Ask the Experts
August 20, 2018 - Updated
September 19, 2011 - Originally Posted

Mixed Technology - Which First

We have just put in place our first assembly line and are assembling a mixed technology circuit board. We assemble the through hole section first using our wave system, then the SMT parts using a reflow process. Is it always done in this order? Can the order be reversed?

S. S.

Expert Panel Responses

You could potentially run the through hole first. However, you should read the specs on the components beforehand. Many through-hole components are not compatible with SMT because they simply cannot take the heat of the reflow oven without being damaged. But it all depends on the boards you build. If they are single sided I would always run SMT first then go to a stuff line for the operators to put in through-hole parts. Then I would run it in the wave. It's a little different if you run double-sided boards. In those cases, I would run the boards as normal in the reflow with one exception: On the simple side of the board -- typically the bottom side -- I would glue the through-hole components to the board.( I define "simple" to mean mostly resistors and capacitors.) After the glue cures and exits the reflow oven,send the board to the stuff line with the leads protruding from the simple side of the board. If yours is a dual wave machine, the SMDs and through-hole parts will be soldered simultaneously on the pass. If both sides of the board are extremely complex you can continue the procedure you are now following so long as parts are not being damaged. Better yet, invest in aselective soldering machine. Most have around a 1mm keep-away and can solder through-hole parts easily.

Chris Ellis
Sales Manager
Chris Ellis is a Sales Manager/Engineer for Manncorp with 19+ years experience in the PCB assembly equipment industry. Chris worked as a Manufacturing Engineer for 5 Years. Prior to going into sales, he was with PCB assembly equipment suppliers. Before joining Manncorp, Chris was an independent rep in the Carolinas for several years.

First should be SMT, then we have to go for THT using Selective Soldering or Wave Soldering. There are a lot of influencing factors like Pick n Place. Optimization will be easy for further Mass Production.

Keshava Murthy
Process Engineer
Bosch Automotive Electronics India Ltd.
Keshava Murthy is a Process Engineer in the PCB assembly industry working in Bangalore, India.

SMT assembly is usually done prior to Wave assembly. You will need "Wave Fixtures" for your PCBA designed to protect the components from the Solder Wave and causing them to fall off the board.

Peter Greenland
Global Sales Director
Photonis USA
Mr. Greenland has 20 years electronics industry experience ranging from circuit board design to product support & repair services. Peter has spent the last 4 years at Pycon Inc. supporting customers that require high mix, low volume manufacturing solutions.

This is or could be a dangerous precedent as it must be known as to whether or not the PTH components can take the thermal excursion of the surface mount process. If the PTH components cannot take the heat then the process has to be reversed, do the SMT first then do the PTH. Fixtures can be modified to protect the SMT components during the wave solder or selective soldering process. In certain cases the PTH components may have to be soldered manually, if they are not accessible to the automated soldering process.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

We always smt first and then thru hole for repeatability due to thru hole component size issues. SMT components are designed for multiple thermal cycles where thru hole in some instances has multiple thermal cycle issues.

Mark McMeen
VP Engineering Services
STI Electronics Inc.
Mark T. McMeen is STI Electronics Inc.ʼs Vice President of Engineering Services. He oversees the daily operations of the Engineering Services division of STI. He has over 18 years experience in the manufacturing and engineering of PCBs.

The order can definitely be reversed - actually this isthe traditional way to do it - the SMT first and then the through holecomponents. If you are running a "single side" board through SMT, andthe TH parts are going on the same side with the SMT parts, then the wave isthe way to go. If you are running "double sided" assemblies inSMT (SMT components on both sides) then you want to design your processaccording to the equipment that you are going to use later for TH parts as wellas the type of components that you are populating in SMT. For chip components(resistors, caps and some SOT packages) placed on the secondary side of theboard, you can use a glue/adhesive process in SMT and after populating the TH components,use a wave process to solder the SMT components (turn the "chip" wave on) andthe through hole parts. Now, if the population on the secondary side of theboard does not withstand solder bath (part type, height, etc.), then SMT can bedone as a reflow, reflow (for both sides of the board) and then solder the THcomponent by hand or using a selective soldering equipment (point to pointsoldering robot, selective soldering). You can also use solder wave pallets(fixtures) to protect areas of the board. We are successfully using thetraditional ones as well as the ones with titanium inserts for the componentsthat are suitable to them.

Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
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