supplementary

Scenario for Assignment 2

Introduction

Advanced Manufacturing Services (AMS) has a range of different equipment. As described in the Scenario for Assignment 1, this varies in age and utility, so is not all of the latest standard: some of the equipment will be suitable for lead-free; other items may need to be adapted or even replaced during the lead-free implementation. Certainly, as you will have discovered in Unit 6, there are some aspects of each one of the processes that need to be considered by the company during the lead-free transition.

As describing specific items of representative equipment might convey an unacceptable and unintended commercial preference, we have described the equipment available in generic terms, and indicated characteristics and performance criteria, supplementing this with photographs of typical equipment.

If you would like to look at some examples of what a small-medium volume contract manufacturer’s factory would look like, and the sort of equipment it would have, then you may get further ideas from these web sites:

http://www.acl.co.uk/
http://www.cil-uk.co.uk/
http://www.coltergroup.co.uk/index.htm
http://www.hbmodules.co.uk/
http://www.intercole.co.uk/subcontract/default.htm
http://www.offshore-electronics.co.uk/
http://www.saturneng.co.uk/index.htm

These are listed in alphabetical order and were ‘self-selected’ from those companies on the list at http://www.aforall.com/directory/Business_and_Finance/Electronics_and_Electrical/Contract_Manufacturers that had included on their web site a description of the equipment and facilities available. [A missed opportunity for others!]

General arrangements

The factory floor is organized as three basic flow lines for surface mount, supplemented by:

One of the surface mount lines is conveyorised and is arranged to handle single-sided SM assembly. Where second side assembly is needed, parts are returned to the start of the line. This dictates batch operation unless the design has been arranged on panels where circuits alternate top and bottom sides in such a way that they can be flipped and the second side populated by repeating the print/paste/reflow operation on the same set-up. [This practice is occasionally used in such situations, as it allows an element of continuous production rather than batch processing]

The two remaining lines both have surface mount capability, including a glue station, and are associated with a single small reflow oven, a wave soldering machine and an aqueous cleaner. Because these lines are used primarily for small batch manufacture, the equipment is not interconnected by conveyors. The overall layout of the area is indicated in Fig 1.

Schematic layout of AMS assembly area

not to scale

Equipment

Printing

Line A has an automatic printer capable of printing panels up to 500mm square. While a conventional metal squeegee assembly is currently used, the printer can be retrofitted with a sealed head unit. The printer is fitted with a comprehensive wet/dry under-stencil cleaner and a fiducial correction system whose camera doubles as a 2D post-print inspection device. Depending on the set-up, full inspection takes approximately 25 seconds per board, but the cycle time for the printer in the full automatic mode is 12 seconds, so inspection is generally carried out only on a sample basis.

Figure 2: A typical automatic printer

Figure 2: A typical automatic printer

A platen is used to support the substrate for side 1 printing; for second side printing, custom nests are used to provide board support yet having clearance around underside components [This is typical of a line used for relatively long runs as this tooling method is cheaper than more flexible programmable pin supports]

The printers on Lines B and C are smaller semi-automatic machines, that on Line B being the older. The maximum board size is 430mmx400mm and the alignment, though motorized, is carried out manually, based on the images from fixed fiducial cameras. [This is typical of machines used for small-quantity production] Whilst the stencils for these machines are smaller than those used on the Line A printer, a conversion chase has been made so that, where necessary, they can be used on the automatic machine.

Figure 3: A typical semi-automatic printer

Figure 3: A typical semi-automatic printer

No automated under-screen cleaning is fitted, and inspection is by the operator. A supplementary piece of equipment is available to measure paste height.

Paste is applied manually, and the metal squeegee assembly cannot be upgraded to a sealed-head unit.

Placement

Line A has three items of placement equipment, two ‘chip shooters’ and one precision placement machine that is capable of handling the largest fine pitch components currently used by the company. The equipment was provided by a single manufacturer and the control software allows a degree of flexibility of feeder allocation between the machines. The optical system is grey-scale and arranged so that an underneath view of components can be taken, allowing accurate positioning of BGAs and a check before placement that all balls are present.

Figure 4: A three-station placement line

Figure 4: A three-station placement line

Reflow

The reflow oven on Line A is a 5-zone type with one cooling zone and a pin conveyor. Made in 1997, it has a clam-shell construction, and was bought with the idea that it might be retrofitted to run with nitrogen. However, the manufacturer has ceased trading!

Profiling is carried out with a standard board mounted with three long thermocouples that are plugged into the oven and use its in-built monitoring software. The company is considering using an external ‘mole’ for monitoring in order to be able to make more representative measurements.

Figure 5: Five zones of control on a convection furnace

Figure 5: Five zones of control on a convection furnace

Lines B and C share the same smaller and older 4-zone oven, which was made by a different manufacturer to that on Line A. It cannot be converted to operate with an inert atmosphere, and there are concerns about the maximum operational temperature of the oven, which is quoted as 240°C.

Figure 6: Assemblies leaving a belt conveyor oven

Figure 6: Assemblies leaving a belt conveyor oven

This second oven has just a belt conveyor, so can be used only for single-sided reflow. Neither oven has the capability of being retrofitted with board support between the rails, and some problems have been experienced when reflowing thinner boards.

Figure 7: Centre board support on a reflow oven with a pin conveyor

Figure 7: Centre board support on a reflow oven with a pin conveyor

Wave soldering

The wave soldering machine dates from the early 1990s, but has been well-maintained. Shared between all the lines, but primarily used on Lines B and C, it has a foam fluxer, a two-stage infrared pre-heater and a dual wave.

Figure 8: Dual wave pot on a machine with finger conveyor

The solder pot can be wheeled out for maintenance, and could potentially be retrofitted with a nitrogen diffuser to give a less-oxidising atmosphere around the wave. From the lead-free perspective, it is made of a type of stainless steel that was current during the 1990s but may be subject to mild erosion with some types of solder.

To meet the requirements of a specific major customer to have a visibly clean product, a three-stage aqueous cleaner has been installed.

Figure 9: Small batch cleaner with automated handling

Figure 9: Small batch cleaner with automated handling

The soldering/cleaning process has been evaluated with a water-washable VOC-containing flux that gives excellent results, even when the components are not perfectly solderable. Whilst this flux is thus generally a benefit as regards process yield, there are pressures from other customers for AMS to adopt a VOC-free no-clean flux. The difficulties and costs associated with running two fluxes have encouraged the process engineer to investigate retrofitting a spray fluxer.

The machine is a UK/American style with a single sloping finger conveyor, from the top of which soldered assemblies travel on a short downward conveyor past a ‘home-brew’ fan-cooling assembly in order to bring the part to handleable temperature ready for inspection. Bridging occurs on some designs, so a ‘view and touch up’ operator is generally assigned to repair assemblies before moving to the aqueous cleaning machine.

Figure 10: Single-conveyor construction wave-soldering machine

Figure 10: Single-conveyor construction wave-soldering machine

Rework and hand assembly

Through-hole insertion can be carried out by hand, often with the aid of laser-pointer assembly stations, for higher volumes of radial, axial and DIP format parts, using automated equipment.

Figure 11: Laser-pointer style of hand assembly aid

Figure 11: Laser-pointer style of hand assembly aid

The hand assembly area is well-equipped with suitable temperature-controlled irons. Some of these need the tip to be changed in order to alter the temperature of operation; others have power supplies with controls that allow adjustment. For some tasks, involving larger-gauge terminals, heavy duty irons are also employed, although there is no means of providing background heat.

Hand assembly typically uses adjustable jigs of the type shown in Figure 11. These can be inverted after insertion, with the components retained by pressure pads, to allow the board to be soldered by hand if necessary. [Some ‘non-wet’ components need to be assembled after wave soldering, and using no-clean flux, because they will not survive either the machine soldering and/or the cleaning process]

Most boards are reworked at assembly stations, but AMS also have one special-purpose machine that uses hot air to reflow the joints to rework BGAs and fine-pitch QFPs.

Figure 12: Hot air tool reworking a QFP

Figure 12: Hot air tool reworking a QFP

Inspection and test

Quality is important to AMS, and operators are trained in-house to IPC standards. Typically projection systems are used for detailed work.

Figure 13: Vision system for inspecting assemblies

Figure 13: Vision system for inspecting assemblies

AMS has considered purchasing AOI equipment, but the only investment possible to date has been a machine of the ‘comparator’ type where a soldered assembly is compared with the stored image of a good part. Despite the modest investment of around £30,000 this has proved very effective in detecting faults such as missing or reversed components. The equipment is claimed to be able to spot solder defectives, but typically only succeeds with gross defects such as bridges or joints without solder. As is typical of this type of machine, the lighting and optical systems are limited, though adequate for purpose. The production manager is seeking authorization to purchase a more sophisticated system than can scan joints and detect their shape, as a way of automating the task and reducing the heavy inspection burden on the more complex high-reliability products in the range.

When AMS first assembled BGAs, and needed to optimise their process, they made use of the X-ray inspection facilities at a local university, but this is too inconvenient and costly for ongoing monitoring. Realising that this is an area of exposure, the Quality Manager has been looking at alternative ways of examining BGA joints, although an automatic system may be more than the company budget can sustain.

Figure 14: System for X-ray inspection

Figure 14: System for X-ray inspection

AMS carry out in-circuit test using custom ‘bed-of-nails’ fixtures, and have a range of test equipment for functional test. Their resources for stress screening are limited to operational (but non-monitored) high temperature soak and manual twin chamber rapid temperature change; any other customer requirements are subcontracted to a local test house.

Figure 15: Test fixture for functional test

Figure 15: Test fixture for functional test

Capacity

As Figure 1 has indicated, there is a general shortage of room for expansion and new equipment, short of taking over additional premises nearby on the trading estate. Most equipment is, however, somewhat under-utilised, provided that batch changeovers are kept to a minimum and the set-up organised for efficiency.

The exceptions are on Line A, when certain high-volume products are being made. Which process is the bottleneck depends on the design; for a densely-populated double-sided assembly, it will be the chip-shooters; in the case of a simple but physically large and heavy board, the limiting process is reflow.

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