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PCB Manufacturing Tolerances
PCB Fabrication Tolerances: Drawings to Inspection
The manufacturing tolerances of printed circuit boards (PCBs) provided to SUGA before quoting a fabrication project define the limits of each toleranced dimension for thickness, drilled holes, copper thickness, mechanical geometry, and impedance-related features. By identifying these limits and required measurements, SUGA provides the OEM with a quote for the project based on the information provided within the submitted files.
PCB manufacturing tolerances are verified using the following methods: a review of the submitted drawings and Gerber files, a review of the layer construction and drill chart, verification of fabrication notes, and how the stated inspection requirements align with the limits set forth in the submitted files.
Tolerance Review Inputs:
review of submitted drawings and Gerber files
review of layer construction and drill chart
verification of fabrication notes
confirmation of measurement method to be used.
What PCB Manufacturing Tolerances Mean Before Quotation
Manufacturing tolerances indicate how much variance is acceptable regarding the dimensions of a fabricated board. Tolerances are more than just a set of numbers next to a dimension; they provide the manufacturer with information on how the dimension will affect fit, electrical performance, mechanical assembly, inspection effort, and final acceptance.
To assess possible impacts on the price of a project, SUGA reviews the fabrication drawing and the accompanying Gerber files, looks at the drill charts and layer construction, identifies any fabrication notes provided by the OEM for the project, and determines how closely each of these files corresponds with the stated inspection requirements. At each step, SUGA evaluates the dimensional limits published for the project and determines whether additional engineering or manufacturing action will be necessary to make the project manufacturable and supportable. Quotations are based on the information provided within the same set of files, and these same files are used throughout the fabrication and inspection process.
When Standard PCB Tolerances Are Enough
Standard PCB tolerances will generally suffice for all areas of a PCB where connector fit, controlled impedance, enclosure alignment, plated hole reliability, or panel break-out do not drive the specification. The need for a narrower specification should only be justified if it protects a function or an agreed acceptance need.
The board outline specified to provide general clearance is not the same as the milled slot that positions a connector. In addition, a mounting hole that is not critical does not carry the same risk as a plated hole that has been tied to a lead fit. A numeric limit associated with the mounting hole or the plated hole may carry different risk based on the location of the hole and the method of measuring it.
Tolerance Must Be Manufacturable, Measurable, and Acceptable
A useful tolerance note should show that the dimension is manufacturable, measurable, and acceptable according to the agreed fabrication files. Once these items are documented and understood, the quote can separate standard conditions from those that require additional process attention, measurement planning, or inspection record requirements.
| Capability Parameter | Covered Range | Application Scope |
|---|---|---|
| Rigid PCB layer count | 1–30 layers | Rigid FR-4 PCB tolerance review |
| Finished board thickness | 0.4–3.2 mm | Thickness tolerance scope |
| Mechanical drilled hole range | Up to 6.35 mm | PTH and NPTH drilling tolerance |
| CNC milled hole and cutout | Above 6.35 mm | Milled hole, cutout, and outline tolerance |
| Bow and twist basis | SMT boards and through-hole-only boards | Mechanical flatness acceptance |
| Copper tolerance scope | Inner copper, outer copper, and heavy copper defined in the final fabrication files provided | Copper thickness and etching tolerance |
The above ranges should be used to evaluate capability coverage, not as manufacturing guarantees. Unusual materials, unusual layer constructions, more stringent project criteria, or conditions outside the listed ranges should be verified before a quotation is considered final.
PCB Thickness, Hole Size, and Copper Weight Tolerances
PCB thickness, hole sizes, and copper weights are best checked first when determining manufacturing tolerances for PCB quotations. Thickness, hole size, and copper weight impact the methods and materials used to produce a PCB and therefore must be considered before determining whether a specific configuration can be produced by the intended manufacturing method, including drilling, plating, and assembly, and whether it will fit into the final assembly configuration. The amount of tolerance, or the variation of the finished product’s thickness, hole type, or copper weight, will affect the path to measurement and acceptance records. Therefore, tolerance scope should be confirmed before quotation for each PCB type.
When determining manufacturing tolerances for PCB configurations, use the manufacturing tolerances of finished PCB configurations only as guidance. The manufacturer’s standard manufacturing tolerances and the submitted drawings, Gerber files, drill charts, layer construction notes, and inspection requirements must be validated and confirmed before finalizing the pricing, tooling, or inspection planning.
PCB Finished Thickness Tolerance by Board Type
Finished PCB thickness varies by PCB configuration. PCB thickness for a standard PCB and a thin PCB will typically have different tolerances associated with them. All these factors, including laminate construction, copper balance, and plating, determine the final PCB thickness. Therefore, the manufacturing tolerances should identify whether the value is intended to be a target for finished thickness, fit tolerance, or acceptance tolerance.
Drilled Hole vs Milled Cutout Tolerance Differences
When evaluating PCB hole tolerance or PCB milled cut-out tolerance, the requirements for PCB holes should be evaluated based on the hole manufacturing method. For example, a hole drilled with a drill will have a tolerance associated with the size of the drill, as well as the plating condition of the PCB and whether the hole is plated. A CNC-milled opening or routed cutout is more accurately classed as a profile feature than a drilled hole, and should be reviewed under a different logic from drilled holes.
Inner, Outer, and Heavy Copper Tolerance Differences
Inner, outer, and heavy conductors are affected differently from each other, as lamination, etching, and plating processes do not control all layers in the same way. The line-width tolerance on an outer 2 oz conductor should take into consideration how outer-layer plating varies and the agreed inspection plan, as opposed to treating it as just another trace specification.
| Geometry Parameter | Tolerance / Limit | Scope | Measurement |
|---|---|---|---|
| Board thickness | ±10% or ±0.10 mm, whichever is greater | Finished thickness ≥1.0 mm | Micrometer at 5 points; coupon cross-section by inspection plan |
| Thin board thickness | ±0.075 mm (±0.003 in) | Finished thickness 0.4–1.0 mm | Thickness map by lot |
| Mechanical drilled hole | ±0.075 mm (±0.003 in) | PTH or NPTH; drilled diameter up to 6.35 mm | Plug gauge; drill chart cross-check |
| CNC milled hole or cutout | ±0.15 mm (±0.006 in) | Milled hole or cutout above 6.35 mm | CNC first-article measurement |
| Finished copper, inner layer | Min. 24.9 µm (1 oz start copper); agreed Class 2 acceptance after processing | Inner copper after lamination | Cross-section coupon |
| Finished copper, outer layer | Min. 33.4 µm (1 oz start copper + plating); agreed Class 2 acceptance after plating | Outer copper after plating (base copper + plating combined) | XRF test pad; coupon cross-section |
| Heavy copper, outer layer | 2 oz or 3 oz finished copper minimum defined in the final fabrication files provided | Power or thermal copper layers | Coupon cross-section |
Copper thickness values and measurement records should be traced back to the submitted files and the inspection plan. If the order specifies an IPC class, copper thickness should be verified against the relevant IPC standard text and the purchase order, rather than inferred from this table alone.
PCB Layer Registration and Annular Ring Tolerances
The location of a drilled hole is determined by much more than just its diameter. As soon as copper pads, inner layers, and plated features get involved, the registration of the drilled hole must match the registration of the copper pads and the design of the layers closely enough to create a usable annular ring. This means that registration and copper clearance are part of the same manufacturing decision.
When producing multilayer PCBs with dense via construction, small pad sizes, small clearances, or several layers that must be aligned after lamination and drilling, there is an increased risk of manufacturing errors. Prior to accepting tight requirements, SUGA reviews the relationships between the fabrication drawing, Gerber data, drill chart, and layer information against the manufacturer’s evaluation.
Factors That Determine Annular Ring Tolerance
The copper area surrounding a drilled hole is the annular ring. A larger pad does not necessarily diminish the risk created by drill position, layer alignment, and clearance. A drill size that works safely in one layout may be difficult in another due to the limited space available for registration shift surrounding the drill.
Drill-to-Copper Clearance in Multilayer PCBs
The clearance from drill to copper becomes especially critical on the internal layers; a small position change may reduce insulation distance or create reliability issues around the plated holes. In a dense fan-out area or an area with many layers, clearance should be determined using the information of the complete final-layer construction and conductor pattern, not only from the top-layer view.
| Alignment Dimension | Held Value | Board Area | Coupon / CAM Check |
|---|---|---|---|
| Layer-to-layer registration | ±0.075 mm | Multilayer boards ≤6 layers | X-ray registration coupon |
| Layer-to-layer registration | ±0.10 mm | Multilayer boards 8–12 layers | X-ray registration coupon |
| Layer-to-layer registration | ±0.125 mm | Multilayer boards ≥14 layers | X-ray registration coupon; released stack-up |
| Drill-to-copper clearance, outer | ≥0.15 mm (6 mil) | Outer copper to PTH wall | CAM clearance check |
| Drill-to-copper clearance, inner | ≥0.20 mm (8 mil) | Inner copper to PTH wall | CAM clearance check |
| Annular ring, external | ≥0.05 mm (2 mil); SUGA target; acceptance per drawing and applicable IPC requirement | Outer pad to drilled hole | automated optical inspection (AOI); coupon measurement |
| Annular ring, internal | ≥0.025 mm (1 mil); SUGA target; acceptance per drawing and applicable IPC requirement | Inner pad to drilled hole | Coupon cross-section |
| PTH pad diameter, component | Drill +0.50 mm | Through-hole component pads; the component footprint on file | CAM pad-to-hole verify |
| Via pad diameter | Drill +0.40 mm | Non-HDI via pads; the Gerber set | CAM pad-to-hole verify |
| Dense via pad diameter | Drill +0.30 mm | Dense fan-out; approved stack-up | CAM pad-to-hole verify |
Pad-to-hole relationship verification for dense fan-out and high-layer registration should connect with the individual layer stack-up, the Gerber set, CAM check, and inspection plan. Internal annular ring acceptance should follow the drawing and applicable IPC criteria. In instances where the order states criteria per IPC class, the allowable breakouts or reductions must conform to the actual standard text before qualification begins.
PCB Bow, Twist, Profile, Slot, and V-Score Tolerances
Mechanical tolerances determine how well a PCB is able to sit flat, fit into an enclosure, align with connectors, and separate from the panel after fabrication. Unlike copper and hole dimensions, mechanical tolerance risks often appear after the PCB has been fabricated, typically during mechanical fit, assembly, and panel breakout.
SUGA will evaluate bow, twist, profile, slot, and V-score requirements to determine if they fall within the tolerance limits of the fabrication drawing, panel design, and stated inspection requirements. Bow and twist limits of 0.75% each for boards populated with SMT components should be treated as specified in the table below, as measured on the defined diagonal of the board. This tolerance should not be expanded as a universal IPC class promise unless the standard scope has been confirmed.
Bow and Twist Tolerance for Flatness and Fit
The importance of flatness is that a finished PCB must match the overall outline of the PCB and still remain usable throughout handling and following assembly processes. SMT component assemblies are particularly susceptible to mechanical distortions such as bow and twist because they can affect solder paste contact, component placement, connector alignment, and PCB support during assembly.
Profile, Slot, and V-Score Tolerance Notes
Mechanical fit is often dictated to a large degree by the outline and slot limits. While the outline of a general PCB, a connector slot, and a cutout used to locate an enclosure do not carry the same mechanical fit risk, V-score conditions should be evaluated carefully. As with other tolerance parameters, the limits for the residual web and breakout edge are dictated by board thickness and panel design.
| Mechanical Profile | Acceptance Limit | PCB Condition | Verification Method |
|---|---|---|---|
| Bow | ≤0.75%; IPC-TM-650 2.4.22 diagonal basis | Boards carrying SMT components | Flat datum plane; diagonal calculation |
| Twist | ≤0.75%; IPC-TM-650 2.4.22 diagonal basis | Boards carrying SMT components | Flat datum plane; diagonal calculation |
| Bow and twist | ≤1.5%; IPC-TM-650 2.4.22 diagonal basis | Through-hole-only boards | Flat datum plane; diagonal calculation |
| CNC milled profile | ±0.15 mm (±0.006 in) | Standard board outline | CNC first-article; perimeter sample |
| CNC milled profile, tightened | ±0.10 mm (±0.004 in) | Tight-tolerance outline; the fabrication drawing | CNC first-article; fabrication drawing note |
| Internal slot width | ±0.10 mm (±0.004 in) | Milled slot width ≥1.0 mm | Pin gauge; CMM by inspection plan |
| Narrow slot width | ±0.15 mm (±0.006 in) | Milled slot width <1.0 mm | Pin gauge |
| V-score residual web | 0.30–0.40 mm | V-scored panels; 1.6 mm board thickness | Cross-section sample |
| V-score profile after break | ±0.20 mm | Break-out edge after de-paneling | First-article edge sample |
Tightened profile, narrow slot width, and V-score residual web values should be referenced back to the appropriate drawing note, panel design, board thickness, and method of inspection. They should not be reused as fixed values for an entirely different panel design.
PCB Etching, Trace Spacing, and Controlled Impedance Tolerances
There are many tolerances associated with conductors on PCBs, and while there is a common way to define a tolerance set for each conductor type, there are also many ways to evaluate the tolerances. Conductor widths, trace spacing, copper-to-board-edge distance, controlled impedance widths, and solder mask thickness are examples of different manufacturing conditions. Therefore, grouping them as part of the same PCB tolerance set can obscure the real likelihood of conductor-related risk.
SUGA uses conductor dimensions to evaluate the copper weight used for fabrication, layer position, the Gerber data used to create the PCB, the approved layer construction required to produce the PCB, and the inspections specified for the PCB. The goal is to differentiate between normal variances associated with etching and those dimensions that contribute to the electrical performance of the PCB, the clearances needed at panel edges, panel separation, or the solder mask applied over each conductor.
Copper Weight Changes Etching Tolerance
Etching tolerance control differs between inner-layer conductors and outer-layer conductors. A ½ oz inner-layer conductor and a 2 oz outer-layer conductor do not have the same process condition. Additionally, the process conditions for etching and plating for inner-layer conductors differ from those of outer-layer conductors. As copper weight increases, all variances associated with plating and etching should be considered as part of the inspection plan rather than buried in a generic line-width notation.
Trace Spacing, Edge Pull-Back, and V-Score Clearance
While same-layer trace spacing, copper-to-board-edge distance, and copper-to-V-score clearance serve different design requirements, they provide electrical separation from other conductors, milling clearance, and panel break-out clearance, respectively. Therefore, as these distances come near the stated limits in the fabrication drawing, the drawing must clearly define the location of the critical area to meet the design requirements, rather than being based on a broad, generic trace-and-spacing statement.
Controlled Impedance and Solder Mask Need Different Evidence
While conductor width is required for controlled impedance, it does not, by itself, determine it. Controlled impedance requires consideration of layer construction, dielectric data, copper geometry, and the agreed test method. A separate issue related to solder mask thickness is coverage. A given value for the solder mask should serve as a planning guide unless otherwise stated on the drawing by a specific inspection requirement.
| Etch Area | Reference Value | Application Scope | Verification Method |
|---|---|---|---|
| Inner conductor width, ½ oz | ±15% down to 4 mil (0.10 mm) | Signal traces; non-impedance-controlled | Etch coupon; AOI width sampling |
| Outer conductor width, 1 oz | ±20% down to 5 mil (0.125 mm) | Signal traces; non-impedance-controlled | Etch coupon; AOI width sampling |
| Outer conductor width, 2 oz | ±25% down to 8 mil (0.20 mm) | Power traces; non-impedance-controlled | Etch coupon; AOI width sampling |
| Controlled-impedance conductor | ±10% impedance tolerance; line width per approved stack-up | Controlled-impedance nets; approved stack-up | time-domain reflectometry (TDR) coupon |
| Trace-to-trace spacing | Minimum 3 mil (0.075 mm) outer; 4 mil (0.10 mm) inner | Same-layer copper features; released stack-up | CAM DRC; AOI |
| Outer copper to board edge | 0.25 mm (10 mil) | Milled board edge to outer copper | CAM clearance check |
| Inner copper to board edge | 0.40 mm (16 mil) | Milled board edge to inner copper | CAM clearance check |
| Copper to V-score line | 0.40 mm (16 mil) | V-scored panel edge | CAM panelization check |
| Solder mask over flat copper | 10–25 µm typical | Liquid photo-imageable mask; ≤2 oz copper | Mask thickness measurement; coupon cross-section |
| Solder mask at copper corner | ≥5 µm | Copper edge or corner; fabrication drawing note | Coupon cross-section |
Etching limits define what copper geometry can be verified or held to requirements. Impedance requirements define what needs to be proven electrically. Mask requirements define what must be confirmed for coverage or coating thickness. Separating each of these values helps avoid using one value as a shortcut for another.
How Tight PCB Tolerances Affect Cost and Lead Time
The effects caused by tight tolerances on a PCB are increased costs and longer lead times due to the extra work created during the manufacturing or verification process.
The strongest tolerance note on a PCB is not necessarily the narrowest value on the drawing; rather, it is the label on the drawing that provides distinction regarding which limit protects the function of the PCB and which dimension has been left within the standard manufacturing capability. That distinction allows SUGA to calculate the pricing associated with the work performed without classifying every dimension as a special requirement.
When Standard PCB Tolerances Avoid Unneeded Cost
Standard tolerances are usually a better match for dimensions that do not have the potential to impact electrical characteristics, mechanical fit, plated-through-hole integrity, or the acceptance of the results of an inspection performed on a PCB. Tightening a PCB clearance that is not critical will cause additional clarification but does not necessarily improve the finished PCB in this situation. The connector slots, dense via areas, controlled conductors, and flatness requirements of a PCB, on the other hand, are dimensions that require enhanced scrutiny since changing the precision of these dimensions may change the way that the PCB is manufactured.
What Tight PCB Tolerances Add to Verification
When a PCB has a tightened profile value, narrowed slot condition, dense annular ring condition, or controlled impedance on a conductor, additional verification may be required that is not necessarily required with a normal PCB file. These requirements may include CAM comparison, first-article measurement, coupon evidence, TDR coupon testing, or inspection records associated with that PCB drawing. If the acceptance method is not clearly stated, it may be difficult to determine what measure is being used to interpret the numeric limit even if it appears to be concise.
When detailing a quotation, critical dimensions, the reason for the tight tolerances, and the expected inspection results should be included. Treating the estimated price and lead time as final without these details can create quotation, verification, and scheduling risk.
PCB Tolerance Notes That Make Quotation Clearer
Providing clear PCB tolerance notes is essential for SUGA to determine which project requirements are considered standard manufacturing limits and which are critical to the function, fit, electrical performance, or acceptance of the product. The best way to provide these notes is to tie them to a specific dimension that relates to the area of the PCB, to state the reason for the limit, and to indicate the expected inspection results.
A note stating only the phrase “tight tolerance” does not provide a manufacturer with enough direction. A note that identifies a connector slot, a controlled conductor, a dense via area, a plated hole fit, or a panel break-out edge gives the manufacturer a practical point to evaluate.
Marking Functional PCB Features That Need Tight Tolerance
It is essential to mark all critical-to-function PCB features on the drawing. The reference to a board-wide thickness note is markedly different from the reference to a fit requirement near an enclosure. A standard via has different requirements from a dense via field with very limited annular ring margin. A general profile requirement has different characteristics from a slot that locates a connector.
Required Files for PCB Tolerance Evaluation
The file set required to perform a proper PCB tolerance evaluation includes the fabrication drawing, Gerber data, drill chart, layer construction or stack-up, fabrication notes, and any special inspection requirements. In addition, when plated and non-plated holes are present in the same area, or routed cutouts could potentially be interpreted as drilled features, the drill chart and drawing notes must separate those conditions clearly.
PCB Fabrication and Assembly Tolerances Are Different
PCB fabrication and assembly have different tolerances. A fabricator has finished fabrication tolerances based on finished part parameters, including thickness, hole size, copper layout, registration, annular ring, flatness, product outline, slots, and related parameters, while an assembler’s tolerance covers the assembly operation, including component placement, solder joint, mounted component position, and process confirmations.
The fabrication drawing defines a plated through-hole, which may affect the fit of the leads of a component during the assembly process. Therefore, the two tolerance types should be reviewed separately and accounted for in the PCB drawing.
PCB Tolerance Measurement Methods and Inspection Records
PCB fabrication tolerances, including thickness, hole size, registration, copper geometry, and assembly tolerances, including component placement, solder joint, and mounted component position, cannot be measured or recorded using the same method. Each of these types of tolerances requires a verification method to match the tolerance required by the fabrication order.
SUGA aligns the verification of each PCB tolerance with the dimensions that are being evaluated and the acceptance criteria agreed upon for the fabrication order. The purpose of this alignment is not to create unnecessary records for every fabrication order but to ensure that when critical limit conditions affect function or final acceptance, the limit condition can be verified as being within a defined limit.
Match the Measurement Method to the PCB Feature
PCB feature measurement methods that are typically aligned with dimension type include:
- Finished thickness — defined-point measurement via micrometer or thickness map as required by the inspection plan.
- Plated hole size — plug gauge check and drill chart cross-reference for plated and non-plated separation.
- Layer registration — X-ray coupon or cross-section data when internal layer alignment records are required.
- Outer conductor width — etch coupon and AOI sampling according to the inspection plan.
- Controlled-impedance conductor — TDR coupon to verify the required controlled impedance for the PCB.
- Final outline and slot — first-article measurement via CNC, coordinate measurement, or pin gauge when the drawings identify critical fit dimensions.
Acceptance depends on the drawing, inspection plan, and records required to define acceptable quality.
Matching Inspection Records to PCB Acceptance Requirements
The inspection process can also be used to clarify what is required for acceptance of a PCB. When inspection records are used to validate the acceptance of PCBs, they should provide detailed evidence of those processes and verification methods.
Creating a large number of records is unnecessary; rather, early identification of the critical tolerance points will allow a smoother quotation, fabrication, and final inspection process without requiring that every dimension be created as a special control item.
Send Critical PCB Tolerance Notes with Your Project Files
By including the critical tolerances for each dimension, SUGA can determine whether a PCB is ready for quotation, is in need of engineering clarification, or requires additional inspections before starting fabrication. Make it clear in the submitted records which dimensions must stay within limits and state why that dimension is critical to function or acceptance.
If every dimension is marked as tight, it adds additional questions rather than providing confidence. When only the functional areas are marked as tight, SUGA can concentrate on those areas to produce a PCB that meets the required tolerances and dimensions. Include notes on controlled impedance, tight-profile dimensions, special conductor weights or finishes, bow and twist, slot fit, V-score conditions, and acceptance records.
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PCB Manufacturing Tolerances Questions
PCB manufacturing tolerances specify the allowable dimensional variance after the PCB has been manufactured. PCB tolerances are based on measurable fabrication outcomes of the finished PCB or a coupon, including finished thickness, drilled hole diameter, conductor width, annular ring, flatness, and related dimensions. Each of these dimensions is defined by a measurement method. When SUGA receives an inquiry to quote PCB fabrication with specifications, it reviews those specifications to determine if the requested limits can be manufactured within the stated fabrication tolerances and identifies any requirements that need clarification before providing a quote.
There is not a standard PCB dimension tolerance for all dimensions on a PCB. PCB tolerances vary based on thickness, hole size, copper width, profile, slot, bow, twist, and conductors whose impedance is related to the function performed by those conductors. The specification of a standard value may be acceptable when the dimension does not affect the fit or electrical performance of the PCB, the reliability of plated holes, or acceptance records. A tighter tolerance value should be specified based on the specific function performed by the PCB.
Standard tolerances for PCB thickness are generally specified in two ways depending on the thickness of the PCB. For PCB thicknesses that are equal to or greater than 1.0 mm, a percentage-based tolerance such as ±10% is considered the most useful way of expressing tolerances, as laminate, copper, and plating each contribute to the final PCB stack and the total thickness of the combined material. When final PCB thickness is below 1.0 mm, it is more logical to set a standard tolerance of ±0.075 mm than to express that as a percentage for a much thinner PCB. To ensure that the PCB has a nominal, fit, or acceptance limit, the drawing must indicate which application of the tolerance is appropriate. SUGA will confirm which application will be used before providing a quote.
The finished hole size tolerance of a PCB varies depending on the type of feature, the plating condition, and the function. PTHs, non-plated features, mechanically drilled features, milled openings, and routed cutouts cannot be evaluated using a single simple rule. There should be clear requirements for each of these hole types. By separating the plated and non-plated dimensions in the drill chart, and making certain that the drawing indicates the locations where the holes affect lead fit, fastening, alignment, and connector position, it will be easier for the PCB designer to understand and communicate those tolerances to the manufacturer.
PCB bow and twist tolerance regulates how much a completed PCB can bend or twist away from being flat. PCB bow and twist tolerance is critical to the assembly process when mounting a PCB with surface-mount components, mounting a PCB in an enclosure, aligning the PCB with connectors, and ensuring reliable handling during assembly. The manufacturer must establish the acceptable value for bow and twist tolerance based on the condition of the finished PCB and the measurement basis used to determine those values, and any more stringent flatness tolerance should be indicated on the drawing where it will affect board orientation within the enclosure, lead fit, or assembly mechanics.
Annular ring tolerance relates to the copper that remains around a drilled hole after fabrication. Copper annular ring tolerance is affected by pad size, drill position, layer registration, and the amount of clearance between the pad and drilled hole; drilled diameter alone does not determine this tolerance. Different types of inspection evidence for external and internal annular ring conditions may be needed since internal features cannot be determined only by looking at the outside of the PCB. Areas containing dense vias and close relationships between pads and holes must be clearly marked on the drawing.
Tighter PCB tolerances result in increased cost and longer lead times when they require additional work within the manufacturing or verification paths, such as CAM clarifications, tighter tool setup tolerances, first-article CMM dimensions, supplemental coupon cross-section evaluations, TDR coupon evaluations, or engineering approval before production release. A tighter specification in a non-critical clearance section of a PCB design may not affect manufacturing processes if it lies within generic fabrication tolerances. However, if the same tighter tolerance is applied to a connector slot, dense via area, controlled-impedance conductor, or panel flatness, then one or more checks may be added. Only dimensions where tighter limits are beneficial to function, fit, or acceptance should be marked.
PCB fabrication drawings must identify tolerances that can impact functionality, fit with other components, electrical behavior and performance, mechanical use, and acceptance. Finished thickness, plated and non-plated holes, controlled slot dimensions, profile dimensions, bow or twist tolerances, copper annular rings, copper clearance, V-score requirements, special copper conditions, and impedance-related conductors should have specified tolerancing on a PCB fabrication drawing. Each tolerancing value must also clarify whether it is intended to define a target, define a minimum or maximum value, or serve as an acceptance limit.
Related Capabilities for Other PCB Requirements
Tolerancing may also relate closely to many other manufacturing constraints that each order may encounter across different areas of PCB manufacturing. If the greatest concern of PCB fabrication is not dimensional variation only, the following related capabilities can help the order process line up with the appropriate technical discussions.