Homepage » Technology » Products »

Selective soldering – is it the best method?

An integral technique in todays’ manufacturing processes
Selective soldering – is it the best method?

Since regulations were brought in from the European Union’s Waste Electrical & Electronic Equipment (WEEE) and the Restriction of Hazardous Substances (ROHS) to enforce lead-free soldering, the use of selective soldering techniques have increased greatly.

Patrick McWiggin, SolderStar’s Technical Director

Selective soldering is the method of soldering selectively through-hole (TH) components on a mixed technology assembly, which would otherwise need to be hand soldered, or passed over a wave soldering machine following the soldering of the SMT elements within a reflow oven. It is commonly used where a number of conventional technology components or connectors need to be added to high quality medical, automotive or aerospace SMT assemblies. To combat the potential damage to the surrounding parts on the PCB, the requirement for the selectively soldered region needs to be exact; this resulted in the need for process control technologies to be developed which would ensure the procedure was carried out within a defined production specification.
The use of selective soldering for lead-free TH soldering applications requiring higher process temperatures has many advantages over other methods like conventional wave soldering due to the potential of increased reliability. Because components are exposed to lower thermal contact during the process it eliminates the potential internal thermal damage.
Some mixed-technology assemblies soldered with standard wave solder equipment would need to use custom designed carriers and masking, however many assemblies, for example, stacked and double-sided SMT assemblies, and even some TH devices, are too complex to be processed this way.
With the need for selective soldering increasing, SolderStar has developed a dedicated range of process profiling products for the selective processes. One of the most advanced systems is the WaveShuttle PRO Selective, designed to analyse, improve and gain control of the mini-wave selective soldering process quickly This instrument is unique due to it incorporating two measurement systems ensuring that the most comprehensive and repeatable measurement results are obtained.
The WaveShuttle PRO Selective was designed to extend the use of existing SolderStar datalogger’s and could be used on the selective soldering machines. Using the Smartlink system, it included a dedicated measurement fixture/pallet. This included three arrays of solder contact sensors, including;
  • X Speed Sensor – which measure and verify the speed of the X axis of the machine
  • Y Speed Sensor – which measure and verify the speed of the Y axis of the machine
  • Height Array – Contact sensors that are set at height increments of 0.2 mm to 2.4 mm to allow the height of the solder ‘bubble’ to verified
In addition to these innovations, a pre-heat temperature sensor and a solder temperature probe was added to verify all key parameters in a single pass.
The most recent addition to the company’s selective soldering range is the WaveShuttle Selective and is now the tool of choice for setup and daily testing of mini-wave selective machines for many. What makes this solution unique is the specially configured array of titanium wave contact sensors arranged on the underside of this measurement fixture to allow quick and comprehensive verification of the machine functionality.
Nozzle strikes on clinched components of the PCB due to PCB warp or general tolerances can result in the positioning mechanism of the machine becoming mis-aligned. X and Y positioning mechanism and speed on the WaveShuttle Selective are verified by sweeping the selective soldering nozzles between pre-defined datum’s. A range of stepped contact sensors are employed to allow nozzle wave height measurements to be made and periodically verified.
Other common problems reported by manufacturers was crystallisation occurring in the selective fluxer nozzle resulting in the mis-fluxing of components causing problems with poor wetting or leaving un-activated flux on areas of the assembly. To combat this problem the company developed flux measurement sensors, which can be optionally installed to the measurement fixture. This allows for flux presence and position to be obtained as part of the daily tests.
All parameters can be quickly analysed with the good and bad limits defined easily to allow periodic process verification to be performed without the need for specialist knowledge.
The other main method seen in selective soldering is the multi-wave selective/dip soldering machine, which trades flexibility for increased throughput. With this style of machine custom wave tooling is produced to expose the molten solder selectively to the area required in a single dip. Analysis of the performance of this process needed a different approach as it was found that engineers needed the ability produce their own analysis sensor configuration to match the tooling design.
The company’s Multi-Wave PRO was developed and comprises compact and flexible instrument design to allow dedicated test fixtures to be produced quickly and cost effectively for multi-wave soldering machines. Measurement sensors are fitted to a customer’s specific location, which corresponds to the soldering areas within the multi-wave solder pot tooling, this technique provides contact time and difference data quickly allowing engineers to identify any problem areas within the tool design. Any differences in nozzle to PCB contact can be easily measured providing a powerful platform for process capability analysis, improvement and on-going testing.
High speed measurements required to yield accurate results
The X/Y speeds in selective soldering machines can be fast which requires a quick responding measurement system to provide accurate and repeatable results.
The contact sensor system used with the company’s development scanned every 10 milli-seconds and measured digitally to give extremely accurate results.
The innovative contact measurement system relies on a ‘Ground’ contact to be made to the machine, to detect any contact with a solder wave and it was found that on some mini wave machines the ground was not easily achievable. To combat this a ground pin was produced and placed next to each measurement sensor so repeatable and accurate measurement was always attained.
This measurement and verification is paramount in the soldering process. To produce electronic assemblies without a known thermal profile is detrimental, as you ‘cannot manage what you do not measure’. There can be changes in preheat performance, settings can be accidently edited, or nozzle build up can occur. Also if the nozzle strikes the PCB during production, either due to PCB warpage, or incorrect machine setting, the X/Y mechanism can be thrown out of alignment and soldering will be off position.
Having developed and evolved the selective soldering process with this ground-breaking technology, the company trialled the new system with a customer, who reported all pre-heat and nozzle measurements to be accurate. It was also noted that the wave height measurement was accurate to 0.1mm. But the one area of the machine that was not tested was the fluxing stage. This is notoriously difficult to analyse electronically.
Fluxing-the difficulty with selective soldering process
Flux facilitates the soldering process and has the primary purpose to clean the components that are to be soldered. To obtain a successful solder any impurity including dirt, oxidation or oil must be removed.
Flux can be removed through mechanical or chemical cleaning, but the increased temperatures required to melt the solder can make the assembly re-oxidize. This can be accelerated as the soldering temperatures increase and can prevent successful soldering.
On a conventional wave solder machine, the entire assembly generally is fluxed, preheated and passed over the solder bath. With selective soldering, only the area to be soldered by the nozzle is fluxed, with dropjet technology to spray small amounts of flux onto the PCB.
As the assembly only comes into contact with the solder nozzle in those places, any flux deposited elsewhere will not be burned off and are left on the assembly, which is not desirable and can cause failures.
After 6 months of intensive development and trials due to the many types of flux, which all perform differently, the company found a way to prevent this problem. The final solution was a new sensor, manufactured from high temperature thermoplastics, with an embedded stainless steel surface resistance sensors. It was developed as a configurable measurement system which has the option to be trained on different flux compositions, which can detect flux spots of 2mm upwards, a real break-through in the selective soldering process.
Selective soldering – is it the best method?
Selective soldering utilises many of the concepts of other methods, but it benefits from its ability to localise soldering and fluxing with minimal pre-heating required, making it an innovative and unique technique. The use of selective soldering for lead-free TH and PCB soldering applications includes many advantages over traditional methods like wave soldering, as the thermal shock to sensitive assemblies is potentially reduced. It is a flexible and adaptable process which can be applied to a range of soldering tasks; and with companies like SolderStar developing tools to help setup and further maintain the process to reach specific requirements; it is seen as the answer to many soldering difficulties, which are seen more than ever due to the complex designs being manufactured today.
Current Issue
Titelbild EPP EUROPE Electronics Production and Test 11
Issue
11.2023
READ
Newsletter

Subscribe to our newsletter now

Webinars & Webcasts

First hand technical knowledge

Whitepapers

Find all current Whitepapers here

Videos

Find all current videos here


Industrie.de Infoservice
Vielen Dank für Ihre Bestellung!
Sie erhalten in Kürze eine Bestätigung per E-Mail.
Von Ihnen ausgesucht:
Weitere Informationen gewünscht?
Einfach neue Dokumente auswählen
und zuletzt Adresse eingeben.
Wie funktioniert der Industrie.de Infoservice?
Zur Hilfeseite »
Ihre Adresse:














Die Konradin Verlag Robert Kohlhammer GmbH erhebt, verarbeitet und nutzt die Daten, die der Nutzer bei der Registrierung zum Industrie.de Infoservice freiwillig zur Verfügung stellt, zum Zwecke der Erfüllung dieses Nutzungsverhältnisses. Der Nutzer erhält damit Zugang zu den Dokumenten des Industrie.de Infoservice.
AGB
datenschutz-online@konradin.de