Posted on 20 Nov, 2014 by Neil Sharp
In order to accurately produce a quotation and subsequently manufacture an electro-mechanical assembly, your chosen Contract Electronic Manufacturer (CEM) will need a number of documents from you. Whilst you may be tempted to send across your latest build pack immediately, it’s well worth setting aside some time to review it in detail before doing so.
When transitioning any product from in-house assembly to an outsourced manufacturing solution, it’s not uncommon to find that a degree of ‘local knowledge’ has built up across your shop floor staff. It’s important therefore to make sure all of this valuable knowledge and experience is captured within your current documentation, so that the transition across to your chosen CEM provider is as smooth as possible.
In this blog post we will look at the 5 key pieces of data 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 is 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, that have consistently delivered quality product into 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, which 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 so that the production and engineering teams within the EMS company can clearly identify which BOM refers to which section of your build. This is particularly important on complex machines where multiple sub-assembly builds are built, prior to top level completion.
2) Wiring Schematic
A wiring schematic acts as a basic point-to-point reference diagram to 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, but 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 soldered, or alternatively, they may need crimp terminals and fasteners to be applied. 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 cannot be expected to hold stock of every type of crimp, fastener or crimp 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, there will be decisions made on how and where to route the cabling within your assembly. As cabinet wiring is a specialist skill, without clearly defined instructions the wireman working on your product will determine what they believe to be, in their experience, the best solution. This however could lead to differences in the amount of cabling you have previously used, which could then impact on the material cost.
In order for you 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 raised 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 you produce should reference the associated item numbers and outline the scale that they represent. Where complex electro-mechanical 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 an alternative perspective. Whilst generally having more information on a drawing is 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 format 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 skilled wireman within the EMS company.
5) Manufactured parts drawings
Unlike the electronic components fitted to your printed circuit board assembly (PCBA), drawn items such as enclosures, front panels, overlays, handles etc. are typically produced in much lower volumes and as such, 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 the drawings clearly define the scale and, of course, are revision controlled for completeness so that any changes can be tracked.
It’s important to note that CEM providers experienced in building electro-mechanical 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 in exactly the right format, don’t worry; things like working sample units and photographs can be equally of use and often help answer questions that may typically be detailed on assembly drawings or wiring schedules, for example. 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, create the necessary information for you.
Hopefully, by identifying the types of information required within your electro-mechanical build pack that will enable your EMS partner 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 investing a little time initially in checking these through for completeness, you will be helping to eliminate additional time, cost and resource at both the RFQ and NPI stages.
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Image 2 by 2bluesc.com