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Knowledge Base

PCB Reworking requires knowledge and skill in order to be a successful rework engineer. We hope you will find the following informative and helpful.

 One needs to know the maximum temperatures that must not be exceeded during the reflow process. These temperatures are different for leaded and lead-free solders. Below are the most typical examples:

 

Solder Alloy

Melting Point °C

Temp Max Reflow °C

Sn63Pb37 (leaded)

183

220

Sn96.5Cu3.0Ag0.5 (lead-free)

217

250

 The minimum solder joint temperature for lead-free should be at least 235°C to satisfy accepted industry standards. This implies a package body temperature of 238-240°C. Using Insat 36 or 40 gauge thermocouples make it easy to place the TC on top of the body of most IC packages. Due to the very small thermal mass of the TC bead, the response and accuracy is as good as it gets.

  Tin (Sn) is a soft  metal and has a melting point of 232°C.  Molten Tin has the ability to melt almost all other metals.

 Lead (Pb) melts at 327°C and is also a soft metal. Lead makes very little contribution to the bonding of metal to metal during the soldering process. When mixed at a ratio of 63% tin and 37% lead, the resultant melting point is 183°C, which is lower than that of tin and lead.

 During the soldering process, the dissolution of tin and copper forms an intermetallic layer which is brittle. The ideal intermetallic layer is 1 to 2 microns which forms the strength of the bond of the solder joint. The intermetallic layer gets thicker with every reflow cycle. Thicker intermetallic layers are likely to crack and cause unreliable solder joints. Cracking occurs due to expansion and contraction of the PCB during its normal daily operations. The thickness of the intermetallic layer is controlled by the time above the liquidus temperature of the solder alloy. Typical times above liquidus is 45-90 seconds.

 Intermetallic layers have very high melting temperatures in excess of 400°C, which means they are always present on the BGA pads and will cause unreliable solder joints if the layer thickness is too much. Manufacturers do not recommend more than 3 reflow cycles for this reason. The intermetallic layer gets thicker with each reflow cycle. The copper pad surfaces in contact with molten tin in the solder causes dissolution of the copper which reduces the thickness of the pads. The rate of dissolving of copper by molten tin is higher with lead-free solders such as SAC305 (Sn96.5Cu3.0Ag0.5).

 image

Oxidation

Oxygen in the air causes the copper solder pads to oxidise which prevents solder wetting on the metal surface. Wetting refers to the spreading of the molten solder over the surfaces to be bonded. Oxidation blocks the molecular attraction between the solder and the metal surface of the copper pads.

  Flux

The basis of flux is usually a solid that has been dissolved by a solvent. The most commonly used solid for many years in PCB assembly was rosin. Rosin is derived from pine trees, which does not conduct electricity at room

temperature. Rosin becomes liquid between 125°C and 130°C. Water white flux refers to pure rosin that has been

dissolved in isopropyl alcohol and can only remove the thinnest of oxide layers. Activators are used to increase the acidic strength of the flux. RMA refers to mildly activated rosin flux and RA refers to more aggressive rosin flux. Using high activity fluxes require cleaning of the residues as they are corrosive.

 The Insat Super RMA liquid flux is a high reliability soldering flux which has been formulated to provide extremely reliable reflowing of laptop PCBs, games consoles and all similar boards. It is a no-clean product and can be left on the board. This flux has legendary status within the reflowing community and is highly recommended by those in the know.

 It is a very fast wetting flux and has the highest acidity allowed for a RMA classified product. The rosin used is of the highest quality and has the highest thermal stability figures. It is non-conductive and non-corrosive. This flux starts to activate from about 80°C and therefore has a wider active temperature window to remove oxides from copper surfaces during the preheat and soak phases of the profile. Due to having the ideal solid content, it forms an oxygen barrier during the soldering process thereby reducing the risk of oxidation. Furthermore, when left on the soldered joints after soldering, it will form both an oxygen and moisture barrier at the solder joints.

 At best, flux can only remove a thin layer of oxidation just a few molecules thick. Even the strongest flux may not remove thick layer of oxidation. 

 While on the subject, it may be a good idea to use an RA type rosin flux when de-soldering BGA devices as the higher acidity will remove any oxidation far better than RMA type. As the PCB will be cleaned during the site dressing process, this will not be harmful to the PCB. However, the same cannot be done when soldering the BGA as it would not be easy to clean under the BGA, which would leave corrosive residues on the PCB.

 Cleaning rosin based flux is done using isopropyl alcohol or custom clean cleaning products.

 Water soluble fluxes usually are based on the oxidation removing qualities of an organic acid such as citric. Organic acid fluxes are more aggressive than RMA, so they provide good cleaning, but they are usually corrosive as well. Because of the corrosiveness of these fluxes, a thorough water wash following solder reflow is necessary. It is normal for water soluble fluxes to leave behind a white residue.

 When re-soldering a BGA onto a board, it is best to use a gel type flux and excessive amounts should be avoided. If excess flux is used, at higher temperatures the flux will splutter and bubble up underneath the BGA as the pressure increases and may cause the BGA to be displaced from its position.

 

 Reflowing

 The term reflowing, refers to the soldering of manufactured PCB boards using solder paste. Below is a profile graphic chart showing the 4 stages of a typical profile. Looking at the maximum reflow temperature (in this case 230°C), we know it must be a leaded solder alloy. If the maximum reflow temperature had been 250°C, then it would have been for a lead-free solder alloy.

 The profile can be represented in a table too:

 

Phase

Temperature

Leaded               Lead-free

Time in

Seconds

Heating

Rate /s

Pre-heat Temperature

150                         150

90-120

1 – 2.5°C/s

Soak temperature

180                         195

90-120

Less than 1°C/s

Reflow Temperature

220                         235(250)

45-90

1-1.5°C/s

Cool Down

Room temperature

 

Less than 6°C/s

 It is very important to note that the actual temperature of the board and the temperature indicated by the thermocouple may be wildly different. The accuracy of the indicated temperatures rely totally on how well the TC (thermocouple) has been placed on the PCB and its position relative to the target device.

 The best place to position a TC is under the BGA in intimate contact with a solder ball. This is not possible. So, the next best place could be the top of the BGA taped with either kapton tape, aluminium tape or high temperature masking tape. Using thin gauge TC such as Insat 36 or 40 gauge is ideal. The accuracy is very high and response is excellent. With this method, an assumption is made that the package body temperature is about 3-5°C higher than the underside of the BGA. A package body temperature of 240°C could be used in this case. Maximum package body is 250°C.

  Effort should be made to note when the melting point is reached, as the temperature ramps up in the reflowing stage. The time above liquidus starts at this point. The aim is to reach the maximum temperature in the next 20-30 seconds, and dwell at the maximum or within 5°C of the maximum for 10-20 seconds and then allow to cool.

The cool down rate should be no more than 6°C/s.

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In reflow soldering, the solder paste must be heated to a temperature well above its melting point to achieve totally molten solder state. The reflow process is carried out in reflow ovens and the temperatures in each of the ovens are accurately maintained at predetermined temperatures.

 Using a desk-top, low cost rework machine differs from an enclosed reflow oven and is an important source of heat dissipation from the PCB during the reflow process.  In hot air reflow machines these heat losses from the PCB and in particular the target device being reworked, requires a much higher air temperature to be used. 

 The increased temperatures can be high enough to cause package body damage to delicate BGA devices.  Reducing the heat losses from the PCB means that the maximum hot air temperatures can be kept within the safe  limits of the device.  The accepted maximum package body temperature is typically 250°C.