Ask the Experts
January 3, 2023 - Updated
March 27, 2012 - Originally Posted

Fastening Surface Mounted Connectors Before or After Refow?

Is there an IPC guideline for attachment of SMT or PTH connectors that are hardware mounted? Do you tighten / torque hardware before soldering, or solder then tighten hardware?

Could not find guidance in J-STD-001 or IPC-A-610.Torquing before soldering is difficult with SMT connectors when using solder paste. Any direction is appreciated.


Expert Panel Responses

There are no IPC guidelines which will state that you should torque before or after soldering. Even torquing values are completely design dependent.

There are requirements that a solder joint is not stressed during or after soldering. If you solder, then torque, the soldered connections are going to be under a stress load right from the moment of torquing. By torquing first, even though it's more difficult, you set the connector leads into position and THEN solder them without stress.

Kris Roberson
Manager of Assembly Technology
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.

Always torque before soldering! If you torque after soldering, there will undoubtedly be some movement of the connector which will place residual stress on the solder joints.

Garry McGuire
Sr. Engineer
NASA/Marshall Space Flight Center
Garry McGuire is a manufacturing process engineer and Chair of the IPC J-STD-001 and IPC/WHMA A-620 Space Addendum committees.

The PTH Connectors should be torqued down prior to reflow, when applicable. Mainly due to stress incurred, if you torque them afterwards, assuming you are attempting to get to single soldering step.

The SMT Connectors could be torqued after soldering, pending the lead configuration. Gull wing would allow for stress relief, more than a PTH component, with solder in cylinder.

Assuming you are using Paste-in-hole PTH on your statement below. If the PTH are running through WAVE SOLDERING, then we would recommend torque prior to both SMT and PTH.

When soldering Power components, with Mechanical Heat Sink or Buss Bar, the TO-220 and TO-525 packages would be torqued to the mechanical components, in the same way connectors would be to PCB laminate. During this process, we would "destress" and Re-Torque the component after soldering, by reflowing the solder. This was requirement, with our customer, to meet Military needs.

You may need to include this 'destress' step in your assembly process, to prevent any premature failure in the field. If the connector is for rigid connector system, like PC104 that might be preferred as it resides on a back-plane for the life of the product. If the connector is a DB style with a harness/wire connection, which can be mechanically isolated from force, you may not need the stress relief.

Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.

As far as I know, there is no general guidance, due to the fact that each connector design is unique. In general, PTH connectors should be anchored prior to soldering. You want to avoid putting stress on the joints after soldering. SMT connectors are another thing altogether.

As you pointed out, fastening hardware prior to soldering can be problematic to say the least. Fastening it after soldering can potentially put stress on solder joints which is not desirable. You need to look carefully at each connector design and assess whether there will be unacceptable stresses created if fastened after mounting.

In certain cases, we have found that we actually need to fasten, then hand solder (!) some poorly-designed connectors.

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

Always secure connectors or any mounted hardware prior to soldering if within the tolerances of the assembly process.

If it is a long connector and on the edge it will help keep the assembly straight but most important is if the device is not secured and the soldering takes place and then tightened down you may or will crack the solder joint if the assembly is not totally flight.

The securing force will place undue stress on the joint and will cause a failure at once or later down the road while it is at your customer site.

Terry Jeglum
Electronic Technology Corporation
Mr. Jeglum has 35+ years experience and is the founder of Electronic Technology Corporation. He is responsible for 22 years of program management for the Company.

IPC-A-610 Section 7.1.8 Component Mounting Connectors states that when assembling the PTH connectors into the board the board locks are fully inserted and snapped into the board.The implication also applies if mechanical hardware, such as screws and nuts, are used.

The reasoning for securing the connectors prior to soldering is to prevent applying forces to the solder joint if the hardware is installed after soldering. Basically, secure the connector and solder in place. You must use the best manufacturing practices when assembling these types of devices.

As for surface mount connectors, many of these types of connectors have locating pins, but I could not find any that requires hardware to secure them to the printed circuit board. Their strength is based upon the configuration of the leads and the proximity of the connector housing to the printed circuit board.

The forces to be applied to the connectors must be determined in the design phases of the product so the proper connector scheme can be used to provide a reliable product.

If there are surface mount connectors that do require being torqued down I would imagine it would have to be a manually assemble component and manually soldered in place. Torquing down a surface mount component inserted in solder paste is not a good manufacturing practice. The IPC-A-610 section for this 8.5 Surface Mount Connectors.

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.

Applying torque to the fasteners of a connector that has been previously soldered will in part stress to the solder joints. This is true whether PTH orSMT.

The fasteners also help align the connector body to the land pattern. The heat of soldering process will cause the board to shrink as moisture is driven off of the board, so that the torque may no longer be in effect. Depending on the environment, the solder joints may sufficient to support the connector, such that you do not have to re-torque the fasteners after soldering.

There is a trade-off between the added stress which will be minimized because the fasteners and connector were aligned before and during soldering versus the added mechanical support that the torque provides after soldering.

Mike Green
Design Engineering
Lockheed Martin Space Systems
Mike Green is co-chairman of the IPC Terms and Definitions Committee. He has been working with board design and manufacturing for 33 years.

Reader Comment
Mechanical Fastening of the connectors is recommended to be done before the reflow/wave soldering processes, to omit stress accumulation. But during practice it is observed that where certain torques are required they need to be re-checked for correct torque after the thermal (reflow/Wave) process.

This is caused when the plastic connector cavities under stress during the thermal process, they tend to deform a bit which cause to jeopardize the torque requirement.
Susantha Wickramarachchi, Variosystems. pvt. ltd, Sri Lanka

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