5 key documents to include in your electromechanical build pack
Author : Neil Sharp, Group Marketing Manager at JJS Manufacturing
03 November 2017
To accurately produce a quotation for, then manufacture, an electromechanical assembly, your CEM will need many key documents from you. Whilst you may be tempted to send your latest build pack immediately, this piece explains why it’s worth reviewing it in detail first.
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When transitioning any product from in-house assembly to an outsourced manufacturing solution, a degree of ‘local knowledge’ will likely have built up across your shop floor staff. It’s important to ensure your documentation reflects such valuable knowledge and experience, so that the transition to your chosen CEM (contract electronic manufacturer) provider is as smooth as possible.
In this article, we will look at the 5 key documents an electronics manufacturing services (EMS) provider will want from you, along with specific details of what each of these documents should include from a best practice perspective.
1) Bill of materials
As a minimum, your bill of materials (BOM) should reference all of the materials required to produce your assembly. It should also contain a description of the parts, and define how many of each are required to produce a single product. It’s also best practice to list all of the approved manufacturer part numbers against each of these items, particularly if more than one manufacturer is acceptable from a design perspective. This will be more common for electronic components, as opposed to drawn items, but allows your CEM provider to make informed procurement decisions – to help achieve both your cost and delivery targets.
You may also want to highlight to your CEM provider the current suppliers for each item. If you have invested time in finding sources of supply that you trust, which have consistently delivered quality product to you, then it makes sense to pass this information on. In addition, a complete BOM will list both circuit reference and item numbers against each part. These help clarify exactly where, on the printed circuit board (PCB) or assembly drawings, the items should be fitted.
Finally, your BOM should clearly state its current revision level. Best practice examples will also reference the assembly drawing or schematic number that they apply to; this way the production and engineering teams within the EMS provider can clearly identify which BOM refers to which section of your build. This is particularly important on complex machines where multiple subassemblies are built, prior to top-level completion.
2) Wiring schematic
A wiring schematic acts as a basic point-to-point reference diagram for your CEM provider. These should, as a minimum, highlight cable size, colour and identification numbers, along with details of where terminations are expected to be made.
Many CEM providers generate a wiring schedule from this schematic in a computer-aided design (CAD) package; however, they will likely need some additional information from you, which they may ask for during the request for quotation (RFQ) stage.
3) Wiring schedule
In addition to basic point-to-point information, your CEM provider will want to understand exactly how the terminations within your assembly should be made. For example, you may want them to be soldered, or they may need crimp terminals and fasteners. If the latter is the case, then it’s important that all of these items are listed on your BOM, so they can be procured by the CEM partner and correctly accounted for.
Depending on the specific type of crimp you require, a new tool may be needed in order to fit them, which will have to be purchased by the CEM provider. Whilst many CEM providers will operate Kanban systems for such consumables, they shouldn’t be expected to hold stock of every type of crimp, fastener or tool. It is suggested that you list these out in advance to help avoid any negative cost or delivery implications further down the line.
Finally, decisions will be made on how and where to route the cabling within your assembly. As cabinet wiring is a specialist skill, in the absence of clearly defined instructions, the wireman working on your product will determine what they believe, in their experience, to be the best solution.
This could however lead to differences in the amount of cabling you have previously used, which could then impact on the material cost. To control material costs and reduce the number of initial questions from your CEM provider during the RFQ stage, you may wish to consider supplying them with a wiring schedule of your own to clarify the points above.
4) Assembly drawings
Your assembly drawings help your CEM provider visualise how to fit together all of the items called up within the BOM, including the application of decals and labels. All of the drawings should reference the associated item numbers and outline the scale that they represent.
Where complex electromechanical assemblies are concerned, it is common for multiple drawings to exist for each level of assembly. All levels should be revision-controlled, and if engineering change notes (ECN) apply, then these should be noted on the drawing too.
You may find that certain elements of your assembly benefit from cross-sectional views, which will help your CEM provider picture the area from alternative perspectives.
Whilst having more information on a drawing is generally better, try to avoid having too many details in one section, as this can become counterproductive. As with the wiring schematics and schedule, a CAD drawing is typically preferred over a PDF document, as this allows your CEM provider to add any additional details they feel will help them during the production stage, such as care points. And finally, if the consistency or aesthetics of internal wiring are important to you or your end customer, then your assembly drawings should describe exactly how you expect the cabling to be routed; otherwise, this will be determined by the CEM’s skilled wireman.
5) Manufactured parts drawings
Unlike the electronic components fitted to your PCB assembly (PCBA), drawn items such as enclosures, front panels, overlays and handles are typically produced in much lower volumes; as such, they will be subject to greater variables during the manufacturing process. When producing drawings for these items, you will need to specify the material type and finish, along with complete dimensions and the tolerances allowed. As with your assembly drawings, make sure they clearly define the scale, and of course, that they are revision-controlled for completeness – so that any changes can be tracked.
It’s important to note that CEM providers experienced in building electromechanical assemblies will work with you during both the RFQ and new product introduction (NPI) stages, to help identify if there are any gaps in your manufacturing data. If you don’t have all the information immediately available and in exactly the right format, don’t worry: resources such as working sample units and photographs can be equally useful – and often help answer questions that may typically be detailed on, for instance, assembly drawings or wiring schedules.
A good CEM provider will be able to carry out an initial audit of your data during the quote stages; and then, by working alongside your engineering team, either enhance, or – in some cases – even create the necessary documentation for you.
Hopefully, by identifying the types of information required within your electromechanical build pack (that will enable your EMS partner to produce a quotation and build assemblies), you are now better placed to verify your own internal documents, prior to sending them across to your third party supplier. By initially investing a little time in checking them through for completeness, you will be helping to eliminate additional time, cost and resources at both the RFQ and NPI stages.
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