Quick Turn PCB Assembly: Plan the Critical Path

What Is Quick Turn PCB Assembly?

When you refer to “quick turn PCB assembly,” you are describing the fast placement and soldering of electronic components (in this case, on a fabricated circuit board), followed by the agreed-upon inspections, programming, testing, and release processes. It is most common for prototype or low-volume production runs; therefore, this term does not define a uniform production quantity, nor is there a standard duration among all manufacturers or suppliers.

• The manufacture of a bare circuit board is quick turn PCB fabrication; no components are installed.
• The assembly of electronic components on available bare PCBs using an approved component kit is a quick turn PCB assembly process.
• A quick turn turnkey PCBA process coordinates fabrication, component sourcing, assembly, and agreed final test.

For a broader explanation of the manufacturing stages, see PCB assembly process guide.

How Fast Can Quick Turn PCB Assembly Be Completed?

Published production times for this type of process can range anywhere from 24 hours to multiple business days, based on individual manufacturer conditions; they are not universal averages or promises for another manufacturer.

A standard rigid PCB is not comparable to a rigid-flex assembly using sequential lamination, the use of a long-lead-time or hard-to-source integrated circuit, manual insertion of THT components, programming of firmware, and development of a new functional test fixture.

Where Does the Clock Start and Stop?

The most beneficial start point for a controlled release is when all relevant documents have been approved, materials needed for manufacturing are present, and all impediments that would prevent moving forward with the next activity have been eliminated. The end points will need to be defined independently for all three stages: factory completion, shipping (including customs and clearances), and receipt.

We cannot compare a quote that ends at factory completion to the delivery date requested at the customer’s dock. For instance, even though manufacturing is complete on Friday, it may still take until Monday to dispatch to the customer due to the additional time required between manufacture and delivery.

What Must Be Ready Before the Clock Starts?

The release package must allow for reproduction of the product without having to rely on email threads or the engineer’s memory of what was done.

The controlled release of any hardware may be recorded in a short project list instead of including it in the full project description. Examples of project lists are:

• PCB fabrication data Rev D
• BOM Rev D
• Placement/CPL Rev C
• Assembly drawing Rev D
• Firmware v2.4
• Functional-test procedure TP-017 Rev B

If you make changes to an item on the list, please determine the impact on downstream programs, fixtures, purchases, inspections, and records, and regenerate all affected items. Even though the filenames may match, this does not guarantee that both files are of the same revision.

Which Factors Determine the Lead Time?

A number of factors influence lead time; however, the last outstanding prerequisite will determine the earliest date to begin work, and the length of the linked sequence will determine the completion date.

A rush fee provides priority services to produce items more quickly than if they were going through regular processing; however, it does NOT:

• Eliminate the time a product will take to cure.
• Make an unavailable laminate available.
• Allow the use of an unapproved substitute, such as replacing the laminate with another material that is unapproved.
• Complete a fixture that has not yet been designed.

If there is no way to avoid using a special process on an assembly, it should be clearly identified on the schedule rather than bundled into one number that applies to all suppliers.

How Does the Quick-Turn Assembly Process Work?

1. Align your quotation boundary. Validate quantities, revisions, start triggers, factory finishes, shipment boundaries, and documentation requirements.
2. Review the assembly for DFM, DFA, and DFT. Review the stackups, panel layout, footprint dimensions for components, polarity of components, accessibility of test points, programming interfaces, and test requirements for all circuit assemblies and prototype assemblies prior to releasing to production. Ideally, have these reviews documented against a checklist such as IPC-2231A so they can be referenced in future assembly reviews, rather than using an informal check.
3. Confirm all material supply risks are covered. Validate that each BOM line is accounted for, ensure the correct consigned kit quantity is used, verify that moisture-sensitive devices are correctly handled, verify that approved alternates are identified, and identify any required excess quantities.
4. Prepare for concurrent activities. If inputs are independently verified, the following activities can take place concurrently: PCB fabrication, stencil preparation, machine programming, fixture work, and incoming material checks.
5. Before the line can be loaded, audit the lot numbers of all parts used in the assembly; validate that the parts have the correct revision and that the required quantity of parts has been received; and validate that there are no discrepancies in packaging or polarity.
6. Conduct assembly. The sequence of assembly may include solder-paste printing, placement of components, reflow, second-side processing, THT insertion, hand soldering or selective soldering processes, cleaning of the assembly, follow-on processes, and controlled rework.
7. Verify all in-process inspections and tests are completed as agreed upon and that all required supporting evidence has been collected.
8. Release and dispatch. Close nonconformances, review approved deviations, complete the required records, package the accepted quantity, and record the carrier handoff separately from factory completion.

How Is Quality Maintained on an Expedited Schedule?

A quick-turn project should reduce the number of queues and handoffs, not lower acceptable quality levels based on the specifications. Quality inspections should follow the assembly design and risk of the product, not apply all methods to all products.

• SPI can be used to verify volume, area, height, and placement alignment of the paste prior to placement of components when required by the process and equipment.
• AOI systems are used for inspection of visible placement, polarity, solder, and component presence, but AOI cannot directly verify hidden BGA joints.
• X-ray inspection performs well for the inspection of hidden terminations (i.e., BGA) or bottom-terminated components. An inspection requirement must be established and a method of acceptance defined.
• ICT, flying-probe, and electrical tests require suitable access, data, programming, and sometimes fixtures. These are not interchangeable with functional testing.
• Functional testing must be performed according to an approved testing procedure and documented with established limits of acceptance, including the corresponding power sequence, electrical connections, and software/firmware used in the device, as well as clear rules regarding the recording and retesting of failures.

The requirements for soldering processes and materials are addressed in J-STD-001J, while IPC-A-610J discusses the acceptability of products after assembly. Because passing a workmanship inspection does not guarantee the intended functionality of the product, the test requirement must be stated independently.

Map the Dependency Network Before Counting Days

One flag indicating readiness can conceal where there is a blockage in the schedule due to dependencies on other readiness flags required to complete the project. Stage gates can be used so that an unfinished test fixture does not automatically prevent SMT assembly when the fixture is only required later. This is the same logic used by GAO and NASA program-management guides in larger project environments, applied to PCB assembly scale.

• The assembly process may begin once the assembly release, bare PCB, components, stencil, and CPL data are all ready, as well as pre-placement programming if it is applicable. The date for initial assembly will be determined by whichever of the above items is completed last.
• The functional test process may begin once the units, test assets, test firmware, and approved test limits are all ready. Again, the date that the functional testing process may start will be determined by whichever of the activities mentioned above is completed last.
• Completion of the factory process shall be defined by whichever finishes last among final assembly release, functional testing, and required recordkeeping.
• The dispatch of products will follow factory completion by the duration of time taken to complete the packing and handoff to the carrier.
• Receipt of the products will follow dispatch by the duration of transit time, customs time, and receiving time.

Pre-placement programming will only be included in the list of items that can affect the start of assembly if the programming is required prior to placing the component or device. If the programming takes place after the assembly process, it will belong on the functional test list. Any items that are not needed for the agreed final deliverable should be eliminated from the lists.

This model also allows inspection, testing, and packing to occur in stages. It is possible for the first items in the production line to be inspected while later products are being placed and for functional testing of the products to begin the moment a unit has been inspected and accepted as conforming.

Which Activities Are Critical, and Which Have Float?

A critical activity is defined as an activity that directly impacts the selected finish of the final project through one or more successor activities. Float refers to the amount of time that can elapse for non-critical activities without adversely impacting the completion of the final project. The values listed below are simply illustrative workday data for a small prototype project and are not intended as an industry benchmark for lead time.

In this example, assembly start will be Day 4. The assembly and inspection will both be complete on Day 6. The fixture will be completed by Day 5. So, there is one workday of fixture float. The functional test and release will be complete on Day 7. If the kit is closed out one day earlier than expected, but the bare boards slip from Day 3 to Day 5, it will result in the bare board branch becoming critical and factory completion will be delayed until Day 8.

Float is shared schedule protection. Therefore, if a noncritical activity takes up a portion of the float, any future changes will have less protection and the forecast will have to be updated.

Turn Engineering Questions Into Scheduled Decisions

Do not rely on an ‘awaiting customer’ status to solve engineering questions.

The difference between the Day 8 conclusion of Q-01 and the prior scenario involving board slip is that there is a distinct delay path in this situation that consumes one workday of shared float, while both share the same date of completion. The response deadline should be set based on working backward from the beginning of the blocked activity to find the latest possible safe start. The decision must also provide a workable determination regarding the released BOM, drawing, deviation, firmware, test limit, or any other controlled record to be considered complete.

How Should a Change Recalculate the Commitment?

1. Consume float. The affected activity is delayed; however, the selected completion date remains the same, and any residual allowance will be documented as a smaller number.
2. Replan. If the sequence or resource plan is altered, then the restated network of activity will have an updated linkage to the selected completion date. You will need to document the new links and assumptions.
3. Pause the blocked activity until the required input, material, approval, or technical input can be provided. Document the owner of that condition and the condition required to resume work.
4. Reset and recommit. The updated estimate will reflect the new selected completion date, and you will document the previous projection and the reason or authorization for the change. You will then issue a new milestone date without deleting history records.

A fixture drawing correction may take one day of test-branch float but does not affect factory completion. Approval of a substitute part that moves kit closure beyond board availability will require you to revise the commitment to start assembly. The same change can therefore have different effects depending on when it occurs and how much float remains.

What Does an Expedited Schedule Cost?

Quick-turn jobs typically have higher costs due to the need for prioritized engineering reviews, designated production capacity, small-lot setup costs, expedited component orders, overtime labor, dedicated QA resources, and express delivery.

However, the most accurate areas of savings within a quick-turn job would result from eliminating wait time caused by in-process delays rather than eliminating inspection activities.

• When designing a quick-turn job, utilize manufacturer-stipulated or industry-standard materials, finishes, stack-ups, or stocked components when your design allows.
• Approve all technically acceptable alternative parts before product release, not just when your company is experiencing shortages.
• When sending in a BOM, fabrication data, CPL, drawings, firmware, and test procedures, submit them as one coherent revision.
• Treat mandatory acceptance testing and optional characterization separately so that both testing schedules accurately reflect when they are to be performed.
• Changes in quantity, firmware, coating, marking, packaging, or delivery boundary require schedule review if they occur after manufacturing capacity has been scheduled.

Although there may be cheaper quotes on products, you must also determine whether they include component verification, programming, functional testing, records, and transit before comparing costs.

When Is Quick Turn Most Useful?

EVT and DVT Design Validation

During the EVT and DVT phases of a project, a very small volume of assemblies is often required rapidly so that electrical, thermal, firmware, and mechanical assumptions may be verified against real assembled products. The final product of this verification for the EVT and DVT phase typically is not a “components soldered” assembly. From what has been learned in the EVT and DVT phases, the project may require programmed units, current consumption limits, interface reviews, duplicate units for parallel testing, and records of failures for future revisions.

A typical EVT lot size of 10 to 20 boards can allow electrical bring-up, thermal soak, and firmware validation to be performed simultaneously rather than queuing behind a single unit, which can be one reason that small batch sizes are requested instead of one sample. Therefore, in this scenario, the fabrication and assembly data should be released early, and the test branch put on its own schedule so that it can mature without incorrectly blocking all production activities.

NPI Pilot and Process Verification

The NPI pilot’s purpose is to validate that all the released data, component selections, stencil, machine program, solder profile, manual operations, inspection plan, test method, and records can be successfully replicated. The customer may want to approve the first article before the remainder of the quantity is produced, so this approval must also show up in the dependency network.

A pilot lot size of 50 to 200 units with first-article approval of the first 5 to 10 boards as a gateway is often the minimum sample size needed to expose problems with the stencil, reflow profile, or fixture that may not be identified in a 5-piece prototype run. The concluding document should clearly indicate the source of each issue: design, supply, manufacture, testing, or documents.

Urgent Replacement or Field Recovery

The replacement assembly for a failed system may have the shortest business deadline but also has high configuration risk. The team needs to verify the revision of the hardware currently installed in the system, confirm that the approved substitute parts are compatible with the revision and that the firmware in the assembly being used is compatible with the installed configuration, verify that the connectors used in the assembly are correct for the installed configuration and that the mechanical fit is correct, and verify that an acceptance test has been performed prior to release of the assembly.

If an assembly is shipped in a timely manner and does not match the configuration in the field, the outage will not be resolved. The project schedule should differentiate between factory completion of the assembly, dispatch of the assembly to the customer, and receipt; therefore, carrier cutoff, customs, site access, and incoming verification may control recovery. When the only requirement for starting a production line is 2 to 3 confirmed-good units, a partial-lot release of the assemblies can separate the immediate requirement from the testing cycle required for the remainder of the order so that the operational downtime can be reduced without representing the entire order as complete.

How Do You Compare Quick-Turn Quotations?

Ask every supplier the same questions to establish a baseline for comparing the quick-turn quotation:

• When does the supplier start the clock? Is it based on the date of upload of the files, engineering approval, receipt of payment, kit completion, or formal production release?
• What does the “finish” or completion actually mean? Does it mean that the assembly is complete, factory complete, dispatched, or received?
• Are fabrication, sourcing, programming, inspection, testing, records, packaging, and logistics included in the quote?
• What working-day calendar, order cutoff time, weekend rule, and carrier handoff will this supplier follow?
• Which engineering questions will place the schedule on hold, and does a revision change restart the quoted tier?
• Which activities will occur internally, which will occur externally, and where will there be potential queue time created by handoffs?
• How does this supplier report progress, shortages, deviations, revised finish dates?

To see examples of this service scope, refer to quick-turn service overview.