with partial coverage to control standoff and voiding.
Bottom line
Read the drawing in this order: find D and E for body size, A and A1 for height and standoff, e for pitch, then b and L to build the land pattern. Trust the toleranced min/max columns for clearance and footprints, treat BASIC and REF as nominal-only, and subtract mold flash from any keep-out. Before you approve a second source, match the JEDEC MS-/MO- code and compare maxima — the name will lie, the outline code will not.
