QFP Removal and Replacement Procedure

Illustration # 1

1. Introduction:

1.1 Quad Flat Pack packages are plastic die encasements with lead contact distribution around the perimeter of the package and can be referred to as “Gull Wing” packages due to the shape of the very fine contact leads.

Great care must be exercised in handling, not to distort or bend the leads, any distortion effects the “Coplanarity” of the overall component lead plane to the contact pads on the board and this distortion can effect the ease of which a chip is placed, for example if just one lead is lower in plane than the others, the whole chip begins to pivot around the lower lead and a swivel sliding effect is found during placement. If any lead is higher than it’s companions then that lead might not be effectively soldered during reflow. So it’s important to handle new packages with caution, no matter what the lead count.

1.2 Removing a QFP is relatively straightforward, as long as the component is no longer required, the two processes discussed in this procedure, cover “Eutectic” removal and “Reflow” removal.

1.3 – Eutectic removal, removes the component at eutectic condition, which means the minimum time and temperature to pull the component from the contact pads when the solder securing the last leads changes to a molten state, as the component is subject to upwards tension during the process, by an Automatic Vacuum Pick Up Assembly a small amount of solder can be dragged up with the component leads, leaving reverse icicles on the contact pads, which need to be leveled prior to component replacement. Also a number of the last leads to be released may be bent due to the drag tension. With the Chipmaster this is reduced due to the linear heating throughout the nozzle area, but this does not compensate for the thermal differences in the overall circuit board mass, so that new solder and leveling may be necessary.

1.4 Reflow removal not only allows the removed chip to be reused but removes the necessity of dressing the contact pads with new solder or having to Hot Air Level between stages of removal and placement, thus reducing time to effect a repair and also reducing possible errors in applying new solder, which is a skilled and somewhat difficult process. With Reflow removal, the operation is delayed until a total reflow condition is reached and the component is removed without drag or transfer of solder from the contact area to the component leads, thereby leaving all contact pads complete with clean coplanar solder.

 

 

(Illustration # 2)

The following procedure is designed around the Chipmaster High Power Low Temperature operation, (Illustration # 2) working at reduced temperatures, which does not degrade solder integrity or original quality, i.e. does not reach the critical temperature level, which begins to accelerate the aging process and crystallizes the tin (SN) and lead (PB) molecules, visible by a grayish pitted appearance. Rework with the Chipmaster results in mirror surface joints, providing the original solder has not been degraded through high temperature rework (Illustration # 2a). The QFP procedure discussed is for lead counts of 100 leads, (Illustration # 1) or greater, however the process is identical for all QFP components, excluding those, which are Conformally Coated

 

Illustration # 2a

Solder joint @ 430 F (221 C) Solder Joint @ 550 F (287 C)

2. Conformally Coated Components:

2.1 A detailed repair process for Conformal Coated components is outside the scope of this procedure as they usually contain smaller lead counts than discussed in this document, and are usually found in products subject to frequency of handling and environmental hazards, refer to Motorola document 68-80309F92, available from Motorola APD or APE.

As a summary, if coating exist, it is necessary to carry out a Eutectic Removal only, the Reflow Removal procedure outlined in 7.0 of this document is not possible. The component is removed by setting the Chipmaster to the removal temperature as explained below, (5.0) and using a probe supplied in the kit, gently pry each corner of the component as the coating starts to break down under temperature, (Illustration # 3), be careful not to apply too much pressure so as to damage the component or contact area. All the coating material should now be cleaned from the QFP contact pads and component leads, (should the component need to be reused), Use the pad prep cleaner included in the Motorola # 81-30303-E45, APE # 8100-1097, SMD Tool Kit to clean the contact area after removal and use the knife probes to collect all residues and finally clean with a lint free Antistatic wipe, APE # 8200-1500.

Illustration # 3

3. Required Materials:

Item Description APE Model # Motorola Model #

1. Chipmaster 8100-1003 R1319A (110V) or R1321A (220V)
2. SMT Tweeze/Solder Iron EX-755 R1366
3. SMT Tool Kit 8100-1097 01-80303E45
4. Nozzle for QFP208 8100-1414 66-80332E92
5. Chipmaster Manual 8100-0609 68-8309F40 (Supplied with Item 1)
6. Lint Free Antistatic Wipes 8200-1500
7. Hand 2.3 Dioptic Magnifier 8200-1200

4. Selecting the correct Profile.

The Chipmaster 8100-1003, includes the optional Profile Storage Controller, which includes four (4) carefully designed and pre prepared profiles for most BGA and QFP/SMT applications, (Illustration # 3a). In reworking QFP components greater than 100 lead count Profile # 2, is recommended and will provide the correct “Temperature Ramp” and “Soak” so as not to stress the component under repair and protect the adjacent components from degradation or damage. Should the circuit board be of a heavy metal or ceramic construction, Profile # 3 is recommended, to increase the calorific energy to the board (Not Temperature).

 

Illustration # 3a

To select Profile # 2, from the “Operator View Display” (Illustration # 4) press the INDEX key on the controller ONCE to enter the “Operator Menu”

then press the INDEX Key, ONCE more, to reach the “Profile Field” ; then press either the “UP” or “DOWN” arrow to reach the desired Profile (RED L.E.D. display. Then press ENTER. Press the INDEX key THREE times to return to the “Operator View Display”. See Chipmaster Manual for complete explanation of controller and features.

Illustration # 4

5. Eutectic Removal Procedure

5.1 Set the temperature control on the ChipmasterŠ to 440F, and set the timer to zero by:

To set or change Set Point temperature, from the “Operator View Display” press the INDEX key on the controller once to enter the “Operator Menu” then press the INDEX Key, TWICE, to reach the “Temperature Field” ; then press either the “UP” or “DOWN” arrow to reach the desired temperature (RED L.E.D. display). To do this, use the “UP” arrow to increase: hold the button in to increase rapidly. To increase temperature in one-degree increments, press the arrow button in individual steps. The same procedure works for the “DOWN” arrow button. Then press ENTER. Press the INDEX key twice more to return to the “Operator View Display”. See Chipmaster Manual for complete explanation of controller and features.

5.2 To reset the timer to zero, in the “Operator View Display” press the ENTER key and the “DOWN” arrow key simultaneously. The display will flash once, indicating that the timer has been reset to zero.

5.3 How The Timer Works:

If the timer is reset to zero, it will START counting UP once the heat cycle begins (Foot Pedal pressed) and will STOP counting once the cycle is stopped by the operator (Foot Pedal released). The timer will display the cycle time, e.g. 1: 45 minutes, or whatever. Each time the foot pedal is depressed to start the cycle thereafter, the timer will count DOWN to zero from whatever cycle time (in this example, 1:45 minutes) was recorded. This is the Timer procedure used for Eutectic Removal.

The procedure for changing cycle time is similar to setting temperature, Press the INDEX key ONCE (from the Operator View Display) to enter the “Operator Menu” and reach the Timer field. Use the “UP” arrow key to increase time in one-second increments; hold the key down to increase rapidly. Once the time is set, press ENTER; then press the INDEX key FOUR times to return to the “Operator View Display” when a specific time has been set, e.g. 2:05 minutes, the Chipmaster’s Timer counts down to zero once the foot pedal to start the heat cycle is depressed. The timer will stop at zero, even if the heating continues (foot remains on the pedal). For Eutectic Removal we do not set time to remove a component, as the time will be recorded once the component is automatically removed and the Foot pedal released. However we will change (add) time to replace a component using this process.

(Illustration # 5)

 

Install Nozzle 8100-1414 , 66-80332E92, 1.40″ x 1.40″ ( 35.6mm x 35.6mm) for a QFP208 in the Chipmaster, or appropriate nozzle for any other size component. (Illustration # 5)

Using the no-clean flux paste provided in the SMT Tool Kit apply a thin 0.025″ line around the leads of the component, ensure all leads have flux applied and note the orientation of the component for installing a replacement.

Place the board in the Board Holder, as described in the Chipmaster manual and center the component under the Chipmaster Nozzle, the body of the component should be between 1/8″ to 1/4″ (3mm to 6mm) below the base of the nozzle. (Illustration # 6)

(Illustration # 6)

5.7 Depress the foot pedal to start the heating cycle. Note that the GREEN display will begin “counting” the seconds while heating. The RED display will indicate the heated air temperature as it ramps.

5.8 Turn on the vacuum pump, then depress the Vacuum Pickup Assembly to capture the component. The component will not, however, lift off at this time.

5.9 Once the component leads achieve eutectic condition and after approx. 2 minutes 25 seconds of heat application using Profile # 2, the component will automatically rise from the circuit board by the tension applied by the Vacuum Pickup Assembly and be drawn up into the nozzle of the Chipmaster. Release the foot pedal to stop the heating cycle. The GREEN timer display will stop counting, and will record the length of time elapsed.

Depress the board holder piston (cylindrical top of the board holder); remove the entire board and holder from beneath the nozzle assembly by sliding the whole assembly sideways and out from beneath the nozzle. Let the component cool for a while and with a flat surface, such as a small card, place the card under the nozzle, turn off the vacuum pump, using the switch on the front of the Chipmaster. The component should release and drop onto the card.

6. Reinstallation of Components

6.1 Hot air level the pads. In most instances, sufficient solder will remain on the pads to attach a new chip; but the surface may be rough, uneven, or have solder “icicles”, making accurate placement of a new chip impossible. To hot air level, Locate the circuit board (still in the Board Holder) under the nozzle, with the component removed, depress the foot pedal to begin the heating cycle. It is recommended to add a little more flux to the contact area to ensure thorough wetting, don’t worry about flux residue as a cleaning process 6.3 will remove residues. The GREEN display will begin counting DOWN to zero. The solder will melt and become smooth; the solder should cover the pad(s) completely, with a slightly rounded “pillow” shape.

6.2 Remove the board and board holder assembly from beneath the Chipmaster nozzle (depress cylinder, slide holder and board out). Let the board cool to ambient temperature.

Using the Cleaning Fluid Pad Prep Pen from the SMT Tool Kit, release sufficient fluid by, depressing the valve at the tip to soak the area and using the cleaning tip proceed to clean entire contact and surrounding area, using an Antistatic wipe 8200-1500 to finally remove all traces of contamination and old material.

6.4 If there is insufficient solder to cover the contact pad(s), add solder using wire solder or solder paste and a soldering iron. Apply no-clean paste flux to aid wetting and clean afterwards with Pad Prep Cleaner as 6.3.

Now that the contact pads are coplanar and free of all residues and contaminates, apply a thin line of no-clean paste flux (0.025″) around all contacts and with the double sided “clean and flux” brush supplied in the SMT Tool Kit, use the softer and longer flux brush end to brush the flux from outside the contacts to the center, ensure all contacts are adequately fluxed, it’s important that too much flux is not applied otherwise the component will float off position during reflow.

Now, take a new component, (the removed component may be reinstalled if required, providing leads are coplanar), and holding the center of the component between thumb and forefinger of the right hand and with clear view of the entire left hand side and half view of the top and bottom of the component, place the component, aligning the Y axis (left side) and then gently aligning top and bottom half sides.

A small hand 2.3 dioptic magnifier 8200-1200 held in the left hand easily provides all the vision needed to align. Once the component is in place gently release the chip out and away to the right, the flux will support the component. Probes supplied in the SMT Tool Kit will aid fine alignment or the entire alignment procedure dependent upon personal preference. Aligning one full side and two halves should align all four sides but each lead must be exactly centered over the contact pads. The no-clean paste flux will hold the component in place during reflow (melting) of the solder. This flux will dissipate, however, by the completion of the heating cycle. The component corners may be tagged with a soldering iron for extra support prior to reflow, if so desired.

6.7 Place the board under the nozzle of the Chipmaster, being careful to center the component under the nozzle as accurately as possible.

Press the INDEX key on the Chipmaster controller ONCE, and verify the eutectic (Removal) time, now add twenty (20) seconds to this eutectic time to provide sufficient time to reflow the entire component. Use the UP arrows as described previously and press ENTER to save the change. Press the INDEX key FOUR times to return to the “Operator View Display”.

Depress the foot pedal to begin the heating cycle, (Illustration # 7). When the timer reaches zero, the reflow is complete, wait for the solder, component and board to cool remove the Board Holder and inspect your work.

(Illustration # 7)

7. Reflow Removal Procedure.

This procedure follows all of the steps detailed in the Eutectic Procedure above with the exception of contacting the Vacuum Pick Up Assembly before eutectic condition so as to remove the component automatically when all leads reach the condition, In the Reflow Removal Procedure the vacuum is turned on, but the Vacuum Pick Up Assembly is not connected until the component is completely re-flowed.

It is critical in this procedure to use the no-clean paste flux and ensure all the leads contain flux before proceeding to remove the component as described previously.

The reflow time can either be calculated by close observation of the component during reflow, so that when the front, (easily visible), leads can be seen to be re-flowing, a time of twenty seconds (20) is added by noting the timer and waiting until the extra 20 seconds is completed, at which time the entire component is at reflow condition and the Vacuum Pick Up Assembly can be lowered Or as can be seen by the foregoing Eutectic Procedure, from learning the time from performing a single Eutectic Removal and the Reflow Time entered into a written procedure for future repair operations on the same type of component on the same circuit board.

Once Reflow has been reached, depress the Vacuum Pick Up Assembly lightly and gently down by hand, do not be heavy handed, a light and slow moving pressure should be applied with the forefinger, (avoid pressing the component into the reflowed solder on the contact pads, which could cause bridging), observe under the nozzle as the vacuum cup almost reaches the component body, the body will lift to the cup and you may slowly draw the component up into the nozzle, don’t release, but control the Vacuum Pick Up Assembly all the way back to the rest position. Release the Foot Pedal and the Reflow Time will be recorded.

It is not necessary to hot air level the contact pads as the contact pads are coplanar and free of spikes or bridging, next clean as previously described and apply new no-clean paste flux.

7.6 Placement of the component is as previously detailed in the Eutectic Removal Procedure with the exception that it is not necessary to add time to replace, when the timer reaches zero, the reflow cycle is complete.

 

Note:

The Chipmaster, (Illustration # 8) is a Low Temperature operation machine and will not damage board, components or degrade solder, providing Low Temperatures are used, this is recommended to be less than 450F (232C). Effectively this increases the “Window of opportunity” to rework a component and if for any reason you don’t succeed first time you may retry with confidence that you will not cause damage. As with any skill related activity, experience breeds success and practice will make the user highly skilled and the Chipmaster will respect the skills you have learned.

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(Illustration # 8)