Dana on Data: Understanding Mechanical Drill Size Capability and Cost

“Information is not knowledge.” — Albert Einstein

Fabricator capabilities are generally provided initially on a one-page summary as part of the general marketing presentation. The technical values that are presented provide the “check mark” information so the potential customer can determine if the fabricators capability is greater than the design  requirements. Often, this is the only method used for design rule knowledge transfer.

Design documentation provides finished hole sizes (FHS) versus drilled hole sizes (DHS) for plated through- or blind/buried holes. The FHS will typically be 0.05 – 0.127 mm less than the DHS. DHSs are provided for non-plated guide pin or tooling holes and some press-fit connector styles.

Mechanical drill hole specifications are often provided by the fabricator as a single variable value in the capability matrix. They may provide both standard capability and advanced capability guidance for a higher cost option. Table 1 shows a typical capability table that is provided in the initial marketing presentation.

KorfTable_1_0921.jpg

Table 1: A list of typical capabilities.

The minimum drill diameter is used by the designer to select the via finished hole size and via pad size, based on the IPC class that is used. The maximum drill diameter provides the limit for tooling or guide hole sizes. The aspect ratio is derived from the PCB overall thickness divided by the drill diameter. Abiding by the aspect ratio specification is critical to ensure that the hole can be copper plated thick enough  to meet the minimum IPC or application requirement.

A common mistake is when the designer uses the minimum drill diameter with the maximum aspect ratio because the table implies that they can be used together. In reality, the maximum aspect ratio varies with the drill diameter. Table 2 shows how the aspect ratio decreases as the drilled hole size decreases.

KorfTable_2_0921.jpg

Table 2: The relationship between aspect ratio and drilled hole size.

The maximum aspect ratio per drill hole size will vary between fabricators based on their specific capabilities. The designer should ask for this table prior to determining the routing strategy. Some fabricators will provide the following design rules to present the aspect ratio to drilled hole diameter relationship (Table 3).

KorfTable_3_0921.jpg

Table 3: Typical aspect ratio design guidelines.

There are other factors that will impact the maximum aspect ratio. Common items are:

  • The total copper thickness for all layers in the hole may limit the minimum drilled hole size and or significantly affect the PCB cost. Removing non-functional pads for vias significantly affect the drilled hole cost.
  • Drill bit diameters are generally metric and standardized through the world. Custom drills can be tooled at a higher cost. FHS’s that are close to each other, e.g., 0.025 mm different, may be combined into a single drill diameter to save cost. Drilled hole sizes that are non-metric, e.g., 10 mils, will generally be converted to a metric size, 9.8 mils, in this example.
  • Buried vias are generally used on a double-sided core or on a thinner sub-laminated PCB. This via will generally have a low aspect ratio.
  • Via filled holes in which the plated diameter is the critical dimension. There is a limitation on the maximum plated diameter where the epoxy will remain in the hole and not slump out of the other end. If it is too large, the mask will leak out the bottom of the hole. Conversely, it will not fill the hole when the aspect ratio and diameter are too small.
  • Via plugged holes have the same limitation as via filled holes. The via plug is not intended to completely fill the hole. It is intended to cover the top surface. A maximum FHS should be provided. The via plug will sag and not properly coat the hole if the diameter is too large.
  • The glass weave style may have limitations for small drill diameters.
  • Surface finishes may have drilled hole aspect ratio limitations to ensure that the plated hole is large enough to allow for proper coating and cleaning. This is common for immersion and OSP surface finishes.

Similar limitations are also applicable to punched holes used in thin FPC or rigid PCBs. The aspect ratio limitation is not restricted by the copper plating process but from the die used to punch the holes.

The designer should ask for the drill diameter to aspect ratio limitations prior to starting the layout to ensure that the design is not outside of that fabricators capability or pushing it into their higher cost advanced capability. Most design and DFM software do not currently have the ability to load aspect ratio versus drilled or finished hole size rules, so the rules must be manually checked.

Dana Korf is the principal consultant at Korf Consultancy LLC. 

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2021

Dana on Data: Understanding Mechanical Drill Size Capability and Cost

09-29-2021

Fabricator capabilities are generally initially provided on a one-page summary as part of the general marketing presentation. The technical values that are presented provide the “check mark” information so the potential customer can determine if the fabricators capability is greater than the design requirements. Often, this is the only method used for design rule knowledge transfer.

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Dana on Data: Understanding Mechanical Drill Size Capability and Cost

09-23-2021

Fabricator capabilities are generally initially provided on a one-page summary as part of the general marketing presentation. The technical values that are presented provide the “check mark” information so the potential customer can determine if the fabricators capability is greater than the design requirements. Often, this is the only method used for design rule knowledge transfer.

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Dana on Data: The Critical Importance of the Fab Product Engineer

07-29-2021

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05-13-2021

PCB fabricator front-end engineering departments are always under great pressure to be kept small, generate production tooling instantaneously from customer data and never, ever, make a mistake. Key performance indicators (KPI’s) emphasis internal process improvements and are generally simple in nature, such as jobs/person/day and scrap dollars/month.

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03-11-2021

IPC APEX EXPO’s emphasis on the Connected Factory Initiative based on CFX and IPC-2581 is underway in a virtual mode this month. One area that has not been addressed is the automation of industry technical specifications from organizations like IPC, ASTM, UL, IEC, etc.

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Dana on Data: Factory 4.0 NPI Data Transfer Improvements

01-14-2021

The recently released IPC Connected Factory Initiative scope is similar to other Factory 4.0 models with the same glaring omission: They all seems to assume that the incoming design data can’t be used as-is and must be reviewed and potentially manually modified prior to manufacturing release.

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2020

Dana on Data: Reducing PCB Specification Interpretation Issues

11-12-2020

The PCB industry has accepted a low-quality level of provided documentation from its customers for the past several decades. In this column, Dana Korf reviews one common fabrication print note and asks, “How do you interpret this note?”

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Dana on Data: A Team Method to Reduce Fabricator Engineering Questions

09-03-2020

Hundreds of PCB designs are released to be quoted or fabricated every day around the world, and most will have engineering questions or technical queries generated once the data package has been received and analyzed. Dana Korf outlines seven fundamental steps based on Lean/Six Sigma concepts to reduce data transfer issues.

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Dana on Data: How Can the PCB Industry Improve From COVID-19 Responses?

07-16-2020

In response to the COVID-19 pandemic, the world transformed a very slow medical approval process into the equivalent of a concurrent NPI process by challenging some of the golden rules. Dana Korf shares his thoughts on four areas the PCB industry can re-evaluate and improve.

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Dana on Data: The Importance of PCB Technology Roadmaps

05-14-2020

Peter Drucker once said, “Trying to predict the future is like trying to drive down a country road at night with no lights while looking out the back window.” Dana Korf explains how it is critical that PCB fabricator technology roadmaps and capacity planning align with their customers’ product development and volume requirements to ensure that optimum cost, reliability, and performance goals are achieved.

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Dana on Data: Automating DFX Transfer and Analysis Using IPC-2581C

03-19-2020

We are inching closer to a world where a complete intelligent PCB data transfer is realized. The IPC 2-16 Digital Product Model Exchange (DPMX) Subcommittee has just sent revision C out for IPC-2581 Consortium review with final industry approval targeted for this June. Dana Korf discusses the significant additions and their impact.

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Dana on Data: Creating IP-protected PCB Design Rules

01-09-2020

One of the primary reasons that data packages aren’t compatible is the fabricator/assembler does not provide a complete set of design rules out of concern of giving away their intellectual property (IP). Dana Korf explores the design rule development hierarchy as well as what should be included in an IP-protected design rule document.

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2019

Dana on Data: The DFM/Data Transfer Process Is Broken

11-14-2019

In a world that is showing great strides toward implementing a Factory 4.0 world, why can’t a design be passed from a designer to the fabricator without errors every time? Dana Korf emphasizes moving the responsibility up in the food chain, examines key design package error categories, and proposes creating a cultural change.

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New Column—Dana on Data: IPC-2581 Intelligent Bi-directional Data Flow

09-12-2019

The IPC Consortium is nearing completion of transferring notes on drawings and working with IPC on converting key IPC specifications into attributes that can be automatically loaded into CAD and CAM systems. This format is extendable to created automated company-specific acceptance files that can be automatically loaded into the CEM’s or fabricator’s engineering systems. IPC-2581 data format is being widely used globally and now needs to become the standard to reduce NPI cycle times by associating critical design information automatically to the physical features.

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