PCB assembly (PCBA) service

01

State the IPC-A-610 acceptance class

Writing for I-Connect007, Neil Sharp of JJS Manufacturing writes that “For many electronics manufacturing services (EMS) providers, IPC-A-610: Acceptability of Electronic Assemblies is the agreed standard”, in three classes. Class 1 “is the most lenient”, for simple low-cost products where function within an acceptable life is enough; Class 2 “is typically the most requested for non-critical electronic assemblies” where long-term reliability is desirable and allows some cosmetic imperfection, such as surface-mount parts slightly off pad that remain electrically and mechanically sound; Class 3 means building to all IPC criteria, including laminate selection, plating thickness, material qualification, processes and inspection. [1]

Sharp notes that Class 3 may need slower surface-mount placement for accuracy, more scrap where parts cannot be reworked, and extra time for inspection or cleaning, all adding cost, and that an OEM using its own criteria should cite them on every RFQ and purchase order. [1] Editorial note: because Sharp lists laminate and plating among Class 3 criteria, state the class for the bare board as well as for the assembly.

Give the assembler the Gerber and drill files, bill of materials with approved manufacturers, centroid (pick-and-place) file, assembly drawing, IPC-A-610 class, quantity and build schedule, who sources components (turnkey or consigned), moisture-sensitive and special parts (BGA, QFN, fine pitch), soldering requirements (lead-free or leaded), cleaning and conformal coating, test requirements (AOI, X-ray, ICT, functional), and packaging.

02

Control moisture-sensitive parts and bare boards

Broadcom's application note on moisture-sensitive surface-mount devices explains that absorbed moisture can crack packages during reflow (the popcorn effect), and reproduces the IPC/JEDEC floor lives out of the bag at ≤30 °C/60% RH: MSL 1 unlimited (at ≤30 °C/85% RH), MSL 2 one year, MSL 2a four weeks, MSL 3 168 hours, MSL 4 72 hours, MSL 5 48 hours, MSL 5a 24 hours, and MSL 6 mandatory bake before use with reflow within the time on the label. All reflow, wave soldering and rework must be completed within the floor life. [2]

Broadcom's packing table per J-STD-033 requires dry packing, moisture barrier bags with humidity indicator cards, desiccant and labels for levels 2a to 5a. [2] The German electrical industry association ZVEI clarifies that J-STD does not classify bare PCBs, that bare boards absorb moisture until saturated and are not dry when delivered, that repackaging does not extend shelf life, and that drying is mandatory for hygroscopic materials such as flexible boards. [3]

Editorial calculation: an MSL 3 reel has 168 hours of floor life; if a two-shift line opens it for 16 hours a day, it must be resealed with fresh desiccant or used within about 10 working days, and exposures add up across openings. Ask the assembler how floor time is tracked per reel and how boards and parts are baked when exceeded. [2]

03

Name the solder alloy and check its melting range

Kester's alloy temperature chart lists each solder alloy it sells with its melting range in °C and °F and the forms available (paste, wire, preforms, bar). A PCBA quotation that says only "lead-free" leaves the alloy open, and the alloys on the chart melt at different temperatures. [4]

Kester's product page for its "R276" no-clean paste shows how an alloy and powder appear on an order line: it is offered in Sn63Pb37 and Sn96.5Ag3.0Cu0.5, is classified as ROL0, and lists items such as "R276 Sn96.5Ag3.0Cu0.5 T4 86% 100g SYR", giving alloy, powder type, metal content and package. [5] Ask the assembler for the same items for the paste on your boards: alloy, powder type, metal percentage and flux classification.

The alloy also links to moisture control. Broadcom's note says all reflow, wave soldering and rework must be completed within the floor life of moisture-sensitive parts [2], and a lead-free alloy that melts at 217-220 °C instead of 183 °C for Sn63Pb37 [4] means the reflow profile runs hotter. Ask whether the components' MSL ratings were checked at the profile used. The IPC-A-610 class [1] sets acceptance, not the alloy, so state both on the order.

Editorial calculation: 217 °C minus 183 °C is a 34 °C difference between the start of melting of Sn96.5Ag3Cu0.5 and the melting point of Sn63Pb37 on the Kester chart. [4]

Values are from Kester's alloy temperature chart and R276 product page; reflow profiles and paste datasheets were not reviewed.

04

Compare what suppliers offer

Suppliers linked to this page list PCB assembly services; each card links to the page reviewed.

Listings include one-stop PCB fabrication and assembly, turnkey PCBA with component sourcing, EMS for industrial and IoT products, and assembly of special boards such as metal-core, flexible and ceramic PCBs; ask each for the IPC class they build to, MSL control, inspection and test coverage.

05

Trace the assembly process

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your boards

A PCBA quotation needs values for your boards. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the files, class and test needs together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Files: [Gerbers; drill; BOM with AVL; centroid; assembly drawing]
Class: [IPC-A-610 Class 1, 2 or 3]
Sourcing: [turnkey; consigned; partial]
Parts: [MSL levels; BGA; QFN; fine pitch]
Soldering: [lead-free or leaded; cleaning; coating]
Test: [AOI; X-ray; ICT; functional]
Volume: [quantity; schedule; packaging]

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