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August, 2015
Reflow in Today’s High-Mix/High-Volume Production Environment
By Bjorn Dahle, President, KIC, San Diego, CA
markets in the US and Europe can be described as high-mix, low-to-medi- um-volume lines. Many of these lines experience more downtime than pro- duction time, creating a serious chal- lenge: how to increase the volume while also increasing the efficiency. The downtime is mainly caused
M
by extended machine setup and changeover times due to the high mix of components on SMT boards. One critical machine that can
contribute to a great deal of the downtime and/or the potential suc- cess of a SMT production line is the reflow oven. Reflow oven setup time can be quite long for a new PCB assembly, often the slowest change - over for any machine in the produc- tion line. Long times to prepare a reflow oven can impact delivery times and profit. While many tech- nologies have been developed for
The production needs of high-mix PCB assembly
dictate savings in reflow oven setup time to boost
the efficiency of any high- mix, lower-volume PCB production line.
lower-mix, high-volume production lines, less attention has been paid to the production needs of high-mix PCB assembly; saving reflow oven setup time can boost the efficiency of any high-mix, lower-volume PCB production line. Manufacturers and suppliers of
solder paste, substrate materials, and electronic components provide tolerance or process windows for the safe use of their products. A reliable manufacturing practice involves the attachment of thermocouples (TCs) to each unique PCB part number to ensure that each manufactured PCB remains in-specification (in-spec) for the process and oven limits deter- mined optimum for that PCB. An in- spec profile is determined by measur- ing a time-versus-temperature pro- file that considers interfaces between multiple process windows:
Jose del Valle, COO/Vice President of NuVal EMS, Inc., counts on reflow oven efficiency.
Consideration of a PCB sub-
strate and its place in a reflow oven process includes the choice of PCB glass transition temperature (Tg) and decomposition temperature (Td) and other factors, such as copper weights and the number of circuit layers. Another factor is the type of surface protectant used with a select- ed PCB material, since a PCB assem- bly may require multiple reflows and cleaning. Jose del Valle, Chief Operating
Officer/Vice-President of contract manufacturer NuVal EMS (Tustin, California), believes that it is not suffi- cient to process MOST components within specification. Rather, ALL com- ponents must be held within the rele- vant process window. This is made more difficult by the large numbers of components mounted on typical PCBs. The relative positions of the compo-
improve quality and productivity while reducing manufacturing costs. Commercial oven setup software tools such as KIC Navigator and KIC Auto-Focus from KIC can dramati- cally reduce reflow oven setup time. KIC Navigator can create optimum oven profiles based on a single exist- ing profile. KIC Auto-Focus works as a
form of KIC Navigator with a data base, searching for the best oven pro- file for a particular PCB. With this software, a technician simply sup- plies the length, width, and weight of the PCB, and the system recom- mends an optimal reflow oven recipe within a few seconds, without run- ning a profile. This software-based approach is an improvement over guessing which oven profile to use for a particular board, and a verification profile can be run afterwards to
any printed-circuit-board pro- duction lines working in sur- face-mount-technology (SMT)
l l l l
A solder paste specification or specifications for other materials such as underfill.
Substrate tolerances and processing requirements.
Design efforts to reduce the process window by creating an effective thermal layout, such as positioning components with larger heatsinks next to smaller SMT components.
Tolerances for different electronic components.
nents and their thermal masses and thermal process requirements can also determine the thermal characteristics of a particular PCB.
Best Manufacturing Practices For companies such as NuVal
EMS attempting to apply best manu- facturing practices to the solder reflow process, such practices as thermal profiling,
traceability,
process control, and inspection are among the tools that can help
ensure that the oven parameters are the best for that PCB.
Oven Setup Software Reflow oven setup software
evaluates the large amounts of data represented by the many different thermal profiles for the many compo- nents mounted on a PCB to be man- ufactured. It can also determine the effects of oven conveyer speed in its search for an optimum reflow recipe. Such software can be used to identify a common recipe that can process groups of different PCBs. One method is to simply attach a large and a small PCB to the same profil- er. The software will search for a recipe that can process both PCBs within specification, so that PCBs between the two limits will be in specification. Depending upon the reflow oven, a single recipe may suit all PCBs. Software can also help, designing a handful of recipes to serve small, medium, and large PCBs. The use of TCs can verify the effectiveness of these reflow oven recipes. Prediction software for reflow
ovens can identify a new oven recipe by only changing the conveyor speed, and keeping the zone temperatures the same. Changing reflow oven recipes via conveyer speed is near instantaneous. But if the tempera- tures of a reflow oven recipe change, an oven may require 30 to 45 min- utes to stabilize to new tempera- tures, especially when changing from hotter to cooler temperatures. Reflow ovens will heat up much faster than they cool down, and some production managers may schedule production runs during a day by starting with the coolest reflow oven recipe and ending with the hottest. This approach requires more pre-produc- tion planning across the organiza- tion, but enables a higher mix envi- ronment to become higher volume by improving efficiency, noted del Valle.
High Reliability Electronics Many electronics manufactur-
ers, especially those involved in high- reliability (hi-rel) and automotive electronics, require the use of best manufacturing practices. The reflow oven must be set up correctly and
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